Nutritional Stress Factors and Food Processing

BY SAMUEL LEPKOVSKY

University of California. Berkeley. California

CONTENTS Page

I . Introduction . . . . . . . . . . . . . . . . . . . . . . 105

I1 . Nature of Certain Stress Factors . . . . . . . . . . . . . . 107 1 . Stress Factors Operating via the Pituitary-Adrenal System . . . 107 2 . Physiological Stress Factors That Do Not Operate via the Pituitary-

Adrenal System . . . . . . . . . . . . . . . . . . 108 3 . Esseiitial Nutrieiits as Stress Factors . . . . . . . . . . . 109 4 . Nutritional Stress Factors . . . . . . . . . . . . . . . 109

110 1 . Nature and action . . . . . . . . . . . . . . . . . . 110 2 . Destruction . . . . . . . . . . . . . . . . . . . . 111 3 . Counteraction . . . . . . . . . . . . . . . . . . . 114

ents of Foods . . . . . . . . . . . . . . . . . . . . . 114 1 . Nature and Action . . . . . . . . . . . . . . . . . . 114 2 . Counteraction . . . . . . . . . . . . . . . . . . . 116

V . Imbalance of Nutrients as a Stress Factor . . . . . . . . . . . 117 1 . Mineral Imbalance . . . . . . . . . . . . . . . . . . 117 2 . Imbalance among Other Nutrients . . . . . . . . . . . . 118

120

2 . Decreased availability . . . . . . . . . . . . . . . . 120 3 . Fractionation . . . . . . . . . . . . . . . . . . . 121

I11 . Nutritional Stress Factors Normally Occurring in Foods . . . . . .

I V . Nutritional Stress Factors Whicli Do Not Normally Occur as Constitu-

V I . Stress Factors Produced in Foods by Processing and Storage . . . . 1 . Decreased Digestibility . . . . . . . . . . . . . . . . 120

4 . Destruction of Microorganisms . . . . . . . . . . . . . 122

V I I . Destruction of Stress Factors by Processing . . . . . . . . . . 122 1 . Use of Heat . . . . . . . . . . . . . . . . . . . . 122 2 . Use of Enzymes . . . . . . . . . . . . . . . . . . 123 3 . Use of Microorganisms . . . . . . . . . . . . . . . . 123

V I I I . Stress Factors and Acceptability of Food . . . . . . . . . . . 124 1 . Role of Flavors . . . . . . . . . . . . . . . . . . 124 2 . Pliysiological State and I t s Effect on Flavor and Acceptability . 124

I X . Summary . . . . . . . . . . . . . . . . . . . . . . . 126

.

References . . . . . . . . . . . . . . . . . . . . . . 128

I . INTRODUCTION

The science of nutrition is entering a new era . Most. though not all. of the essential nutrients are known . The nutritional requirements of various animals. including man. have been determined with a fair degree

105

106 SAMUEL LEPKOVSKP

of accuracy for most of these nutrients. Also, much is known about the disorders which result from an inadequate intake of each of the essential nutrients. Some of these disorders are non-specific and are characterized by a general debility, decreased growth, and decreased food intake j others are sufficiently specific to point almost unmistalr- ably to the nutrient that is deficient in the diet of the afflicted animal.

The time, if ever, when all our foods will consist of purified nutrients seems distant indeed ; but we shall probably continue, nevertheless, to supplement our food with pure' nutrients in ever-increasing amounts. The source of the bulk of our food still consists of living tissues-or derivatives of living tissues-of plants, animals, or microorganisms, all of which are basically similar in chemical composition, since they all contain essential nutrients such as minerals, vitamins, fa t ty acids, and amino acids. It should be realized, however, that these living tissues were not meant to be food f o r animals. For example, the wheat grain was meant to produce more wheat-not to be made into a loaf of bread; and fish were meant to produce more fish-not to be p u t into a can or frying pan, for eating.

Many compounds that normally occur in living tissues and are perhaps even essential constituents of these tissues are toxic when ingested by animals. These deleterious compounds must be reckoned with when we consider nutritional problems, and they are, in short, nutritional stress factors.

The bulk of our information on nutrition has been obtained by study- ing laboratory animals, which are housed under carefully controlled conditions to insure them maximum comfort and protection against un- favorable environmental conditions. The animals are, moreover, care- fully selected; those that are sick or otherwise abnormal are usually discarded. The food is carefully selected, prepared, and often purified.

When research information thus obtained is applied to animals, and more especially to man, the tacit assumption is made that they, too, live under the ideal conditions of the experimental animals. Nothing could be further from the truth. Animals and man must subsist largely on the foods that are available, and the choice is often very limited. Cli- matic and other environmental conditions may be unfavorable. The sick, especially among human beings, cannot be discarded; they must rather be given every opportunity to live. Life, as we know it, is handicapped by mental, physical, environmental, physiological, pathological, and other stresses affecting the well-being of animals. In this discussion, stress factors refer to those compounds or conditions-environmental, physio- logical, or pathological-which adversely affect the well-being and nu-

NUTRITIONAL STRESS FACTORS 107

tritional requirements of animals. the subject completely.

No attempt will be made to cover

11. NATURE OF CERTAIN STRESS FACTORS

1. Xtress Factors Operating via the Pituitary-Adrenal Xystem

The pituitary and adrenal glands act together as part of a mechanism that helps animals resist a large variety of non-specific stresses that threaten their survival (Selye, 1948). Perhaps the first response to most stresses is the secretion of adrenalin by the adrenal medulla. The adrenalin activates the pituitary, which increases the secretion of adreno- corticoprophic hormone, which, in turn, acts on the adrenal cortex, caus- ing an increase of the secretion of adrenocortical hormones; the latter enable the animal to resist a wide variety of stresses by setting in motion many different types of processes (Selye, 1948; Sayers, 1950) such as:

1. Breakdown and metabolism of tissue protein, resulting in an in- creased output of nitrogen in the urine.

2. Stimulation of gluconeogeneses : the carbohydrates thus formed in- crease the levels of blood sugar and liver glycogen.

3. Decreased utilization of carbohydrates by the tissue and increased utilization of fat, accomplished, in part, by decreased sensitivity to insulin.

4. Possible loss of essential nutrients, other than amino acids: such nutrients must be identified so that they can be replaced by appropriate dietary measures.

