A METHOD FOR THE DETERMINATION OF THE ENERGYVALUES OF FOODS AND EXCRETA.

By FRANCIS G. BENEDICT AND EDWARD L. FOX.

(From the Nutrition Laboratory of the Carnegie Institution of Washington,Boston.)

(Received for publication, October 9, 1925.)

The determination of the energy values, specifically the heat ofcombustion, of various foods and excreta forms an important partof many investigations in nutrition. The calorimetric bomb iscommonly used for these determinations, but its complexity ofmanipulation and its almost prohibitive cost greatly narrow theopportunities for making such studies. Moreover the calculationof results is time-consuming and usually involves more mathe-matics than are warranted by the highest limit of accuracy possiblein the preparation of the sample from unhomogeneous material.As an outcome of research on the respiration of man and thedevelopment of simplified apparatus for measuring the actualoxygen consumption during the processes of oxidation in the body,a principle for the determination of the energy values of foods andfeces in the case of both men and animals has been experimentallytested and embodied in an apparatus which is less complicated, lessexpensive, and which enables simpler calculation of results thandoes the bomb calorimeter.

The fundamental principle of this apparatus is the direct de-termination of the volume of oxygen required to burn a knownweight of a substance and the computation therefrom of the po-tential'energy of the substance, based upon a series of factors forthe calorific value of a liter of oxygen previously established bycombustion of similar material in a bomb calorimeter. In a puresubstance, such as sucrose, the amount of oxygen involved in thecombustion of 1 gm. can readily be computed. Since the heat ofcombustion is accurately known, the calorific value of each literof oxygen can be calculated. These relationships for a number of

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commonly metabolized compounds have been computed (1) andare given in Table I., Thus with these pure substances, if the amount of oxygen re-quired to oxidize 1 gm. of substance is known, the heat of combus-tion can be accurately computed by the use of the factors in thelast column of Table I. Since food materials and feces consistfor the most part of the substances listed in Table I, particularlyof protein, fats, and carbohydrates, it would be possible to com-pute the energy values of various foods by estimating the pro-portion of protein, fat, and carbohydrate in the food (based upon

TABLE I.

Gaseous and Energy Relationship in the Combustion of Some CommonlyMetabolized Compounds.

Substance.

Starch........................Cane sugar ....................Dextrose......................Lactic acid ....................Animal fat ....................Human " ....................Protein.......................Acetone.......................6-Oxybutyric acid............Ethyl alcohol

Oxygenrequired to

oxidize 1 gm

cc.

829.3785.5746.2745.9

2013.21990.8956.9

1542.9968.2

1459 .5

Produced in theoxidation of 1 gm.

Carbondioxide.

cc calls.

829.3 4.20785.5 3.96746.2 3.74746.0 3.62

1431.1 9.501420.4 9.54773.8 4.40

1157.2 7.43860.7 4.69972.9 7.08

already existing food analyses) and applying the establishedcalorific value per gm. of each of these substances. The range inthese calorific values is very wide, however,-from 4 to as high as 9calories-whereas the range in the calorific value of a liter ofoxygen is only from 4.6 in the case of protein to 5.0 in the case ofcarbohydrates. Actually determined calorific values of oxygenin the case of various foods and feedingstuffs and excreta of manand beasts would therefore enable more exact approximations tothe true energy values of the substances in question. By compar-ing the oxygen involved in the combustion of 1 gm. of substancedirectly with the heat of combustion as determined in the calori-

Calories perliter

of oxygen.

5.065.045.014.854.724.794.604.824.854.85- .- .... ...... I ____ .- I _

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metric bomb, one can establish such ratios. When the calorificvalues of oxygen have thus been established, it is only necessary

TABLE II.

Calorific Values of Oxygen for Various Foods, Feedingstuffs, and Excreta.

Substances.

High carbohydrate substances.Dried skimmed milk ...................................Oyster crackers .........................................Corn-meal ..............................................Nut bread .............................................Cheese sandwich ....................... ..........Chicken "Salmon salad sandwich ................................Club sandwich ........................................Doughnut.............................................

Highly nitrogenous substances.Glidine (vegetable protein) .............................Ossein................................................Collagen.......................... ..............Plasmon ......... .............. .............

Fats.Olive oil ..............................................Corn ................................................

