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Nutrition 25 (2009) 892–895

Editorial

When less is adequate: Protein and calorie restriction boosts immunity

and possibly, longevity—but how and why?

www.nutritionjrnl.com

Energy balance is a homeostatic system. High-energy diet

consumption and protein–energy undernutrition cause mal-

function of this system and lead to obesity and malnutrition,

respectively. By definition, malnutrition is an imbalance be-

tween the nutrients the body needs and the nutrients it gets.

Thus, malnutrition includes overnutrition, i.e., consumption

of too many calories or too much of any specific nutrient—

protein, fat, vitamin, mineral, or other dietary supplement—

and this is common in the developed world, especially

excess consumption of calories from fat and protein, and

in the developing countries, protein–energy undernutrition

is common, especially in children. Undernutrition causes

stunted growth, decreases muscle mass and strength, causes

smaller internal organs (such as kidneys with a decreased

number of glomeruli, relatively fewer pancreatic b-cells,

etc.), and impairs immunity that renders children more sus-

ceptible to develop insulin resistance, metabolic syndrome,

and type 2 diabetes mellitus in adult life [1]. It is surprising

that even overnutrition produces similar consequences. For

instance, overnutrition not only leads to obesity but also im-

pairs immunity and causes low-grade systemic inflammation

that is associated with progressive atherosclerosis, pancre-

atic b-cell dysfunction in the form of insulin resistance,

type 2 diabetes mellitus, hypertension, and certain forms

of cancer [2]. Conversely, protein and calorie restriction

without malnutrition improves immunity by protecting

against hepatitis B virus and malaria infections, delaying

or preventing development of cancer and metastasis, and

possibly delaying the onset of numerous age-associated dis-

eases including atherosclerosis, diabetes mellitus and greatly

increasing lifespan [3–5]. In this context, the results of Oar-

ada et al. [6] on the beneficial effects of a low-protein (1.5%

casein) diet on host resistance to fungal infection in mice

are interesting. They showed that animals fed a protein-

restricted diet (1.5% casein) had higher antifungal activity

in the spleen and liver and increases in spleen and liver

levels of interleukin-6 (IL-6), interferon-g (IFN-g), and

antimicrobial protein myeloperoxidase, and mediators of

inflammation such as cytokine IL-18, nuclear factor-kB,

inducible nitric oxide synthase, and granulocyte-macrophage

colony stimulating factor were less profoundly increased

E-mail address: undurti@hotmail.com (U.N. Das).

899-9007/09/$ – see front matter � 2009 Elsevier Inc. All rights reserved.

oi:10.1016/j.nut.2009.03.005

compared with those seen in mice fed a 20% casein diet.

The low-protein diet–fed animals (1.5% casein) showed

a less dramatic gain in total body, spleen, and liver weights

compared with those that were fed the ‘‘optimal protein

diet’’ (20% casein). From data provided by the investigators,

it is not clear whether the resistance to fungal infection ob-

served in the low-protein diet–fed animals was due to less

gain in weight. This is an important variable because inappro-

priate weight gain could have a dampening effect on optimal

immune response [7,8], although the mice fed a 20% casein

diet in the study reported by Oarada et al. were not necessar-

ily obese by definition. These results also emphasize the fact

that the composition of the diet is an important factor in the

modulation of immune response(s) because the diets used

were isocaloric except for the change in protein content.

Why would a change in the protein content of the diet influ-

ence immune response? Why, how, and what is responsible

for the beneficial effects of restricted protein intake on stress

induced by infection? Is it possible that some of the lessons

learned from the beneficial actions of 20% to 40% calorie re-

striction on lifespan extension and its protective actions

against cardiovascular diseases, cancer, diabetes, and neuro-

degenerative diseases could be extended to the benefits ob-

served with a low-protein diet?

In a study that evaluated the combination of angiotensin-

converting enzyme inhibitors and a low-protein diet (0.6 to

0.7 g $ kg�1 $ d�1 instead of�1 g $ kg�1 $ d�1), it was noted

that not only did kidney disease not progress but insulin re-

sistance improved remarkably as measured by oral glucose

tolerance testing and glucose, insulin, and C-peptide determi-

nations. Furthermore, these patients showed decreased

plasma triacylglycerol very-low-density lipoprotein concen-

trations, decreased proteinuria, and an increased high-density

lipoprotein concentration [9]. Because angiotensin-convert-

ing enzyme inhibitors do not improve insulin resistance in pa-

tients with chronic renal disease, the change noticed in

insulin resistance in this study could be attributed to a low-

protein diet. In this context, it is interesting to note that

even calorie restriction improves insulin resistance, possibly

by enhancing the activation of sirtuins (that include SIRT1,

SIRT3, SIRT4, SIRT6, and SIRT7), a group of oxidized nic-

otinamide adenosine dinucleotide–dependent deacetylases.