It is not certain whether it is the new carbohydrates that are essential to the survival of the animal under stress, or whether it is some other unknown compounds formed by the reactions set in motion by the acti- vated pituitary-adrenal system. Even less is known of the price the animal pays, in terms of nutrients, for protection against the various non-specific stresses that are part of the hazards of living. Evidence is accumulating to indicate that, in order to restore the protein lost to the body, the protein intake must be increased, but it is also necessary to make certain that the dietary protein reaches the cell, where i t is to be laid down as tissue protein. After protein is digested to amino acids and absorbed from gut into the blood stream, i t is not a t once available to the cell. These amino acids must first pass through the liver where they may or may not be deaminated. The endocrine system is one of the factors which will determine whether these absorbed amino acids will reach the cell where they are needed. The stresses of the environ- ment affect the function of the endocrine system and the liver, and

108 SAMUEL LEPKOVSKY

they must also be reckoned with when considering the metabolism of proteins. The bridge between the gastrointestinal tract and the cell may be complicated and tortuous. Essential nutrients, some of which may as yet be unidentified, may be required for the efficient transfer of the proteins from the gut to the cell. These nutrients must be character- ized and the role they play must be determined if we are to prevent deficiencies of the nutrients used up by animals when they are sub- jected to stresses of various kinds. The diet plays an important role in meeting the losses of essential nutrients during stress, but the exact nature of this role remains largely to be determined (Ershoff, 1951; Samuels, 1951).

The stresses which affect the pituitary-adrenal system are many and variable (Sayers, 1950 ; Swingle and Remington, 1944 ; Selye, 1948 ; Ershoff, 1951 ; Tepperman and Engel, 1942) ; they include :

1. Environmental factors, such as extremes of heat and cold, anoxia, and extremes of high and low humidity.

2. Physiological factors, such as pregnancy, fatigue, excessive work, dehydration, caloric deficiency, and stresses created by emotional states of various kinds.

3. The effects of various drugs, such as atabrine, atropine, nicotine, morphine, thyroid, and dinitrophenol.

4. Pathological states, such as infections, fevers, intoxications of various kinds, shock, surgery, burns, hemorrhage.

5. Metabolic disorders that involve the hyperactivity of the various endocrine glands, or the administration of large doses of hormones to compensate for underactivity of the endocrine glands.

Each of these stress factors may exert its stress action in other spe- cific areas as well as those mentioned, depending on the nature of the stress factor and the physiological condition of the animal involved. The manner in which these stress factors increase nutritional require- ments remains largely to be determined. (Ershoff, 1951 ; Samuels, 1951.)

2. Physiological Xtress Factors That Do Not Operate via the Pituitary- Adrenal System

There are some stress factors that do not necessarily involve the pituitary-adrenal mechanism. For example, rapid growth, because of rapid cell division, increases the nutritional requirements of the animal. So does lactation. Improper pancreatic function, by decreasing the flow of pancreatic juice, decreases the digestion of protein. There are probably many other such stresses which have not yet been clearly defined.

NUTRITIONAL STRESS FACTORS 109

3. Essential Nutr ien ts as Xtress Factors

Under certain conditions essential nutrients become stress factors ; they may or may not operate via the pituitary-adrenal system. These conditions include :

1. Imbalance. The presence of an excess of one essential nutrient in relation to another (McCollum et aE., 1939).

2. Certain metabolic disorders. Phosphorus becomes toxic to para- thyroidectomized rats fed high phosphorus, low calcium diets (Shelling, 1932). Potassium becomes toxic to adrenalectomized rats, particularly when they are fed diets low in sodium (Richter, 1936).

3. Certain nutritional deficiencies. To an animal deficient in thia- mine, carbohydrate becomes toxic by further aggravating the thiamine deficiency (Richter et al., 1938). To rats deficient in pyridoxine or vitamin BIZ, high protein diets may become stresses by aggravating these respective deficiencies.

Thus, whether a compound functions as an essential nutrient or as a stress factor depends on the physiological state of the animal.

4. Nuttritional Xtress Factors

A large number of different kinds of compounds that are present in food act to increase nutritional requirements and will be referred to as nutritional stress factors. These compounds are specific and exert their effects by:

1. Decreasing food intake. 2. Interfering with the digestion of food. 3. Decreasing the absorption of nutrients from the gastrointestinal

4. Decreasing the utilization of or increasing the destruction of

I n short, these nutritional stress factors interfere with the transfer of nutrients from the environment to the cell.

Some of these stress factors are proteins ; many are non-proteins. Some are of known composition; many are still to be identified. These compounds increase the requirements for a large variety of nutrients, such as minerals, amino acids, and vitamins. Most of these nutritional stress factors occur normally as components of the plant or animal tissues that form part of our food supply. Certain of these factors are absorbed from the soil; others are formed in foods during spoilage or processing.

Although commonly found in foods, these nutritional stress factors

tract.

absorbed nutrients.

110 SAMUEL LEPKOVSEY

have escaped widespread recognition for a long time, for the following reasons :

1. Many are destroyed by heat during preparation of the food for the table, particularly those that are protein. The mere process of soaking certain foods, such as cereals, destroys certain stress factors. The stress factor in linseed meal that inactivates pyridoxine is destroyed by soaking in water, and the one in corn that inactivates nicotinic acid is destroyed by soaking in lime water. The practice of those primitive peoples who soaked cereals, such as corn, thus receives impressive scien- tific justification.

2. The effect of a stress factor can be counterbalanced by an excess in the diet of the particular nutrient that is inactivated by the stress factor. Indeed, a well-balanced diet consists of essential nutrients in sufficient quantity not only to provide for the needs of the animal but also to restore nutrients that are lost by the action of the diet’s nutri- tional stress factors.

3. By mechanisms largely unknoyn though assumed to be changes in the enzyme systems of the tissues, the organism is physiologically able to adapt to various nutritional stresses. For example, human beings who are fed whole-wheat diets go into negative calcium balance; however, as a result of adaptation they gradually attain positive calcium balance, without change of diet.

4. Ingestion of a ration containing a nutritional stress factor usually decreases the animal’s intake of food, thus decreasing the danger of intoxication.

The following discussion is limited largely to the nature of these nu- tritional stress factors, to their action, counteraction, and destruction. The role of imbalance among the essential nutrients and the impact on them of processing will also be considered.

