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Effects of protein-energy interactions on growth


* Instituto de Nutricion y Tecnologia de los Alimentos (INTA), University of Chile, Casilla 138-11, Santiago, Chile.

1. Introduction
2. Mechanisms for effects of protein and energy on growth
3. The determinants of catch-up growth
4. Effect of the protein/energy ratio on growth of premature infants
5. Effect of protein and energy on growth of children with primary malnutrition
6. Effect of the P/E ratio on growth of children with malnutrition secondary to chronic renal insufficiency
7. Conclusions and speculations


Growth is determined by genetic and environmental influences among which diet is of major importance. Catch-up growth is accelerated tissue gain beyond that expected for chronological age. Protein and energy intakes can influence both normal and catch-up growth. Hyperplastic growth is severely restricted during critical periods if the diet lacks protein, whereas hypertrophic growth is mainly affected by energy supply. Specific effects of protein-energy interactions on growth are mediated by hormones (insulin, cortisol, growth hormone) and growth factors (IGF-I, IGF-II and EGF) at the organ and whole-body level. Different rates of lean and adipose tissue accretion may result from such effects. Diet and age will influence tissue composition during catch-up growth. The protein/energy ratio is an important determinant of normal and catch-up growth in premature infants and in children suffering from primary or secondary malnutrition.

1. Introduction

Growth can be defined as the process by which living organisms increase their mass and size. Growth is determined by genetic and environmental influences, of which diet is a major determinant. Catch-up growth represents accelerated tissue gain beyond that expected for chronological age; it occurs after a period of slowing or cessation of growth. Protein and energy may influence both normal and catch-up growth (TANNER, 1989).

Length increments are mainly related to hyperplastic growth. Linear growth will be severely restricted if protein is absent from the diet during critical periods, or if overall nutrition (including protein and energy) is insufficient. Catch-up linear growth after malnutrition may not lead to full recovery in length, since arrests in cell division may not be reversible if the nutritional deprivation has been severe, prolonged, and has occurred early in life. Weight gain during the first 12 months of life is dependent on both hyperplastic and hypertrophic growth; later on, hypertrophy is the main determinant of weight gain. Weight gain will be mainly affected by energy supply in relation with energy expenditure. Weight loss is usually fully reversible.

Several excellent reviews on protein-energy requirements and interactions in the human during normal and catch-up growth have been published recently (ZLOTKIN, 1986; WATERLOW, 1986; JACKSON and WOOTTON, 1990). This paper will address selective areas, most familiar to the authors, or areas that have been dealt with less comprehensively in the mentioned reviews:

1. Potential mechanisms by which protein/energy (P/E) ratios may affect growth. The role of hormones and growth factors in modulating normal and catch-up growth will be examined. The effect of dietary protein and energy on hormonal and growth factor responses which modulate growth will be reviewed.

2. Immediate and delayed effects of dietary P/E ratios on growth of prematurely born infants. Metabolic and body composition studies of growing premature infants, fed varying protein and energy intakes, will be discussed. The potential significance of the differential effect on growth of energy from fat and carbohydrate will be examined.

3. The immediate and delayed effects of dietary P/E ratios on catch-up growth of infants with primary malnutrition. Metabolic and body composition studies of malnourished infants fed high-protein and high-energy diets during recovery will be discussed. Long-term follow-up of stature and maturation of recovered malnourished infants will be presented.

4. The effect of the P/E ratio on growth of children with secondary malnutrition (chronic renal insufficiency [CRI] will serve as an example). Low-protein diets have been suggested as a way to delay the progression of renal failure. In children, this may compromise growth. Results of a controlled double-blind study of CRI infants, fed two levels of protein isoenergetically, will be presented.

2. Mechanisms for effects of protein and energy on growth

2.1. Insulin and insulin-like growth factors
2.2. Growth hormone
2.3. Epidermal growth factor
2.4. Corticosteroids

The specific effects of protein-energy interactions on growth are mediated at the organ and whole-body level by hormones (insulin, growth hormone, cortisol) and growth factors (IGF-I, IGF-II and EGF). These effects result in different rates of lean and adipose tissue accretion. Diet and age will influence the composition of the tissue during catch-up growth. The dietary P/E ratio is an important determinant of normal and catch-up growth in premature infants and in children suffering from primary or secondary malnutrition. We will review recent knowledge on the hormonal regulators of growth and then explore how nutrients interact with them during normal and catch-up growth.