Cottonseed oil .........................................Cod liver oil ..........................................Goose fat .............................................Butter ................................................

Mixed foods.Beef stew .............................................Mince pie ................................. ..........

Animal foods.Hay, Specimen I......................................

" " II .....................................Cottonseed meal ......................................Linseed meal ..........................................Gluten " .

Excreta.Human feces ..........................................Steer " .......... .....................

Calories perliter of oxygen.

4.894.904.884.884.954.954.984.934.90

4.674.694.704.65

4.744.714.704.704.754.62

4.844.97

4.804.864.664.764.85

4.974.84

to measure directly the oxygen involved in the combustion of afood and, taking into consideration the general character of thesubstance being burned, to apply the heat factor previously de-

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termined from the actual combustion of similar materials. Arather extensive series of such factors is presented in this paper(see Table II), together with a description of the method forthe direct determination of the oxygen absorbed in the combustion.

This procedure obviously requires a combustion chamber, amethod of ignition, and the control of the combustion, in order toinsure complete conversion of all the carbon and hydrogen in thesample to carbon dioxide and water and the liberation of nitrogenin the free form in the case of nitrogenous materials. A knownweight of substance is burned in a confined volume of nearly pureoxygen, with provision for the rapid absorption by soda-lime ofthe carbon dioxide produced and the return of the purified oxygento the combustion chamber. The contraction in volume due tothe absorption of oxygen in the process of combustion may bemeasured by a small expansion chamber, the top of which isbrought back to its original level by introducing a measuredvolume of air or oxygen after the combustion ceases, or it may bemeasured by noting quantitatively on a calibrated spirometerbell the decrease in the apparent volume of air in the entireapparatus.

The application of this principle in the estimation of the heatingvalue of fuels has already been described elsewhere (2). The ap-paratus perfected for this purpose has been styled the "oxy-calorimeter," since it measures the heat indirectly by referenceto the oxygen consumption. This most precise form recognizesthe necessity for great accuracy in the estimation of the energyvalue of fuels, because of their high economic value and financialimportance. Hence in the measurement of the contraction involume of oxygen in this special form of apparatus every effortis made to secure standard, reproducible conditions of tempera-ture, pressure, and particularly humidity.

When the energy values of foods and excretory products areto be studied, however, a simpler device with a sufficiently highdegree of accuracy to meet all conditions in problems in nutritionmay be employed. Thus, any one of the numerous simple formsof closed circuit respiration apparatus may readily be adapted tothe direct determination of the oxygen consumption involved inthe combustion of a known weight of previously dried food mate-rial, and this type of apparatus is especially recommended for use in

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hospitals, dietetic laboratories, and particularly those institutionsalready possessing some simple form of respiration apparatus.The fundamental principle of the apparatus remains the same.Since, however, the error in the sampling of foods and excretamay be as high as 5 per cent or more, special provisions for thecontrol of temperature, pressure, and particularly humidity areentirely unnecessary and, indeed, not justified.

The combustion chamber and method of ignition are common toall forms of the oxy-calorimeter, but several methods for themeasurement of the oxygen absorption are available.

FIG. 1. The student respiration apparatus employed as an oxy-calorim-eter for determining the energy values of foods, feedingstuffs, and excreta.

Use of the Student Respiration Apparatus as an Oxy-Calorimeter.

Of the various forms of apparatus for the measurement of theoxygen consumption of humans the simplest, accurate form withwhich we are familiar is that described by the Nutrition Labora-tory as a "student form of respiration apparatus" (3). This ap-paratus, when combined with a combustion chamber and a motor-blower device for controlling the ventilation of the system, servesequally well for measuring the oxygen absorption during the com-bustion of a sample of food or excreta. The details of the ap-

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paratus, as thus set up, are shown in Fig. 1. A main feature of thestudent respiration apparatus is a copper can, B, two-thirds filledwith soda-lime and covered with a pure gum, rubber bathingcap, D. When this apparatus is used as an oxy-calorimeter, theusual inspiratory and expiratory valves are removed and the ap-paratus is connected -with a combustion chamber, A, and a motor-blower unit, C.