111. NUTRITIONAL STRESS FACTORS NORMALLY OCCURRINQ IN FOODS

1. Nature and Action

I n Table I a number of foods that contain nutritional stress factors are listed. They vary widely in character and composition. The castor bean contains a protein that is a violent poison, often referred to as a toxalbumin (Breese Jones, 1947). It acts by paralyzing the respiratory and vasomotor systems. Certain feeds, such as alfalfa (Lepkovsky et al., 1950b) and cottonseed meal (Schwartze and Alsberg, 1924), act in an unknown may to decrease the food intake and growth. The compound in alfalfa is probably a saponin (Peterson, 1950) ; its action remains

NUTRITIONAL STRESS FACTORS 111

unknown. The compounds that make nutrients unavailable by interfer- ing with either their absorption or their utilization are : the compound of linseed meal that makes pyridoxine unavailable (Kratzer and Williams, 1948) ; the compound of corn that makes nicotinic acid unavailable (La- guna and Carpenter, 1951 ; Kodicek, 1951) ; the phytin of cereals that makes calcium unavailable (McCance and Widdowson, 1942) ; the oxalic acid of spinach and rhubarb that makes calcium unavailable (McCol- lum et el., 1939) ; the avidin of raw egg white that makes biotin una- vailable (Eakin et al., 1941) ; the conalbumin of raw egg white that makes iron unavailable (Alderton e t al., 1946) ; a compound of raw po- tatoes that makes starch unavailable (Halnan, 1944) ; an inhibitor in soy- beans that makes methionine and possibly cystine unavailable (Hayward et d., 1936; Hayward and Hafner, 1941) ; and the thiaminase of fish that destroys thiamine (Sealock e t al., 1943). An unusual inhibitor is present in soybeans. It causes marked diuresis in chicks (Bouthilet et al., 1950).

A large group of foods contains an antitrypsin that inactivates trypsin and interferes with the digestion of proteins. The antitrypsins of soy- beans (Ham and Sandstedt, 1944), raw egg white (Balls and Swenson, 1934), and lima beans (Klose et al., 1949) have been extensively studied. The antitrypsin of raw egg white has been identified as ovomucoid (Line- weaver and Murray, 1947). Of special interest is the finding that the pancreas hypertrophies when a chick is fed raw soybeans, presumably in response to their antitrypsin content (Chernik et al., 1948). Also, the thyroid enlarges when an animal is fed soybeans (Sharpless et al., 1939), rape seed (Kratzer, 1950), and cabbage (McCollum et al., 1939).

Such physiological action of nutritionaI stress factors, especially as they affect specific organs, must be considered of the greatest importance, even though its full significance cannot be estimated a t present. A curious compound found in buckwheat is a fluorescent dye that causes a rash only in white animals, and only when they are exposed to sun- shine. The reaction can be so severe as to be fatal (McCollum e t al., 1939). Another interesting compound is the lipoxidase of soybeans, which destroys carotene (Sumner and Tressler, 1943).

2. Destruction of Xtress Factors

The destruction of nutritional stress factors is essential for good nu- trition. These in- clude the compounds of raw egg white, soybeans except the goiterogenic factor, navy beans, lima beans, castor beans, and fish. The gossypol of cottonseed meal, though not a protein, is also destroyed by heat under certain conditions (Olcott, 1948). The compounds of alfalfa, buckwheat, the phytin of cereal, the cholesterol of egg yolk, and oxalic acid are not

Those that are proteins can be destroyed by heat.

TABLE I

Some Selected Deleterious Compounds or Inhibitors Which Are Normally Occurring Constituents. of Foods

Inhibitor

Gossypol

Action Nature Inactivation or destruction

Destroyed by oxidation Destroyed by heat Inactivated by iron

Destroyed by wetting

Unknown

for 24 hr.*

Dietary counteraction

Unknown

Food

Cottonseed meal

Decreases food intake

Linseed meal

Alfalfa

Unknown

Unknown

Toxalbumin

Unknown Removes pyridoxine

Unknown

from diet Increasing

Adding cholesterol

Unknown

pyridoxine '

and f a t 6a

Saponin ( Y) BP

Castor bean Protein Paralyzes respira- tory and vasomotor systems

Makes calcium unavailable

Destroys thiamine

Destroyed by heat

Cereals Phytin Inositol

Enzyme,

hexapliosphate

protein

Inactivated by hydrolysis by enzyme phytase

Destroyed by heat

Increasing calcium

Increasing thiamine

Unknown

Fish Thiaminase *

Buckwheat Fluorescent dye

Causes itch, dermatitis, and death in white animals only

Goiterogenic

Unknown

Unknown

Unknown

Thyroxin or

Unknown

iodinated casein lo

Rapeseed meal

Potatoes

Uiiknown

Unknown

Oxalic acid

Unknown

Prevents diges- tion of starch l1

Makes calcium unavailable

Destroyed by heat

Unknown Spinach, rhubarb

Increasing calcium

Egg white Conalbumin Protein Makes iron unavailable l2

Inactivates trypsin

unavailable 15

Deposited in tissues

Inactivates trypsin Hypertrophy of

pancreas

Makes methionine unavailable la

Diuresis 21

Goiterogenic 22

Destroys carotene

Goiterogenic

Makes biotin

Destroyed by heat

Destroyed by heat

Increasing iron

Unknown Egg white

Egg white

Ovomucoid 13 Antitrypsin, protein l4

Protein Avidin Destroyed by heat

Unknown

Increasing biotin

Biotin deficiency m Cholesterol Sterol

Soybeans Antitrypsin Protein Destroyed by heat Unknown

Z Increasing 3

E L 8

Increase e3.

Unknown Q

8

methionine

Unknown 5 Z Increasing iodine a

Increase u)

vitamin A

iodine Tn

3 * c3

Unknown u1

Nicotinic acid

Soybeaiis Unknown Protein Destroyed by heat

Destroyed by heat

Unknown

Destroyed by heat

Unknown

Destroyed by heat LU

Heat zs Soaking in

lime water

Soybeans

Soybeans

Soybeans

Unknown Unknown

Lipoxidase *

Protein

Unknown

Enzyme, protein

Cnknown Cabbage Unknown

Lima beans Antitrypsin Protein Inactivates trypsiii

Decreases growth%

Makes nicotinic acid unavailable

10 Kratzer, 1950. l1 Halnan, 1944. 12 Alderton e t al., 1946. 13 Lineweaver and Murray, 1947.

l6 Eakin et al., 1941. 16 Okey et al., 1950. 1' Ham and Sandstedt, 1944. 18 Chernik e t al., 1948.