Of these, SIRT1 has been well characterized and deacetylates

U. N. Das / Nutrition 25 (2009) 892–895 893

transcription factor p53, forkhead subgroup O proteins, and

the DNA repair factor KU, thereby increasing the stress

resistance of cells by inhibiting apoptosis and increasing re-

pair [10]. SIRT1 regulates glucose and lipid homeostasis

and increases mitochondrial biogenesis and metabolism. In

DIET

Carbohydrate rich Protein rich

Plasma Glucose Plasma In

Macrophages/Lymp

Motility/adhesion Free radical

GIT Liver

6 5 Desa

Dietary and endog

GLA, AA, EP

Lipoxins, Resolvins, Pr

Humoral and cell mediated I

Fig. 1. Scheme showing possible interaction(s) among diet, EFAs, the gut–liver–

inappropriate increase in plasma glucose and insulin levels that could inhibit the ac

of GLA, AA, EPA, and DHA and as a consequence the formation of lipoxins, reso

rophage/leukocyte function, enhance their adhesion, and augment production o

carbohydrate-rich diet also inhibits neuropeptide Y levels in the hypothalamus

of a high-carbohydrate diet may dampen the production of incretins that could, i

similar, if not identical, scenario may exist with a diet rich in protein. A protein

6-5-desaturases and its consequences, leading to immunosuppression that is, in

and interferon-g. A calorie-/protein-restricted diet that does not produce malnut

enhances the activities of 6-6- and 6-5-desaturases, and thus leads to the forma

esins that protect against infections and enhance wound healing. A calorie-/protein

and other neurotransmitters. Enhanced activities of 6-6- and 6-5-desaturases p

inappropriate production of proinflammatory cytokines. AA, arachidonic acid; D

erance; GLA, g-linolenic acid; MPO, myeloperoxidase; PMN, polymorphonucle

mammals, calorie restriction not only decreases blood glu-

cose, triacylglycerols, and growth factors but also increases

physical activity. The molecular mechanism for this increase

in physical activity is not known, but it has been suggested

that calorie restriction triggers changes in brain regions that

Calorie/protein restricted No protein

sulin Incretins

hocytes/PMNs

s/MPO Cytokines

Hypothalamus/Brain

turases

enous EFAs

A, DHA

otectins, Maresins

mmune response

brain axis, and the immune system. A diet rich in carbohydrates produces an

tivities of 6-6- and 6-5-desaturases leading to a decrease in the production

lvins, protectins, and maresins. High plasma glucose would also inhibit mac-

f proinflammatory cytokines interleukin-6 and tumor necrosis factor-a. A

that could enhance the production of interferon-g. Continued consumption

n part, lead to persistent hyperglycemia and secondary insulin resistance. A

-poor or protein-free diet may decrease protein synthesis, inhibit 6-6- and

part, due to decreased production of interleukin-6, tumor necrosis factor-a,

rition inhibits inappropriate increases in plasma glucose and insulin levels,

tion of need-based concentrations of lipoxins, resolvins, protectins, and mar-

restricted diet may also maintain physiologic levels of hypothalamic peptides

rotect cells from inflammation, infections, and other cytotoxins and suppress

HA, docosahexaenoic acid; EPA, eicosapentaenoic acid; GIT, glucose intol-

ar leukocytes.

U. N. Das / Nutrition 25 (2009) 892–895894

govern physical activity and that sirtuins could regulate this

pathway [11].

It is also interesting to note the close relation between pro-

tein, lipid, and carbohydrate feeding and the metabolism of es-

sential fatty acids (EFAs), the precursors of many biologically

active molecules. Linoleic acid (18:2 u-6) and a-linolenic

acid (18:3 u-3) are EFAs. The conversion of linoleic acid to

its long-chain metabolites such as g-linolenic acid (GLA;

18:3 u-6), and arachidonic acid (AA; 20:4 u6) and that of

a-linolenic acid to eicosapentaenoic acid (EPA; 20:5 u-3)