2.1. Insulin and insulin-like growth factors

Insulin has been proposed as a growth-promoting factor during the fetal period (HILL, 1978). Feeding of protein and several essential and non-essential amino acids stimulate insulin secretion in the fetus and neonate. Normal-term infants fed cow's milk formula (P/E ratio 12%) have higher levels of plasma insulin and urinary C-peptide than infants fed breast milk (P/E ratio 6%) (CHRISTENSEN et al., 1971; GINSBURG et al., 1984). Increasing arginine levels during parenteral infusion have also been shown to increase serum insulin levels in newborn infants (UAUY et al., 1991). Higher protein intakes in low birth weight infants are associated with an increased secretion of C-peptide. The significant correlation of urinary C-peptide excretion with weight gain suggests that insulin may be a growth-promoting factor for infants on high-protein diets (AXELSSON, IVARSSON and RAIHA, 1989).

The evidence from malnourished animals and human infants is also conclusive in establishing an association between growth rates and insulin levels during recovery (PAYNE-ROBINSON et al., 1980). The pathogenesis of marasmus and kwashiorkor has been linked to the diet-induced differences in circulating insulin and cortisol (WHITEHEAD and LUNN, 1979). Diets with a low P/E ratio will induce a decrease in the post-prandial insulin surge, while fasting markedly lowers insulin levels. The low ratio of insulin to cortisol is of major significance in the adaptation to fasting or hypocaloric feeding. The post-prandial rise in insulin is associated with the increase in metabolic rate and the higher rates of protein synthesis observed after a meal (MILLWARD et al., 1986).

The insulin-like growth factors (IGFs), formerly known as somatomedins, are polypeptides in plasma that are chemically related to insulin. According to the classical view, they are produced in the liver in response to growth hormone, circulate in plasma, and act in cartilage at the growing ends of bones to promote skeletal growth. In recent years, this view has been modified in several respects. It is now assumed that IGFs are produced at multiple sites. They may act locally or at a distance, and they exhibit diverse biological actions, including effects on differentiated cell functions as well as on growth. IGF-I and -II both are single-chain polypeptides with a molecular weight of 7500 daltons. They consist of four domains, designated A-B-C-D. IGF-I and II are identical in 60% of their structure, and both have 40% homology to insulin in the B and A chain region (HUMBEL, 1984). This chemical similarity to insulin is of clinical significance. The IGFs possess intrinsic insulin-like activity and interact with the insulin receptor; at high levels they are capable of producing hypoglycemia. Despite their chemical similarities, IGF-I and -II appear to have distinct physiological roles and regulation (HALL and SARE, 1984).

Plasma levels of IGF-I (previously known as somatomedin C) are regulated by growth hormone (GH), consistent with its proposed role as mediator of GH action. IGF-II has been shown to be less dependent on GH. It is present at high levels in fetal tissue and brain, suggesting a role in fetal growth and the development of the central nervous system. IGF-I and -II are the products of single genes, located on chromosomes 12 and 11, respectively. Biosynthetic precursors of both molecules contain a prepeptide (signal peptide) that is removed during translation, and a prepeptide (E domain) at the carboxyl terminus (BELL et al., 1984). The structure of the two genes is similar. Hormonal and developmental regulation of the abundance of IGF mRNAs has been established. The multiple IGF-I RNA species are regulated by GH. They are decreased in hypophysectomized rats and restored by GH treatment (ROBERTS et al., 1986).

IGF-I and -II bind to two types of IGF receptors that differ in binding specificity and structure (RECHLER and NISSLEY, 1985). This conclusion has been confirmed by affinity crosslinking studies, and subsequently by receptor purification and cDNA cloning (MORGAN et al., 1987). An antigenically related binding protein, whose levels show marked diurnal variation (10- to 20-fold rise between midnight and 8 a.m.) but do not appear to correlate with GH levels, has been described in serum (BAXTER and COWELL, 1987). It has generally been assumed that the complex of IGFs and their binding proteins does not bind to receptors and is biologically inactive (KNAUER and SMITH, 1980). If all of the IGFs in human serum were able to bind to the insulin receptor, hypoglycemia would result. In fact, hypoglycemia has only been observed after rapid intravenous infusion of IGF-I and in certain tumors that overproduce IGF-II (GULER, ZAPF and FROESCH, 1987; DAUGHADAY, 1989). These results suggest an important role for IGF-binding proteins in modulating IGF biological activity. The basis for the difference between inhibitory and stimulatory binding proteins is unclear, but may reflect the existence of related but heterogeneous binding proteins.