Balls and Swenson, 1934.

Navy beans

Corn Unknown Unknown

Protein

Unknown

1 Schwarte and Alsberg, 1924. 2 Oleott, 1948. 8 Withers and Carruth, 1917. 4 Kratzer and Williams, 1948. GLepkovsky et R Z . , 1950b.

6a Peterson, 1950. 6 Breese Jones, 1947.

8 Sealock e t R Z . , 1943. MeCance and Widdowson, 1942.

McCollum et al., 1939.

Hayward and Hafner, 1041. Hayward et al., 1936. Bouthilet e t al., 1950.

zz Sharpless et R Z . , 1939. za Sumner and Tressler, 1943.

Klose ct al., 1949. 26 Johns and Finks, 1920. !M Laguna and Carpenter, 1951. n Kodicek, 19 5 1.

c1 c1 w

114 SAMUEL LEPKOVSKY

readily destroyed. The factor in linseed meal is of interest, since it can be destroyed by merely moistening the linseed meal with water for 24 hr. (Kratzer and Williams, 1948). Of equal interest is the inhibitor of maize, which is destroyed by limewater (Kodicelr, 1951).

3. Counteraction

If the nature of the nutrient interfered with is known, the compound is counteracted by simply restoring the essential nutrient to the diet. Thus, biotin counteracts the action of avidin ; calcium, that of oxalic acid and phytin ; pyridoxin, the compound in linseed meal ; nicotinic acid, that of corn; iron, the conalbumin of egg white; thiamine, the thiaminase of fish ; and methionine, one of the nutritional stress factors in raw soybeans. The goiterogenic compounds of soybeans and cabbage are counteracted by iodides, but the goiterogen of rapeseed meal requires thyroid or iodinated casein. Certain of these compounds can be con- verted into insoluble compounds that are not absorbed and so are ren- dered inactive. For example, iron makes gossypol insoluble, and calcium, oxalic acid.

Some nutritional stress factors are counteracted in unusual ways. Cholesterol exerts its deleterious action in many animals by being de- posited in the tissues. This action can be prevented by inducing a mild biotin deficiency (Okey e t al., 1950).

The compound in alfalfa, probably a saponin, can be counteracted by cholesterol (Peterson, 1950), but we do not know a t present the mech- anism of this reaction. It is conceivable that cholesterol either forms an insoluble compound with the saponins or raises the blood-cholesterol level enough to prevent heniolysis by saponin. The methods for coun- teracting many of these nutritional stress factors are still unknown.

IV. NUTRITIONAL STRESS FACTORS WHICH Do NOT NORMALLY OCCUR AS

CONSTITUENTS OF FOODS

Representative compounds in this group are listed in Table 11. They originate in the spoilage or in the processing of food, or in the soil, from which they are absorbed by the growing plant.

1. Nature and Action

Cattle that ingest selenium, a toxic compound absorbed from the soil (Horn, Nelson, and Jones, 1936; Franke, 1934; McCollum e t al., 1939), lose their hoofs, are stunted, emaciated, and anemic; many die. Flour bleached with nitrogen trichloride (agene) contains a toxin to which different animals react with varying degrees of severity. It has been

Food

Flour bleached with agene

Spoiled sweet clover

TABLE I1

Some Deleterious Compounds Found in Foods Which Are Not Normally Occurring Constituents of Foods

Spoiled fa t (rancid)

Grain 7,8

Dehydrated foods

Sulfured foods

Sulfites

Inhibitor Nature Action Inactivation or Dietary destruction counteraction

Sulfoximine 9 Causes running Unknown Addition of

Dicumarin

Unknown

Selenium

Unknown

so,

Foods with spray residues

1 Melanby. 1947. * Reiner et al., 1950a.

4 Reiner ct al., 1950. Boudresu. 1947.

&Link, 1943-44.

fits

Aromatic Decreases blood compound prothrombin,

hemorrhages

vitamins A and E Unknown Destroys

Mineral

Unknown Non-enzymic

Gas Destroys

browning

thiamine Causes hemorrhagic

diathesis

Possible toxic eff ects remain to be determined

6 Mattill, 1927. ‘Franke, 1934. SHorn, Nelson, and Jones, 1936. 9 Gortner, 1940. lo Fels and Cheldelin, 1948.

methioniiie

vitamin K Unknown Adding

Unknown Administering vitamins A and E separate from the rancid fa t

Unknown Increasing protein or methionine,” or adding arsenic

Unknown Unknown

Unknown Increasing thiamine

Increasing vitamin E l4

Probably very little

11 Dubois et al.. 1940. a Stadtman, 1948. 18 Morgan et al., 1935. l4 Norris, 1952.

116 SAMUEL LEPKOVSKY

identified as sulfoximine (Reiner et uZ., 1950a). This toxin produces running fits in dogs (Melanby, 1947), and, when it is administered in sufficient concentrations, rats are equally susceptible. Rabbits, cats, and ferrets are quite susceptible, but chickens, guinea pigs, and hamsters show no toxic reaction. So far, no reaction has been demonstrated in human beings who ingest “agenized” flour (Boudreau, 1947). Appar- ently all animals are susceptible to the toxic action of sulfoximine if enough is administered; this has been demonstrated only with the pure compound.

Spoiled sweet clover contains dicumarin, which by decreasing the prothrombin content of the blood prevents clotting (Link, 1943-44) ; animals that ingest dicumarin suffer from hemorrhages.

Because of the complexity of the process of rancidity, the problem of the effects of rancid fats on the nutrition of animals, though studied extensively (Mattill, 1927), is not entirely understood. However, it is well known that rancid fats are detrimental by destroying vitamins A and E, and possibly other vitamins.

Sulfur dioxide when used as a preservative in foods-especially in the dehydration of fruits and vegetables-destroys thiamine (Morgan et ul., 1935) ; the carotene and vitamin C are preserved (Morgan et al., 1931). Sulfur dioxide also causes hemorrhagic diathesis which is coun- teracted by vitamin E (Norris, 1952).