Combustion Chamber.-The combustion chamber consists of awater-sealed, glass vessel, a lamp chimney of standard size,preferably of a glass such as Pyrex with a low coefficient of ex-pansion. The lower end of the chimney is sealed in water in abrass cup, through which is passed a inch standard brass pipeand two rods of nickel or nickel alloy, fitted with electric bindingposts at the bottom, one of which is insulated from the brass baseby hard rubber washers. Into the upper part of the brass pipewide slots have been sawed so as to make three prongs into whicha nickel crucible is easily placed, thus providing sufficient spaceto allow the free passage of oxygen between the bottom of thecrucible and the opening in the pipe. The upright inch pipein the base is screwed into a standard inch elbow. In the top ofthe lamp chimney is placed a one-hole rubber stopper carrying astandard inch brass pipe, 15 mm. in internal diameter. Thisis fitted to a tee at the top provided with a rubber stopper, e,and a side connection of inch standard pipe, which is connectedby a rubber hose to the blower, C. At no point in the circuit isthere an increased or decreased pressure sufficient to cause thewater to leave the water seal. This seal does away with allpossibilities of leaks at this point.

Rotary Blower.-The circulation of the air in the apparatusshould be not far from 30 liters per minute, and a rotary blower ofgood construction is essential. In this combustion system asimple rotary air impeller is sufficient. A very satisfactory simpleblower' connects directly with a 110 volt motor, and the lengthof bearing through the housing of the blower and the lubricationare such as to insure absence of leaks. The rate of speed may becontrolled by a simple resistance in the line. The actual dis-

1 This blower, and indeed the entire apparatus, can be obtained fromW. E. Collins, 584 Huntington Avenue, Boston, Mass.

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charge of oxygen to the combustion chamber may further beregulated by placing a large screw pinch-cock on the rubber tubeconnecting the blower and the lamp chimney.

Ignition.-The electrical method of ignition, commonly em-ployed in the calorimetric bomb, is likewise here used. Fromthe diagram in Fig. 2 it is seen that by closing the electric circuitwith a key or push-button a 110 volt current is caused to passthrough a heating resistance of about 20 ohms, through an in-sulated nickel alloy post in the base of the combustion chamber,and across a fine iron wire (75 mm. long and No. 33 B and S gauge,i.e., 0.18 mm. in diameter) which is attached between the two

FIG. 2. Details of electric circuit used in ignition of substance in thecombustion chamber of the oxy-calorimeter.

nickel alloy posts. On closing the circuit by the key, the wire isburned by the 5 amperes of current and the circuit is automati-cally broken. On releasing the key or push-button there is noarcing. The iron wire used for ignition is simply held in place byinserting the two ends in holes drilled in the ends of the two nickelalloy rods, and then putting bits of alloy rod, slightly smaller indiameter and tapered, in the tops of these holes to crowd down andhold the wire in place. The holes are 4 mm. in diameter and 10 or15 mm. deep. Any resistance of 20 ohms, capable of withstanding5 amperes, may be used. A common, inexpensive heating re-sistance employed in small, portable, electric stoves or electric

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heaters has been found very satisfactory. This resistance screwsinto an ordinary lamp base. The oxygen required to burn theiron wire, computed to be about 5 cc., should be taken into ac-count in the final calculation of results.

Soda-Lime.-With the student form of respiration apparatusthe quality of the soda-lime is entirely immaterial, so long as itabsorbs carbon dioxide. We have commonly used either someof the patent soda-limes on the market or a modified form ofHaldane soda-lime.

Preparation of Samples.-As in any bomb calorimeter, thematerial must be sufficiently dry to burn readily, and preliminarydesiccation is therefore generally necessary. Approximately 1or 2 gm. of the air-dry material, accurately weighed in a nickelcrucible, are ample for the combustion. Usually sufficient ma-terial is employed to insure that the apparent volume of oxygenabsorbed is between 2 and 3 liters.

Crucible.-The sample is laid loosely in a pure nickel crucible,spun or stamped out of sheet nickel, 0.5 mm. thick. These cru-cibles are 22 mm. deep, with a diameter at the top of 32 mm., andhave a total capacity of 9 cc.