EMLER and SCHALCH (1987) reported that changes in rat serum IGF-I during fasting and refeeding are paralleled by changes in the liver IGF-I mRNAs. This suggests that one post-receptor mechanism for the decreased IGF-I is impaired IGF-I transcription or decreased IGF-I mRNA stability. There may also be a translational block, indicating that IGF-I biosynthesis may be controlled at multiple levels (EMLER and SCHALCH, 1987). In normal subjects, fasted for 5 days, then refed diets containing adequate energy (35 kcal/kg) but inadequate protein (0.2 g/kg/d), plasma IGF-I increases but nitrogen balance does not improve providing indirect evidence for IGF-I resistance (ISLEY, UNDERWOOD and CLEMMONS, 1984).

IGFs may act locally or at a distance. Although circulating IGF levels are readily determined, the significance of these values rather than tissue IGF levels is uncertain if local IGFs are primarily responsible for their biological effects. IGF levels are regulated by age and nutritional status. Levels are low at birth, and increase progressively during childhood and steeply during puberty. IGF-I levels are low (despite elevated GH) in states of even modest malnutrition (UNDERWOOD et al., 1986). Indeed, IGF-I is an excellent marker of nutritional status. By contrast, IGF-II levels in human plasma are relatively unaffected by changes in GH or nutritional status and show little fluctuation with age (HALL and SARE, 1984).

2.2. Growth hormone

Growth hormone (GH) is the main hormonal determinant of skeletal linear growth in childhood. Isolated GH deficiency results in proportionate dwarfism that is responsive to GH treatment. In an effort to understand how GH exerts its effects on cartilage at the growing ends of long bones, SALMON and DAUGHADAY (1957) made the crucial series of observations that gave rise to the somatomedin hypothesis. They observed that the GH did not act directly on cartilage, but led to the production of GH-dependent factors in serum that mediated its effects. According to the somatomedin hypothesis, GH from the pituitary gland stimulates the production of somatomedins at a distant site (thought to be the liver), from where they circulate in blood to their ultimate target organ, the cartilage.

This hypothesis led to the purification of the somatomedins from human plasma. After purifications of IGF-I and -II were achieved, it became clear that IGF-I and -II are synthesized in multiple tissues, not just the liver (D'ERCOLE, APPLEWHITE and UNDERWOOD, 1980). ISAKSSON et al. (1987) demonstrated that local infusion of GH into the epiphyseal cartilage or femoral artery caused widening of the tibial epiphyseal cartilage or femoral artery and bone deposition on the same but not the opposite side. These results suggested that the infused GH was acting locally, possibly by stimulating the local synthesis of IGF-I.

It was later described that GH exerted dual effects in an adipogenic fibroblast line: (a) GH promotes the differentiation of stem cells to pre-adipocytes which are more responsive to IGF-I, (b) GH stimulates the synthesis of IGF-I by these cells, resulting in a net clonal expansion (ZEZULAC and GREEN, 1986). Thus, GH appears to act both directly and indirectly by stimulating the synthesis of IGF-I to promote longitudinal bone growth. Diverse biological roles of IGFs can affect linear growth. In addition to participating in fetal and postnatal skeletal growth, IGFs may play a role in wound repair, organ hypertrophy and tumor growth.

Beyond its capacity to stimulate linear growth, GH has a variety of other important effects (BERCU, 1988). It causes the retention of nitrogen and accumulation of lean body mass secondary to the stimulation of protein synthesis (RABINOWITZ. KLASSEN and ZIERLER, 1965) and relative slowing of protein breakdown (WARD, HALLIDAY and SIM, 1987). GH acts on adipose tissue to promote lipolysis and stimulate free fatty acid oxidation. These effects may serve to provide the necessary substrate for the increased energy needs imposed by anabolism (GOODMAN, GRICHTING and COIRO, 1980). The actions of endogenous GH and the efficacy of GH used therapeutically, however, are modulated by nutrient intake and nutritional status (WILMORE et al., 1974).