Certain dehydrated foods deteriorate during storage, because o f a non-enzymatic chemical reaction involving the condensation of proteins (and/or amino acids) and carbohydrates, a reaction referred to as the Maillard reaction (Stadtman, 1948). The dehydrated food becomes un- palatable and darkens j and its physical characteristics are so changed that reconstituting it is difficult. The effect of such deterioration on the nutritional value of the food is largely unknown.

I n agriculture, as increasing numbers of new sprays are used, the problem of the toxicity of their residues becomes ever more serious. Un- like most naturally occurring nutritional stress factors, many of which can be destroyed, few of these compounds are destroyed by the usual processing methods.

2. Counteraction

Dieumarin acts as a competitive inhibitor of vitamin K, and it can be effectively counteracted with this vitamin. Sulfoximine, the inhibitor of agenized flour, acts as a competitive inhibitor of methionine and can be counteracted by massive doses of methionine (Reiner e t al., 1950b). The deleterious action of rancid fats can be counteracted, a t least in part, by restoring to the diet the vitamin A or E which has been de-

NUTRITIONAL STRESS FACTORS 117

stroyed by the rancid fats. It is advisable, as far as possible, to prevent these vitamins from coming into contact with rancid fat, even in the stomach (Lease e t al., 1938). Similarly, the destructive effect of sulfur dioxide on thiamine can be easily counteracted by restoring the lost thia- mine to the diet, Of unusual interest is the ability of vitamin E to pre- vent the hemorrhagic diathesis caused by the presence of excessive sulfite in the diet (Norris, 1952).

The many ways by which the toxic action of selenium can be counter- acted are also of unusual interest : namely, by methionine (Fels and Cheldelin, 1948), by arsenic (Dubois e t al., 1940), and by high levels of protein (Gortner, 1940).

V. IMBALANCE OF NUTRIENTS AS A STRESS FACTOR

1. Mineral Imbalance

Balance and imbalance among nutrients have been studied most ex- tensively with regard to minerals, largely because of their availability and ease of introduction into diets. Imbalance among minerals occurs when one mineral, present in excess, creates a deficiency in another, and thus upsets the physiological balance between them. The logical way to counteract this imbalance is to incorporate in the diet the deficient min- eral in sufficient amounts. Excess iron or manganese causes a phos- phorus deficiency, which can be overcome by increasing the phosphorus content of the diet (McCollum et al., 1939). When calcium is present in excess, phosphorus becomes deficient in the diet; the deficiency can be overcome by means of additional phosphorus. Conversely, when phos- phorus is present in excess, calcium becomes deficient; and this de- ficiency can be overcome by increasing the calcium content of the diet (Maynard, 1947).

Many unexpected interrelations occur when minerals are out of bal- ance with each other.

1. The calcium deficiency caused by excess magnesium can be corrected by the addition of phosphorus. The phosphorus removes the excess mag- nesium by forming an insoluble salt with it (McCollum e t al., 1939).

2. Excess phosphorus may cause an anemia by decreasing the avail- ability of iron. This condition can be counteracted either by increasing the intake of iron or by increasing the calcium intake; the calcium re- moves the excess of phosphorus by combining with it to form an insoluble salt (McCollum et al., 1939).

3. A high intake of calcium and phosphorus makes dietary manganese unavailable by adsorbing it, thus preventing the normal absorption of

118 SAMUEL LEPKOVSKY

TABLE 111

Imbalance as a Stress Factor

Action

Phosphorus deficiency

Calcium deficiency

Phosphorus deficiency

Calcium deficiency

Iron deficiency anemia

Nutrient out of balance

Excess iron or manganese

Excess magnesium

Excess calcium

Excess phosphorus

Excess phosphorus '

Excess minerals

Excess protein

Excess casein, protein

Excess nicotinamide

Excess methionine

Excess glycine or gelatine ; or tryptophan-deficient protein lo

protein

proteins

Excess lysine-deficient

Excess methionine-deficient

'McCollum et al., 1959. 2 Maynard, 1947.

Manganese deficiency

Vitamin B,, deficiency

Aggravates pyridoxin deficiency and vitamin B, deficiency

feed intake

feed intake

feed intake

Depresses growth and

Depresses growth and

Depresses growth and

Increases lysine

Increases methionine

requirements

requirements

3 Schaible and Bandemer, 1942. 4 Caskey and Norris, 1939. 5 Hartmsn et RZ., 1949.

Counteraction

Increase phosphorus

Increase phosphorus

Increase phosphorus

Increase calcium

Increase calcium or increase iron

Increase manganese or

Increase vitamin B, Increase pyridoxin

inject normal amount '

Increase methionine

Increase protein or add glycocyamine

Increase tryptophan or nicotinic acid

Increase lysine

Increase methionine

6 Cereeedo and Foy, 1945. 7 Handler and D a m , 1942. 8 McKittrick, 1947. 9 Grau and Kamei, 1950. 10 Elvehjem and ICrehl, 1947.

this nutrient through the intestinal tract (Schaible and Bandemer, 1942). This condition can be counteracted by increasing the oral intake of manganese, or by giving the needed amount parenternally, thus circum- venting the difficulty of its absorption from the gastrointestinal tract (Caskey and Norris, 1939).

To prevent a mineral deficiency, the various interrelations among the minerals must be watched closely.

2. Imbalance among Other Nutrients

Protein, by entering into many interrelations with other nutrients, in- creases or decreases nutritional requirements. Frequently, excess proteins will counteract stress factors (Gortner, 1940 ; Grau and Kamei, 1950).

NUTRITIONAL STRESS FACTORS 119

Excess protein may itself act as a stress factor, aggravating a pyridoxine deficiency (Cerecedo and Foy, 1945) or a vitamin BU deficiency (Hartman e t al., 1949). The excess protein in these cases may be counteracted by increasing the intake of pyridoxine and vitamin B12, respectively.

A deficiency of methionine in rats results from an excess of nicotin- amide. This excess is methylated and excreted, and in the process the methionine level is depleted, resulting in a methionine deficiency. This deficiency can be counteracted, however, by feeding more methionine (Handler and Dann, 1942). An excess of nicotinamide is innocuous for rabbits and guinea pigs, because these animals do not methylate it (Handler, 1944), a revealing example of the existence of the close relation between the injuriousness of a compound and the physiological reactions of the animal to it. Results obtained with one species must be applied to another species with great caution, and particular care is essential in applying results obtained with animals to man.