Oxygen-Measuring Device.-In the arrangement as shown inFig. 1 the soda-lime for the absorption of the carbon dioxide isinside of the can, and the contraction in volume, or the oxygenabsorbed, is measured by noting the amount of air which must beadmitted to the respiration apparatus to bring the index buttonon top of the flexible rubber bathing cap, D, back to its initialpoint at the beginning of the test; i.e., in contact with the indexneedle. This measurement is made in a most simple manner bymeans of the pump, E, which is an automobile grease pumpslightly modified (4). At the beginning of the experiment thesystem is filled by admitting pure oxygen through the pet-cock, n.At the start the bathing cap, D, is well depressed into the can,B, and the stopper, e, at the top of the combustion chamber isremoved. On replacing the stopper, the bathing cap is distendedgradually by the further introduction of oxygen, until the indexbutton just touches the needle. The motor-blower is then startedand the substance is ignited. At the bottom of the combustionchamber is an elbow with a rather long pipe, b, leading to thepipe in the base of the can, B. It is essential that the pipe, b,

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should telescope well over the pipe in the bottom of the can, inorder to have a "metal to metal" joint, the final closure beingmade by a short piece of rubber tubing. With this type of closurethe heat from the combustion will never be sufficient to damagethe rubber, much less ignite it. Ordinarily no cooling is neces-sary, although one can, if necessary, easily lay a wet cloth on thepipe, b.

Standardization by the Combustion of Sucrose.

A description of the use of the student respiration apparatusin the combustion of pure sucrose, which serves as an excellentsubstance for standardization tests, will illustrate the generalmethod of employment in burning any foods or excreta. From2 to 2.5 gm. of pure sucrose are put in the nickel crucible, whichis then placed inside the combustion chamber. The ignitionwire is attached to the two upright nickel alloy rods, and a smallpinch of powdered pumice stone is sprinkled about the wire whereit rests on the sucrose. After the lamp chimney has been replacedin the water seal, the whole system is filled with oxygen, the rubberstopper is inserted in the top of the lamp chimney, and the bathingcap is brought to the position of contact with the index needle.The motor is then started and the substance is ignited. Thebrilliancy of combustion is such that colored glasses should beworn by the operator.

The small amount of powdered pumice stone on the surfaceof the sugar makes the ignition certain. The inch pipe conduct-ing oxygen into the combustion chamber restricts the dischargeso that the oxygen impinges directly upon the surface of the burn-ing sugar. This tends to control the combustion, prevents thesubstance from frothing unduly, and prevents particles of charredsugar from being blown out of the crucible. Usually the combus-tion is complete, leaving no residue except the ash from the pumicestone, at the end of 1 or 2 minutes.

After the combustion has ceased and the combustion chamberhas returned to its original temperature and the ventilation isstopped, the bathing ap is considerably depressed, correspondingto the decrease in volume of the gas inside of the system, whichrepresents the apparent volume of oxygen consumed. To deter-mine the oxygen consumed the pump, E, is used. This pump is

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connected directly with the 3-way valve, f, which communicatesthrough a pipe with the calcium chloride tube, d, at the top, andlikewise communicates with the can, B. With the valve in theposition shown in the diagram, the piston of the pump is drawnout and the pump becomes filled with dry air, dried by the calciumchloride in d, and at the end of the stroke this air is at the prevail-ing atmospheric pressure. The temperature is obtained from athermometer placed with its bulb resting on the barrel of thepump or placed in a small tee in the rubber hose connecting thepump and the 3-way valve. The operator's hands must nevertouch the barrel of the pump. By turning the valve, f, 90° ,this pumpful of dry air may be discharged into the can, B, thusrefilling and finally distending the rubber cap, D. With the typeof pump commonly furnished with this apparatus each completestroke corresponds to not far from 350 cc., or exactly 1.872 cc.per mm. length of stroke. By repeating this operation successivepumpfuls of dry air are discharged into the can until, at the end,a fraction of a pump stroke will be necessary to bring the indexbutton on top of the bag just in contact with the vertical needle.From the number of full pump strokes, plus the actual length ofthe fractional pump stroke at the end, the total amount of dryair (at room temperature and the prevailing atmospheric pressure)required to replace the oxygen absorbed by the weight of sub-stance burned is easily calculated.

This particular form of oxy-calorimeter was controlled bynumerous combustions of pure sucrose, lactose, benzoic andsalicylic acids, and typical nitrogen-containing substances suchas uric and hippuric acids, and gave results in excellent agreementwith the theoretical amount of oxygen required in the combustionof 1 gm. of the substance.