2.3. Epidermal growth factor

Epidermal growth factor (EGF) has been characterized as an important growth factor in mammalian development and function, but its precise role is not yet completely clear (BEARDMORE and RICHARDS, 1983). The ingestion of natural species-specific milk causes dramatic enhancement of visceral and somatic growth (BERSETH, 1987b; BERSETH, LICHTENBERGER and MORRIS, 1983; HEIRD, SCHWARZ and HANSEN, 1984) and may also influence intestinal enzyme maturation in several mammalian species (YEH and HOLT, 1985).

The mitogenic effect of human milk can be blocked by EGF-specific antibodies (CARPENTER, 1980). Furthermore, EGF has been shown to enhance growth, differentiation and maturation of rodent intestine (MOORE et al., 1986). The EGF in breast milk remains structurally intact and biologically active after gastric passage (THORNBURG et al., 1984). EGF most likely has a role in embryogenesis and organ growth, since receptors have been identified in fetal tissues. However, EGF mRNA has not been shown in the fetus, and it seems likely that the related peptide, alpha-transforming growth factor (TGF), serves a developmental role in the fetus. The mRNA for TGF has been demonstrated in the fetus (BERSETH, 1987a), and TGF acts via EGF receptors. In the post-natal animal, EGF mRNA, immuno-reactive EGF, and EGF receptors are present in many tissues.

In the rodent, the highest levels of EGF mRNA are found in the salivary glands and kidneys, and EGF is secreted in large amounts in saliva and urine. A role for salivary and urinary EGF in the maintenance of stomal, gut and urinary epithelial surface integrity seems likely although not yet proven (DEMBINSKY and JOHNSON, 1985). Many other tissues presumably produce EGF, and tissue EGF concentrations (and probably synthesis) are hormone-responsive in many tissues. The trophic effects of EGF have been potentiated by retinoic acid (WIDDOWSON, COLOMBO and ARTAVANIS, 1976), prostaglandins (KONTUREK et al., 1981) and thyroxine (LAKASHMANAN et al., 1985), all of which are present in breast milk.

Thyroid and gonadal steroid hormones have been shown to influence EGF and/or EGF receptors in various tissues (LAKASHMANAN et al., 1985). EGF can exert mitogenic or developmental actions autonomously or in response to hormonal signals. An endocrine role also is likely but of secondary importance, as is the case for the insulin-like growth factors. The precise details of these EGF actions and effects in immature and adult animals are being explored further in many laboratories.

2.4. Corticosteroids

One of the most conspicuous effects of the glucocorticoid hormones is an inhibition of somatic growth in immature animals. By the early 1960s, several carefully controlled clinical studies had shown that doses corresponding to twice the average endogenous daily secretion of hydrocortisone are sufficient to suppress somatic growth in children. The use of corticosteroids in large doses results in a loss of tissue mass and in negative nitrogen balance.

MUNCK (1971) has demonstrated that glucocorticosteroids produce an early rise in blood glucose by inhibition of glucose uptake by skin, adipose and lymphoid tissues, rather than by enhancement of hepatic gluconeogenesis. The suppression of glucose transport is dependent on the induction of new mRNA and protein synthesis by the hormone. These inhibitory effects of glucocorticoid on peripheral tissue rather than stimulatory effects on hepatic gluconeogenesis were first conclusively demonstrated in vitro (LOEB, 1976).

The mechanism by which glucocorticoids suppress cell division in growing tissue is not clear. It could be a direct action of the hormone on proliferating cells or the effect of steroid-induced changes in growth hormone secretion. Glucocorticoids are known to inhibit growth hormone secretion under certain conditions in vivo, and growth hormone itself has been shown to have profound stimulatory effects on both DNA synthesis and DNA polymerase activity in liver. If the inhibitory effects of glucocorticoids on liver DNA synthesis were due to suppression of endogenous growth hormone secretion, one would predict that these effects could be blocked by the simultaneous administration of pharmacologic amounts of growth hormone, but such blocking has not been demonstrated (HANDERSON and LOEB, 1974).