These effects can be counteracted by glycocyamine, which presumably uses UP the excess methionine in the synthesis of creatine (McKittrick, 1947). The excess methionine can also be counteracted by increasing the protein intake (Grau and Kamei, 1950), though the mechanism involved in this counteraction is still obscure.

An excess of glycine or gelatin, or of a protein low in tryptophan, such as zein, also depresses growth and food intake. These imbalances cause deficiencies of tryptophan or nicotinic acid. Both deficiencies can be corrected by administering tryptophan or nicotinic acid (Elveh- jem and Krehl, 1947).

The increased intake of some proteins deficient in specific amino acids presents an interesting case of imbalance. This condition has been studied with proteins deficient in lysine and methionine (Grau and Kamei, 1950). The dietary requirement for lysine is 0.9% of the total diet, when the proteins of sesame meal are fed at a level of 20%. When the level is increased to 30%, the lysine requirement is increased to 1.1%. Similarly, the methionine requirement is increased when the intake of a methionine-deficient protein is increased. As the protein-level is in- creased, the lysine and methionine requirements also increase, but a t a slower rate (Crau and Kamei, 1950). A protein that a t the level of 20% is slightly deficient in lysine or methionine may prove satisfactory when the level is increased. NO matter how high a level is fed, however, pro- teins markedly deficient in these amino acids are unsatisfactory, even when the intake of lysine or methionine is adequate a t the 20% level.

Excess methionine depresses growth and food intake.

120 SAMUEL LEPKOVSKY

VI. STRESS FACTORS PRODUCED IN FOODS BY PROCESSING AND STORAQE

1. Decreased Digestibility

The bio- logical value of proteins is very often greatly reduced, because of a de- crease in the digestibility of the protein and in the availability of its amino acids.

It is difficult to interpret evidence showing a decrease in digestibility of proteins in foods processed with heat. The methods for determining digestibility of proteins involve the measurement of the nitrogen in the food and in the feces; the assumption is that the nitrogen in the feces represents ungbsorbed food nitrogen plus the “metabolic” nitrogen fraction of the feces as determined on a protein-free diet. No allowance is made for the increase in the “metabolic” nitrogen fraction caused by the ingestion of the food composing the experimental diet. It is difficult, if indeed not impossible a t present, to differentiate what fraction of the feces originates from the food, and what fraction comes with the secre- tions from the body. In the words of Murlin: “It is an old dictum that the ‘feces are essentially unabsorbed residues of the digestive juices’- not unabsorbed food primarily. It has been a moot question for a t least seventy-five years whether feces should be regarded as waste from the body or waste from the food” (Murlin et al., 1938).

Aside from the complications introduced by the nature of feces in determining the digestibility of food, it is also important to understand the reaction evoked in the body by the ingestion of overheated foods. Since “the large stools from torn wheat are caused not so much by the addition of bran and fibre as by the stimulation of the secretion from the alimentary lining” (Murlin et al., 1938), we must inquire into the nature of this body I ‘ waste” to determine whether it has already served an essential physiological need before its excretion into the alimentary canal to swell the fecal output. If increased amounts of digestive enzymes are included among these increased secretions from the body, then i t is possible that this body “waste” may have been associated with improved digestion, and it may, therefore, have a positive value.

2. Decreased AvuiZu6iZity

Moist and dry heat are frequently used in processing food.

Just what is meant by decreased availability of the nutrients of processed foods is also hard to determine or evaluate. “The digesti- bility of the toasted proteins was but little different from that of the raw, particularly in the older animals; and the unexplainable loss of nitrogen occurred chiefly in the urine, indicating that the change pro-

NUTRITIONAL STRESS FACTORS 121

duced by the heat treatment lies probably in the assortment or availa- bility of amino acids absorbed” (Morgan, 1931). The inability of animals properly to utilize proteins that they are able to digest is probably caused by the destruction or unavailability of one or more essential amino acids.

Some light is thrown upon this problem by studies with the protein zein, which is readily digestible both in vivo and i l z vitro, but very poorly utilized unless first subjected to acid hydrolysis (Geiger and Hagerty, 1949). During processing, linkages presumably were formed that made the amino acids unavailable.

A similar phenomenon is observed when proteins are heat processed. Under this condition, lysine is apparently rendered unavailable ; as a result, there is reduced growth of the laboratory rats. These rats re- spond with increased growth, however, to the addition of lysine (Greaves e t al., 1938). Yet, since the same amount of lysine can be isolated chem- ically from heat-processed as from unprocessed proteins, the amino acid itself may not necessarily be damaged by the processing (Block et al., 1934). Other amino acids besides lysine may also be damaged or ren- dered unavailable in the heat processing of foods (Greaves et al., 1938; Block e t al., 1946). Similar results have been obtained with skimmed-milk powder that has been subjected to storage for several years (Henry e t al., 1948).

3. Fractionation

During processing which involves the fractionation of food, nutrients are frequently removed. For instance, in the milling of white flour from wheat, the bulk of the essential nutrients is removed. I n the in- dustrial production of sugar from beets, sugarcane, or corn, all the es- sential nutrients contained in the plant material are removed in the fractionation process. Milk is also fractionated, for butter is considered, economically, the most valuable past of milk. The non-fat milk residue, which carries almost all the essential nutrients of milk, is termed “skimmed milk”; too little of i t is consumed by the human population. These nutrient components that are fractionated out of foods are not lost entirely because they are used by the feed industry in the production of meat, milk, and eggs.

During processing, imbalanced foods are produced, making an other- wise adequate diet inadequate (Grau and Kamei, 1950). Imbalance in proteins, such as wheat gluten, zein, and gelatin, has already been dis- cussed.

122 SAMUEL LEPKOVSKY

4. D e s t r u c t i o n of Microorga,n isms

When foods are processed, the most painstaking care is taken to elim- inate the largest possible number of microorganisms. Yet animals are conditioned to take microorganisms in with their foods; and the in- testinal bacteria may play a decisive role in their health and well being. At any rate, such bacteria should not be taken for granted, even though we do not know much about them. I n germ-free rats, wheat bran is constipating; it is laxative only after it has been acted upon either in vivo or in. vitro by certain intestinal bacteria (Reyniers, 1946). Wheat bran was not laxative in about 25% of normal rats studied, indicating that not all rats contain the microorganisms that can elaborate the laxa- tive principle in wheat bran.