Calculation of Results of a Combustion of Salmon Salad Sandwich,Using the Student Form of Apparatus.

A typical calculation of the heat of combustion of a sample ofmixed food is that for a salmon salad sandwich. The apparentvolume of oxygen absorbed during the complete combustion of thesample was measured when almost six full strokes of the air pumpwere needed to bring the bag back to its original position. Theapparent volume of air introduced is computed from the internal

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diameterof the pump and the length and numberof strokes. Thereis no aqueous tension to be considered, for the air drawn into thepump is first dried by passing it over calcium chloride. Theapparent volume may then be reduced to 0°C. and 760 mm. bythe formula

273 p273 + t 760

in which t is the temperature of the pump barrel, and p the ob-served barometer.

By using the tables prepared by Carpenter (5) for reducingvolumes of dry air to 0°C. and 760 mm. the formula can be moresimply stated:

(IXKXm)-5

in which is the total length of strokes through which the pumptraveled, K is the constant apparent volume of a mm. stroke ofthe pump, m is the reduction factor at t and p, 5 represents thecorrection for the cc. of oxygen used in the ignition of the wire,and W is the weight of the dried sample of salmon salad sandwich.

In this particular case represents five full strokes of the pump,each of 185.5 mm., plus a fraction of a stroke which was 180 mm.long. The total length of strokes was therefore (5 X 185.5 mm.)+ 180 mm. or 1107.5 mm. K equals 1.872 cc. per mm. length ofstroke. The temperature, t, was 26.50 C., and the pressure, p,was 756 mm., making the reduction factor, m, equal 0.907. Thecorrection for ignition of the wire is 5 cc. The sample burnedweighed 1.9932 gm. The formula therefore becomes

(1107.5 X 1.872 x 0.907)-5V =1.993 = 941

1.9932

The nitrogen in the sample was found to be 3.1 per cent byweight, or 25 cc. per gm. of substance. This volume of nitrogentook the place of oxygen absorbed and should therefore be addedto the amount of oxygen measured per gm.; i.e., 941 + 25 = 966cc. The last figure represents the volume of oxygen required toburn completely 1 gm. of the salmon salad sandwich.

Although no proximate analysis of the material was made, one

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can readily assume that it was made up in large part of carbohy-drate and hence the calorific value of oxygen would probably besomewhat less than that for pure starch, i.e., 5.06.

In this case and in the case of all the substances burned in con-nection with this paper direct estimations of the heat of combustionby a standardized bomb calorimeter were made. According tothe combustion in the bomb calorimeter, each gm. of the sampleof salmon salad sandwich had an energy value of 4826 calories.The calorific equivalent of 1 liter of oxygen in the case of thismaterial therefore is 5.00 calories (4826 + 966).

Use of Standard Respiration Apparatus of the Closed Circuit Typeas Oxy-Calorimeters.

Numerous laboratories in the United States possess one of themany types of respiration apparatus using some form of spirom-eter for measuring the oxygen consumption of humans. In suchlaboratories, obviously, it is extremely simple to attach the com-bustion chamber, A, and the motor blower, C, to the respirationapparatus and with equal facility obtain the data with regard tothe oxygen consumption per gm. of substance burned. Suchrespiration apparatus is usually provided either with respiratoryvalves or with a mechanical device for circulating the air. Thesoda-lime for absorbing carbon dioxide is generally inside of thespirometer. In some of the Nutrition Laboratory's earlier formsof apparatus the soda-lime was placed in a bottle outside. Byfar the greater number now have the soda-lime inside of the spirom-eter. The attachment of the combustion chamber to this typeof apparatus is extremely simple. In the case of those apparatushaving the blower inside, it is necessary to remove the blower andattach the combustion chamber and external rotary blower to thetwo openings in the bottom of the spirometer in such a mannerthat the air leaving the combustion chamber (being rich in carbondioxide) enters the base of the soda-lime can and is purified duringits passage through it. Some spirometers have two separateelbows at the bottom; in others there is one casting or fitting com-bining both exit and entrance.