The fact that the effects of glucocorticoids are dominant over those of growth hormone indicates that the inhibition of liver-cell proliferation in the treated animal cannot be attributed to a steroid-induced suppression of growth hormone secretion. The observation of DNA synthesis in weanling-rat liver as an index of tissue growth is consistent with earlier studies on the antagonism of ACTH to the effects of GH on weight gain and bone growth in hypophysectomized rats, and with clinical studies showing that the inhibition of somatic growth in children on chronic steroid therapy cannot be overcome by even prolonged treatment with large doses of GH.

Severe caloric deprivation both in animals and in children has long been known to depress the rate of somatic growth. Not only does restoration of calories permit the resumption of growth, but growth proceeds at an accelerated rate until a size is attained comparable to that of age-matched non-fasted subjects. A similar phenomenon has been described after the cessation of either exogenous corticosteroid therapy or the cure of spontaneous Cushing's syndrome in children, in which a period of glucocorticoid-induced suppression of somatic growth may be followed by a period of accelerated growth, as measured both by weight gain and by height increase. The cessation of corticosteroid administration after a period of growth suppression can be followed by true and virtually complete catch-up growth that is accompanied by a corresponding acceleration of cell proliferation, although such catch-up growth is not invariable. Indeed, large doses of glucocorticoid lead to some permanent stunting in laboratory animals, perhaps through a direct action on epiphyseal chondrocytes. Large doses of glucocorticoid suppress cell division in both bone and cartilage.

In skin adipose tissue, thymus and certain lymphosarcoma cells, exposure to glucocorticoid rapidly inhibits glucose transport, and a decreased uptake of glucose appears to account for many of the familiar catabolic effects of the hormone (MUNCK, 1971). Thus, a decreased glucose entry into adipose cells could trigger lipolysis and fatty acid release, and glucose starvation might well account for the inhibition of proliferation and the involution of lymphoid tissues. The inhibition of glucose uptake in sensitive lymphoid tissues is followed, for example, by a marked inhibition of both DNA and protein synthesis, and thereafter by cell death, autolysis and tissue involution. Glucocorticoid-induced inhibition of glucose entry in both adipose and lymphoid tissue can be blocked by either actinomycin D or cyclohexamide, and hence appears to be dependent upon an early stimulation of specific RNA and protein synthesis.

It is evident that new cell accretion in a variety of growing tissues composed of stable cell populations, is markedly suppressed by the administration of exogenous glucocorticoids. The fact that the amount of hormone required for this effect is small, and indeed corresponds to little more than the normal production rate, suggests that even physiologic levels of these compounds may influence the rate of normal somatic growth by exerting a tonic suppressive action on cell proliferation and thereby play a part in the regulation of normal growth.

3. The determinants of catch-up growth

Growth is a complex process. It depends on adequate nutritional substrates, the action of a variety of hormones, including GH, thyroid hormones, and gonadal steroids, and the mediation of an array of growth factors such IGF-I and -II, epidermal growth factor, fibro-blast growth factor and presumably others (UNDERWOOD and VAN WYK, 1985). Other hormones such as insulin and Vitamin D exert important and permissive effects (HILL and MILNER, 1985). Fetal growth and growth during the first 6 to 12 months of extrauterine life appear to be dependent on growth factors and are independent of growth hormone. Linear bone growth and bone maturation are modulated by growth hormone and thyroid hormones in childhood, and by gonadal steroids during adolescence.

The major hormones modulating linear growth (growth hormone, thyroid hormones and gonadal steroids) have different effects on bone maturation. Growth hormone has a minimal effect, and gonadal steroids have the most effect, the effect of thyroid hormones is intermediate. With a deficiency of one or more factors in the complex growth program, the impairment of growth is variable, but its extent and severity increase with the duration of the deficiency. With treatment there is usually a period of accelerated catch-up growth (TANNER, 1981).

The completeness of catch-up growth depends on three factors: the relative deficiencies of height and bone maturation, assessed as height age and bone age; the relative effects of treatment on linear growth and bone maturation; and the timing of the effects of gonadal steroids on bone maturation and the duration of treatment before the onset of epiphyseal closure (FISHER, 1988). Although traditionally it has been thought that stunting in early life secondary to malnutrition cannot be fully reversed with nutritional rehabilitation, under exceptional conditions the catch-up growth phenomenon has been demonstrated up to 22 years of age (FLOMBAUM and BERNER, 1989).