VII. DESTRUCTION OF STRESS FACTORS BY PROCESSING

1. Use of Heat

When man discovered fire, he employed it to process his food, and he has kept right on using it for this purpose. Only now are we beginning to appreciate its role in the improvement of food. Many of the dele- terious compounds present in foods are proteins that are destroyed by heat (Table I). Without the use of heat, beans and potatoes could not possibly occupy the place that they do today in the diet of both animals and man.

Hea.t must be applied cautiously and intelligently in the processing of foods. Those foods containing toxic proteins must be heated sufficiently to destroy their toxic properties; but they must not be overheated, be- cause excess heat decreases the biological value of non-toxic and detoxi- fied proteins. Frequently, there is little difference between just enough and just too much heat, so that careful control must be exercised in the heat processing of food.

A consideration of heat-processed foods, from the point of view of bio- logical value as well as of popularity, reveals that there may be no apparent relation between nutritional value and acceptability by the consumer. Nutritional investigations show that toasting, “puffing, ” or “exploding” cereals reduces their nutritional value (Morgan, 1931) ; yet such cereals are popular breakfast foods. It is possible that, while reducing the nutritional value of food itself, heat processing nevertheless creates other positive nutritional values. For example, the effect of heat-processed foods in increasing fecal bulk (Murlin e t al., 1938) sug- gests that some compounds formed during heating may stimulate the

NUTRITIONAL STRESS FACTORS 123

flow of the digestive juices and so contribute to “digestive comfort.” This problem needs further investigation.

2. Use of Enzymes

The phytase in cereals can be used to hydrolyze the phytin (McCance and Widdowson, 1942), to prevent its interference in calcium metabolism, and, a t the same time, to make the phosphorus of the phytin available.

The enzymes of meat are used to tenderize i t by “hanging” it under carefully controlled temperature conditions.

Enzymes may also cause food to deteriorate; this effect occurs when lipase hydrolyzes fats. Care in the use of enzymes is essential in order to achieve a maximum improvement of food with the least possible damage.

Enzymes can be used effectively to improve foods.

3. Use of Microorgafiisms

Microorganisms have been used for a long time to process foods. The fermentation of dough to leaven bread, and thus to introduce porosity into it, is an old process. The same ingredients made into a loaf of bread are more acceptable than when they are baked into biscuits. The question may well be raised whether porosity is the chief contribution of the leavening process to the acceptability of bread, o r whether other factors are also significantly involved. During fermentation of bread dough, especially when allowed to proceed for a sufficiently long time, changes are produced in the dough by the activity of the microorganisms as well as by the enzymes present in the flour. Many compounds, in- cluding various flavors, are contributed to the bread by the action of the microorganisms engaged in the fermentation process. Compounds present in flour may be altered by the microorganisms. We have seen an example of such alteration in the formation from wheat bran of a substance with laxative properties (Reyniers, 1946). The longer-extrac- tion flours provide a more extensive opportunity for the activity of microorganisms than does white flour. The compounds thus contributed may impart peculiar characteristics to the resulting bread and may be decisive in determining its acceptability.

Flavors can and do play a decisive role in the acceptability of food and possibly upon its nutritiona1 value. It is very significant that Lim- burger cheese is popular despite the difficulty of getting it past the nose to taste it.

124 SAMUEL LEPKOVSKY

VIII. STRESS FACTORS AND ACCEPTABILITY OF FOOD

1. Role of Flavors

It is no mere academic question whether flavors stimulate the taste buds superficially, or whether the stimulus is a deep-seated physiological phenomenon. The same question may be raised about stress factors in foods, especially since it is known that many of them decrease food intake in animals, presumably as the result of an unpleasant taste and/or odor. Unless we understand the role of flavors in foods, good or bad, whether they act psychologically or physiologically or both, there can be little hope of grappling successfully with the problem of food ac- ceptability.

2. Physiological State and Its Effect on Flavor and Acceptability

Intrinsically, food tastes neither good nor bad. One concept of taste in food is that it is a reflection of the internal chemistry of the body and finds its expression through the brain as a conscious sensation. It is commonly referred to as the psychological reaction to food. (This is not to be confused with psychocultural reactions to foods which do not have their origins in physiology.) The following examples may be cited as illustrating these ideas,

1. Sugar has a well-known “sweet” taste, an expression often used to define a degree of pleasantness in food. The acceptability of sugar has been shown to be related to the level of blood sugar. Human sub- jects with a normal blood sugar level rejected a 30% solution of sucrose as “sickeningly” sweet. The same subjects with their blood sugar re- duced to about half their normal levels no longer found the 30% sucrose solution “sickeningly ” sweet, but it became, instead, quite acceptable as a “long drink” (Mayer-Gross and Walker, 1946). Thus, there was established a direct relationship between the acceptability of a 30% sucrose solution and the physiological state of human subjects, specifi- cally, in this case, the blood sugar level. I n other words, a state of stress (namely, a low blood sugar) rendered the sugar solution accepta- ble, whereas in the same individuals not under this specific stress the same sugar solution was unacceptable.

2. Excess fat is objectionable to many people or is accepted without enthusiasm. Human subjects subsisting on low fat diets for long periods of time develop a craving for fat, often referred to as “ fa t hunger.” I n this condition, which may be considered a state of stress, human beings find fat very delicious. The taste of fa t is determined by the physiologi- cal state of the human subject, in this case a state of fa t deficiency, and

NUTRITlONAL STRESS FACTORS 125

this nutritional deficiency or state of stress has exerted a profound role in determining the taste and acceptability of fa t (Ritter, 1950).