The use of this latter type of spirometer is pictured in Fig. 3.The combustion chamber, A, and motor-blower unit, C, are con-

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nected with the fittings at the bottom of the spirometer. Theair is conducted from the combustion chamber through the pipe,b, with its "metal to metal" connections, is forced upwards throughthe soda-lime in the can, B, passes into the spirometer bell, D, andthen down around the soda-lime can and out through the fittingconnecting directly with the blower, C. The air delivered to thecombustion chamber, therefore, is always free from carbon dioxide.The contraction of the spirometer bell, D, is noted by a pointerattached to a counterweight traveling over a mm. scale. The

FIG. 3. Another type of simplified oxy-calorimeter for determiningthe energy values of foods, feedingstuffs, and excreta.

humidity of the air inside the bell is determined by the moisturecontent of the soda-lime. The contraction of air, which representsthe apparent volume of oxygen used, is in ordinary respirationexperiments calculated by rather empirical formulas, based on alarge number of actual control tests (6).

This apparatus, as pictured in Fig. 3, was likewise controlledby several combustions of pure sucrose, and most satisfactoryresults were obtained. Its use in the actual combustion of a food

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is exemplified by the following typical calculation, based upon thecombustion of a sample of doughnut.

Calculation of Results of a Combustion of Doughnut, Using theBenedict-Collins (7) Portable Respiration Apparatus, with

Wilson Soda-Lime inside the Spirometer.

For this calculation the same formula would be used as givenfor the student form of apparatus of Benedict and Benedict,namely

(V (IX KX m)-5W

In this particular combustion the apparatus used was the Benedict-Collins apparatus (8) similar to the Roth modification (7); i.e.,the blower was not inside the bell and the carbon dioxide enteredthe soda-lime before entering the bell proper. The volume of airin the spirometer is larger than that in the usual Roth modifica-tion, however, and the correction for the rise in temperature ofthe bell (which is applied directly to the spirometer reading) istherefore 1.8 mm. for each degree (Centigrade) change in tempera-ture. Therefore I would equal the corrected fall of the spirometerbell; i.e., the apparent fall plus the temperature correction. Kis the bell factor, and m is the reduction factor for temperature andpressure. In finding the value of m in the standard tables pub-lished by Carpenter (5) it should be taken into consideration thatin this apparatus the air is partially saturated. The reductionfactor should therefore be calculated on the basis of dry air, butRoth's correction (7) of minus 2 per cent for saturated air shouldbe applied to the final reduced volume. The subtraction of 5 cc.,due to the ignition of the wire, should be made as usual.

In the particular combustion used for illustration, equals108.0 mm. plus 7.7 mm. (correction for the rise of 4.3°C. in tem-perature) or 115.7 mm. The constant, K, is 21.32 cc. for thisapparatus. The average temperature during the combustion was22.9°C., and the barometer read 770 mm. The reduction factor,m, for this temperature and pressure is 0.935. The weight, W,of the sample was 2.0570 gm. The formula thus becomes

(115.7 X 21.32 X 0.935 X 0.98) - 5V -= - 1,096

2.0570

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The nitrogen content was found to be 1.1 per cent by weight or9 cc. per gm. of substance. The total amount of oxygen requiredto oxidize 1 gm. of the doughnut is therefore 1096 plus 9 or 1105cc. In view of the well known fatty nature of the doughnut acalorific value of oxygen lying somewhere between 5.06 (that forstarch) and 4.7 (that for fats) should be used. An average figureof 4.85 would be plausible. Actual determination of the factorwas made by combustion of the substance in a bomb calorimeter,in which it was found that each gm. of the sample of doughnuthad an energy value of 5331 calories. The calorific equivalent ofI liter of oxygen in the case of the doughnut, therefore, is 4.82calories (5331 - 1105).

Calorific Value of the Oxygen Involved in the Combustion of VariousFoods, Feedingstuffs, and Feces.

The calorific value of oxygen, as found with the oxy-calorimeterand the bomb calorimeter, for pure cane sugar agrees perfectlywith the theoretical value. The calorific values of oxygen de-termined with the oxy-calorimeter and the bomb calorimeter,when burning the pure substances, lactose, benzoic, salicylic,hippuric, and uric acids, have also been found to be in full accordwith the theoretical values. Of special moment in connectionwith this article, however, are the actually determined calorificvalues of oxygen in the case of various foods and feedingstuffs andthe excreta of man and beasts. A number of such substanceswere burned and the details of the calorific values of oxygen aregiven in Table II.