Experimental findings tend to support the hypothesis of TANNER (1989) that catch-up growth is controlled by a central regulator, which senses body size, relates body size to a set point (reference point) for target size, and accelerates or decelerates growth rate accordingly. Observations made during studies of catch-up growth show that skeletal size and soft tissue mass are often disproportionate during growth disturbances. Proportions may rapidly return to normal during recovery or may remain abnormal, depending on the experimental conditions (MOSIER, 1989). The relationship between catch-up growth and proportionality of growth and the degree to which they are interdependent during normal growth are unclear.

Bilateral irradiation of solely the head, in neonatal rats, results in dose-related stunting. This effect does not result from the nonspecific effects of tissue injury, undernutrition before or after weaning, or hypopituitarism. Stunted head-irradiated rats are capable of catch-up growth after fasting but only to the stunted body size (MOSIER, 1988). Experimental data tend to support the hypothesis of a central control of catch-up growth. Stimulation of catch-up growth does not appear to be related to plasma-integrated GH levels per se. However, growth stimulation does occur in association with a pulsatile pattern of GH secretion. Disproportions in skeletal length and body weight occur during interruptions of growth. Normal proportions are promptly restored during recovery after nutritional and metabolic disturbances, even if skeletal length is permanently stunted. This suggests that proportionate growth is under active control (MOSIER, 1990).

Exogenous GH increases insulin secretion in both normal and GH deficient humans (LIPPE et al., 1981). GH increases plasma free fatty acid release within hours after being injected, but the increased insulin that follows GH injection causes compensatory lipogenesis. Growth hormone also causes resistance to the effects of insulin on carbohydrate metabolism, including decreased rates of glucose transport. A major mechanism by which GH exerts its anabolic effects is by stimulating the production of insulin-like growth factors, which correlates well with the capacity of GH to cause nitrogen retention. For example, in animal and human models of nutritional deprivation, the rise in serum IGF-I concentration in response to GH is attenuated, as is the capacity to retain nitrogen (MAES et al., 1988; SNYDER, CLEMMONS and UNDERWOOD, 1989).

With nutrient replenishment, the IGF-I response to GH is restored, along with the ability to retain nitrogen. The response seems to be particularly sensitive to carbohydrate and protein feeding. When human volunteers are fasted for 10 days, IGF-I concentrations decline by 70% (CLEMMONS et al., 1981) despite evidence that GH secretion is increased (Ho, VELDHUIS and JOHNSON, 1988). The capacity of GH to stimulate IGF-I synthesis is impaired. Likewise, when exogenous GH is injected into subjects who have been fasted for three days, there is no increase in serum IGF-I (MERIMEE, ZAPF and FROESCH, 1982).

Although the relationships between dietary energy intake and GH responsiveness have not been determined fully, a daily intake of approximately 18 kcal/kg body weight is needed for adult humans to maintain normal IGF-I serum concentration and to respond maximally to GH injections (SNYDER, CLEMMONS and UNDERWOOD, 1988). At intakes of 12 kcal/kg, IGF-I responses to GH are attenuated significantly and are greatly dependent on the composition of the diet ingested; specifically the response is enhanced if the non-protein calories are provided as carbohydrates. The protein intake also determines the degree of responsiveness. The serum IGF-I concentration increases incrementally when dietary protein intake is raised from 0.2 to 0.8 g/kg body weight/d. Diets rich in essential amino acids are more effective in restoring IGF-I and nitrogen balance after fasting than diets deficient in them.

The mechanisms by which nutrient intake controls IGF-I responsiveness to GH are complex and not well defined. They probably involve both GH and IGF receptors, IGF-binding proteins, and post-receptor effects. One possible mechanism for the reduced IGF-I during fasting is reduction in GH binding to liver cells. In general, the fall in serum IGF-I is paralleled by a diminution in GH receptors (UNDERWOOD et al., 1986). Other studies, however, suggest that this is not the only mechanism for GH resistance in nutrient-deprived animals, because fasting causes serum IGF-I to decline before there is a significant decline in GH receptors (MAITER et al., 1988) and injections of GH fail to stimulate IGF-I responses maximally, no matter how high the dose of GH given (MAES et al., 1988). Taken together, these findings suggest that, in addition to diminished GH receptors, post-receptor defects in GH action are involved in the GH resistance accompanying nutritional deprivation.

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