3. Milk powder was unacceptable to troops in World War 11. The unacceptability was a consequence of processing, storage, or both, be- cause a t the same time fresh milk was in very great demand. The changes in the powdered milk that caused its low acceptability are not completely understood. There may have been the loss of characteristic flavors which make fresh milk acceptable, the development of unpleasant flavors or other deleterious compounds, or changes in texture, all of which probably played a role in converting a highly acceptable food into one that was highly unacceptable (Coulter et al., 1951). Yet this powdered milk became highly acceptable to troops who had previously rejected it, when these troops were placed under stress. This happened to a group of American soldiers in a German prison camp. They were slowly starving to death, and they knew it because a medical officer in the group calculated their caloric intake and estimated it as about 200 calories short of their basal requirements. They were therefore under severe stress, with an altered physiological state which accompanies in- adequate caloric intake. These men were saved from death by food parcels sent to them through the International Red Cross. The food parcels contained a variety of items, among them powdered milk, cheese, chocolate, canned meat, etc. The powdered milk was found to be the most acceptable item, since ‘‘it satisfied-even more than the chocolate- the prisoners’ craving for something rich to eat.” (Englander, 1945). Here, clearly, is a case in which the physiological state of a human being, specifically a state of stress, had a profound effect upon acceptability of a food.

I n the examples just cited, foods that were normally unacceptable become highly acceptable when a state of stress developed. Conversely there is abundant superficial information to indicate that foods accepta- ble to human subjects under normal conditions become unacceptable under conditions of stress. Unfortunately, there is little good evidence with human subjects to illustrate this. The experience obtained with raw soybeans, although not good, may, however, be cited. When raw soy- beans were fed to well-nourished human subjects (Lewis and Taylor, 1947), there was no particular difficulty encountered with their accepta- bility. The chief observation made was that there was a 20% greater nitrogen retention by these humans of heated soybeans than of the raw. When raw soybeans were fed to human subjects in a condition of stress, in this case Americans who were Japanese prisoners of war (Cartwright and Wintrobe, 1946), they caused nausea, vomiting, and diarrhea and

126 SAMUEL LEPKOVSKY

were so unacceptable that these American prisoners refused to eat them even under the severest conditions of starvation. It is, of course, un- known whether these differences in the reactions to raw soybeans were due to the difference in nutritional condition of the subjects, to differ- ences in the raw soybeans fed, or to some unknown causes.

There is better evidence with animals that foods acceptable under normal conditions became unacceptable under stress. The best example of animal experimentation along these lines concerns fresh-water eels. These animals are under stress when put in salt water. I n fresh water, bits of fish and dead worms are accepted by the eels, but in salt water they will starve to death rather than eat pieces of fish or dead worms. Here a stress condition renders food which is normally acceptable com- pletely unacceptable. Under stress these eels accept a “tempting mor- sel,” a worm that wiggles (Lovern, 1951).

It is a t present unknown just what the mechanism is by which stress factors act to make food more or less acceptable.

1. Affecting the flavor of food, decreasing or increasing its palata- bility.

2. Influencing the flow of digestive juices, increasing or decreasing them.

3. Affecting motor phenomena in the digestive tract, increasing or decreasing peristalsis, gastric emptying time, etc.

4. Changing the composition of the body fluids which bathe the hypo- thalamus and other tissues playing a role in the basic phenomena of food intake such as hunger, appetite, palatability, etc.

They may act by :

IX. SUMMARY

1. The characterization, identification, and synthesis of most of the essential nutrients and the progress made toward the elucidation of their mode of action is a fabulous and revolutionary achievement which has opened up new research vistas to the attention of nutritional investi- gators.

2. Of equal importance is the realization that the decisive event in good nutrition is the delivery to each cell of all the essential nutrients required by the cell in proper amounts and in the proper balance to each other.

3. At least three processes are involved in the transfer of essential nutrients from the environment to the cell:

This process is under the control of various physiological and psychological mechanisms through ex-

a. The ingestion of food.

NUTRITIONAL STRESS FACTORS 127

pressions such as hunger, anorexia, appetite, and satiety, all of which help to determine, a t least in part, acceptability of food.

b. Digestion of food and liberation of the essential nutrients. This is effected largely by the secretions of the mouth, the stomach, the pancreas, the liver, and the intestinal tract.

c. The transfer of the essential nutrients f r o m the gastrointestinal tract t o the cell. This process is largely under the control of the secretary, hormonal, and enzymic mechanisms of the liver, the pancreas, and the gastrointestinal tract.

4. Every one of these processes is subject t o disturbance b y m a n y d i f - ferent kinds of unfavorable factors, namely, stress factors, which increase nutritional requirements b y :

a. Increasing the loss of essential nutrients from the body. b. Interfering with the metabolism of absorbed essential nutrients. c. Decreasing the absorption of essential nutrients. d. Decreasing the digestion of food. e. Decreasing the intake of food.

5 . Many of these stress factors originate in t h e :

a. Environment, including extremes of heat and cold, extremes of high and low humidity, anoxia, etc.

0. Playsiologicul factors, such as caloric deficiency, dehydration, excessive fatigue, disturbed emotional states, rapid growth, preg- nancy, and lactation.

1. Disease, involving infections, intoxications, shock, surgery,

2. Metabolic disorders such as hypo or hyper function of various

3. Administration of various drugs such as atabrine, morphine,

d. Food. Many stress factors may be associated with food. Among

c. Pathological states.

burns, and hemorrhage.

glands.

atropine, nicotine, thyroid, and dinitrophenol.

them are: 1. Deficiencies of essential nutrients in the diet. 2. Imbalance among the essential nutrients. 3. The presence of deleterious compounds which:

a. Destroy essential nutrients. b. Make essential nutrients unavailable. c. Interfere with the utilization of essential nutrients. d. Interfere with the digestion of food. e. Act in unknown ways to decrease the intake of food.

128 SAMUEL LEPKOVSKY

6. P r o c e s s i n g may d e s t r o y or c r e a t e stress factors.

a. Heat, properly applied, will destroy a great many stress factors, especially those that are proteins, and in addition may create positive values such as the synthesis of desirable flavors. Im- properly applied heat will create stress factors by destroying essential nutrients or making them unavailable and by decreas- ing the digestibility of food.

6 . Fermentation will frequently destroy stress factors and in addi- tion may create positive values which will counteract them.

c. Fractionation of food may eliminate stress factors by removing deleterious compounds or create them by removing essential nutrients.

ACKNOWLEDGEMENTS

The writer wishes to acknowledge with thanks the help given in the preparation of this manuscript by Mrs. Edith K. Ritter of Chicago and Dr. S. Morgulis of the University of Nebraska Medical School, Omaha.

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