From Tables I and II it is seen that the range in the calorificvalue of oxygen is extremely small, the lowest value being that forprotein (4.60) and the highest values those for carbohydrates.But since much of the food of man and practically all of the foodof domestic animals is of a high carbohydrate nature, the calorificvalue of most foods will be not far from that of carbohydrates,that is, circa 5.0 calories per liter of oxygen consumed. Whilethere are occasionally some seeming irregularities in the values inTable II, in general the factor is not far from 4.68 for the nitrogen-rich substances such as protein, 4.70 for fats, and nearly 5.0 forthe substances of a high carbohydrate nature.

In the determination of the basal metabolism of humans it is

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common custom to assume that each liter of oxygen consumedunder basal conditions has a calorific value of 4.825. This impliesthat 12 hours after the last ingestion of food there is a metabolismof mixed carbohydrate and fat. In general the use of this factor,4.825, would be thoroughly justified in all estimations of thecalorific value of foods. Slightly greater refinement may be made,however, by classifying the foods somewhat as in Table II. If afood substance is obviously of a fatty nature, with visible fat, onewould be justified in using the factor 4.7. If a composite sampleof all the meals during a day were to be taken, it would be thor-oughly justifiable to consider that the calorific value of each literof oxygen required to burn such a sample would be represented by4.825, while in the case of substances containing a great pre-ponderance of carbohydrate the high value of 5 would be morerepresentative. In any case it is clear that these factors wouldlie far inside of the limit of accuracy which it is possible to obtainin preparing a sample of mixed food for combustion.

SUMMARY.

A method has been developed and tested in the NutritionLaboratory for the indirect determination of the energy values offoods, feedingstuffs, and excreta. The fundamental principleinvolves the direct measurement of the oxygen consumed duringthe combustion of a known weight of a substance and the computa-tion therefrom of the potential energy of the substance by meansof a series of factors for the calorific value of a liter of oxygen pre-viously established with a bomb calorimeter. The apparatusembodying this principle has been styled the "oxy-calorimeter."A brief description is given of the combustion chamber and thedevice for ignition of the substance, and the adaptation of simplerespiration apparatus as oxy-calorimeters is discussed in detail.Illustrations of the calculation of results are given, together withtables listing the calorific values of oxygen for some commonlymetabolized compounds and for a number of various foods andexcreta. This simple form of oxy-calorimeter is recommendedespecially for hospitals, dietetic laboratories, and those institutionsalready possessing one of the numerous simple forms of closedcircuit respiration apparatus. Average calorific values of oxygenfor use in connection with determinations with the oxy-calorimeter

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are suggested as follows: for nitrogen-rich substances 4.68 caloriesper liter of oxygen; for fats 4.7 calories; for carbohydrate-richfoods 5.0 calories; for a mixed diet of carbohydrates and fats4.825 calories; and for feces 5.0 calories.

BIBLIOGRAPHY.

1. Carpenter, T. M., Carnegie Institution of Washington, Pub. No. 03 A,1924, Table 34, 124.

2. Benedict, F. G., and Fox, E. L., Ind. and Eng. Chem., 1925, xvii, 912.3. Benedict, F. G., and Benedict, C. G., Boston Med. and Surg. J., 1923,

clxxxviii, 567; Skand. Arch. Physiol., 1923, xliv, 87. Benedict, F. G.,in Abderhalden, E., Handbuch der biologischen Arbeitsmethoden,Berlin, 1924, Abt. IV, Teil 10, 608.

4. Benedict, F. G., Boston Med. and Surg. J., 1925, cxciii, 813.5. Carpenter, T. M., Carnegie Institution of Washington, Pub. No. 303 A,

1924, Tables 9 and 10, pp. 71-102.6. Carpenter, T. M., and Fox, E. L., Boston Med. and Surg. J., 1923,

clxxxix, 551.7. Roth, P., Boston Med. and Surg. J., 1922, clxxxvi, 457, 501.8. Benedict, F. G., and Collins, W. E., Boston AMed. and Surg. J., 1920,

clxxxiii, 449.

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Francis G. Benedict and Edward L. FoxEXCRETA

THE ENERGY VALUES OF FOODS AND A METHOD FOR THE DETERMINATION OF

1925, 66:783-799.J. Biol. Chem. 

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