and docosahexaenoic acid (DHA; 22:6 u-3) depends on the

activities of 6-6- and 6-5-desaturases [12,13]. AA, EPA,

and DHA form precursors to potent anti-inflammatory and

immunomodulatory molecules, i.e., lipoxins, resolvins, pro-

tectins, and maresins. In addition, GLA, AA, EPA, and

DHA possess antibacterial, antiviral, and antifungal actions

[15–17]. Of the several factors that modulate the activities

of 6-6- and 6-5-desaturases, dietary glucose, protein, and

plasma insulin play a major role. A 96-h fasting produces

a significant reduction, whereas calorie restriction enhances

the activity of 6-6-desaturase. When protein is the only

source of calories in the diet, a marked increase in 6-6-

desaturase is observed. In contrast, glucose administration

results in a significant decrease in its activity. Insulin seems

to have two principal effects in vivo: an enhancement of the

activity of 6-6-desaturase, probably by enhancing protein

synthesis, and a decrease in 6-6-desaturase through a stimu-

lation of glycolysis. However, when an insulin dose that is

sufficient to increase protein synthesis without producing

detectable changes in the blood glucose level is employed,

insulin enhances 6-6-desaturase activity [12,13,18]. Thus,

the effects of various types of diets and insulin on the metab-

olism of EFAs appear to be complex and the final result might

depend on the balance between plasma glucose and insulin

levels and the major source of calories. In the study reported

by Oarada et al. [6], the observation that the group that

received a 1.5% casein showed higher antifungal activity

compared with groups that received 20% and 0% casein diets

could be attributed to lower plasma glucose and optimal insu-

lin levels. In contrast, the group that received 0% casein

obtained all their calories from sucrose that may have resulted

in higher plasma glucose levels leading to suppression of 6-

6- and 6-5-desaturases. It may be noted here that hypergly-

cemia is cytotoxic and suppresses immune response. These

proposals could be verified by studying the plasma and tissue

(especially liver and macrophage) concentrations of EFAs

and their metabolites. It is important to note that GLA, AA,

EPA, and DHA and their products lipoxins, resolvins, protec-

tins, and maresins suppress excess production of IL-6, tumor

necrosis factor-a (TNF-a), macrophage migration inhibitory

factor, and free radicals [12,13]. Thus, it is possible that under

physiologic conditions there is a delicate balance maintained

between plasma/tissue glucose, protein synthesis, plasma/

tissue EFAs and their metabolites, cytokines, and free radicals

that ultimately determines the local and systemic responses to

infection and injury and recovery and tissue repair. Further-

more, EFAs, lipoxins, resolvins, protectins, maresins, and in-

sulin possess cytoprotective actions [12,13,19], whereas

higher concentrations of IL-6, TNF-a, macrophage migration

inhibitory factor, and free radicals are cytotoxic in nature

despite the fact that optimal amounts of cytokines and free

radicals are essential for normal immune response and to

eliminate the invading organisms.

The role of the food–gut–brain–liver axis in the regulation

of food intake, energy homeostasis, and immune response

also needs attention. When nutrients are delivered into the

gut, homeostatic mechanisms in place there are activated so

that blood glucose levels are not unduly perturbed. The reason

for this effect is that ingested nutrients stimulate the release of

gut peptides called incretins, which enhance the secretion of

insulin. This link between nutrient sensing in the gut and

insulin secretion and action in the liver involves an intes-

tine–brain–liver circuit within the parasympathetic nervous

system. Because the hypothalamus is an important regulator

of appetite, satiety, and food intake by elaborating several

peptides that include neuropeptide Y (NPY) and leptin, it is

important to know whether these peptides have any immuno-

modulatory actions. NPY being an orexigenic peptide, its

levels will be low after food intake and high during fasting.

Studies have shown that NPY has a regulatory role in innate

immunity [20] and suppresses the phagocytic and leishmani-

cidal capacities of macrophages [21]. Furthermore, stimulat-

ing non-adherent splenocytes and helper T-cell clones with

antigens in vitro in the presence NPY greatly enhance IL-4

production and inhibit IFN-g [22,23]. Thus, reinterpreting

the data provided by Oarada et al. [6], it is tempting to suggest

that groups that received 0% and 20% casein diets showed en-

hanced hypothalamic NPY production that, in turn, altered

IFN-g and IL-6 secretion. It is possible that plasma and tis-

sue concentrations of insulin and glucose also have a modu-

latory effect on IFN-g and IL-6 secretion. Thus, the effects of

various nutrients including their carbohydrate, protein, and

lipid content on immune response is complex and no simple

explanation will be sufficient to explain all the variables

noted. A better and proper understanding of the relations

between nutrients, the effects of individual component of

various diets on immune response, and the response of the

host to infection(s) needs a very comprehensive study that

includes not only cytokines and free radical generation but

also the behavior of macrophages and other immune cells,

modulatory influence of insulin, glucose, and fatty acids,

sympathetic and parasympathetic nervous systems, and the

role of various hypothalamic peptides and neurotransmitters

on cell and humoral immune responses, as depicted in

Figure 1.

Undurti N. Das, M.D., F.A.M.S

UND Life Sciences, Shaker Heights, Ohio, USA; andDepartment of Medicine, Bharati Vidyapeeth University

Medical College, Pune, India

U. N. Das / Nutrition 25 (2009) 892–895 895

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