Metabolic Response to Malnutrition: Its Relevance to Enteral Feeding
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Gut: first published as 10.1136/gut.27.Suppl_1.9 on 1 November 1986. Downloaded from Gut, 1986, 27, Si, 9-13 Metabolic response to malnutrition: its relevance to enteral feeding A H G LOVE Department of Medicine, Queen's University, Belfast, Northern Ireland SUMMARY Malnutrition results in a wide variety of metabolic responses, depending on circumstances, from reactions to pure deprivation of nutrients to include the added stress of injury and sepsis. Important differences of response exist between adults and children. Weight loss with changes in carbohydrate, fat, and protein metabolism are well documented. Disturbances of fluid and electrolyte balance are newer areas of interest as are changes in requirements for micronutrients such as trace metals. Many of these metabolic changes are under hormonal control. The intestinal tract shares in the response to malnutrition, and the consequent changes in mucosal function determine the ability of the intestine to handle enteral feeds. Such a route for nutritional support is important in protecting intestinal function not only in absorption but also in hormone production. Enteral feeding is increasingly having an important role in the interactions between acute diarrhoeal disease and malnutrition. The clinical circumstances in which malnutrition and the adaptive processes depend on the nutri- may arise vary enormously and will influence both tional onslaughts. Responses to starvation and the signs and the metabolic responses: primary injury must be clearly distinguished but may coexist protein calorie deprivation is common because of in any patient. Table 1 details the major features. environmental forces in many countries; secondary http://gut.bmj.com/ semistarvation is often found in patients with cancer Metabolic response to malnutrition cachexia or the cachexia of other chronic diseases, those with anorexia nervosa, and inpatients, who for BODY WEIGHT one reason or another can take little by mouth. In Weight loss is perhaps the oldest and most addition to inadequate supply, there may also be the commonly accepted sign of a tissue catabolism that added metabolic insult of the catabolic process of injury and sepsis. Furthermore, the responses may on September 25, 2021 by guest. Protected copyright. differ between adults and children extending from Table 1 Comparison ofeffects ofstarvation and injury weight loss to failure of growth, and in extreme forms, the protein calorie deprivation syndromes of Starvation Injury kwashiorkor and marasmus. Earlier work in this area has concentrated on Metabolic rate Decreased Increased deficiency of macronutrients, but with the advent of Weight Slow loss Rapid loss Energy Almost all from fat 80% from fat; new techniques and the application of parenteral remainder from nutrition the importance of essential fatty acids and protein various micronutrients is being recognised. Nitrogen Losses reduced Losses increased When placed in a situation of nutritional depriva- Hormones Early small increases Increases in in catecholamines, catecholamines, tion, the body undergoes a series of changes that can glucagon, cortisol, glucagon, cortisol, be divided into three stages: (i) depletion of reserves; growth hormone, growth hormone (ii) metabolic adaption; (iii) deterioration and ulti- then slow fall mate death. The rate of evolution of these stages Insulin decreased Insulin increased but relative insulin deficiency Correspondence to: Professor A H G Love, Department of Gastro- Water and Initial loss, late Retention enterology, Royal Victoria Hospital, Grosvenor Road, Belfast, Northern sodium retention Ireland, BT12 6BJ. 9 Gut: first published as 10.1136/gut.27.Suppl_1.9 on 1 November 1986. Downloaded from 10 Love has lasted more than a few days. The gain or loss of mg/kg/day occurs in a protein free diet. Overall, body weight is the sum of changes in three body protein synthesis rates in man range from 2 to 5 components - protein, fat, and water. Because body g/kg/day: this is approximate due to difficulties in content and daily fluctuations for water are so much methodology.5 It is generally agreed that in protein larger than for body fat or protein, weight change calorie malnutrition the rate of catabolism of must be interpreted with this in mind, particularly as albumin is reduced to about half of the rate found handling of water and electrolytes may be abnormal after recovery.6 Globulins are probably unaffected in the malnourished state. by nutritional state.7 Plasma transferrin concentra- tion is severely reduced in malnutrition and is due to CARBOHYDRATE METABOLISM a decreased rate of synthesis. Total body protein Glucose occupies a central role in carbohydrate turnover studies indicate up to 30% reduction after metabolism.' It serves as an optimal fuel for many five to six weeks on low protein diets. In the tissues and an obligatory fuel for brain, red cells, presence of injury or sepsis increased rates of and renal medulla. The circulating blood glucose turnover exist. Protein breakdown also seems to would sustain these functions for only a few hours respond to changes in ketone body concentrations without calling on reserves. Reserves are found in when these are high, such as, in starvation, when the form of glycogen in many tissues, but only the protein breakdown is inhibited. These interrelations liver can use this to provide new circulating glucose. are important in determining nutritional support This reserve in the absence of other sources would programmes.8 run out after about 12 hours.2 Amino acids are other major glucose precursors. LIPID METABOLISM Recently it was shown that alanine is quantitat- Lipids provide several useful fuels that are used ively the most important of the amino acids as a directly by tissues - that is, non-esterified fatty acids gluconeogenic precursor.3 It is derived from the (NEFA) and ketone bodies. Triglycerides are also animation of pyruvate in peripheral tissues, particul- distributed throughout the body by means of the arly muscle; but half of the amino acid groups lipoprotein carrier system and either used for released from muscle are in the form of alanine. An storage or degraded to fatty acids and glycerol. important fraction of alanine is also derived from Normally, there is a continuous cycle in the adipo- glutamine metabolism in the gut. Intestinal mucosal cyte, whereby triglycerides are broken down (lipo- integrity therefore contributes to this pathway. The lysis) through the action of lipase and resynthesised. final precursor is glycerol released from adipose Lipolysis forms NEFA and glycerol. This is an tissue. Under normal circumstances this is a minor important fuel source. They may be converted by http://gut.bmj.com/ contributor to glucose production and only assumes the liver to ketone bodies. Ketone bodies cannot be importance in chronic deprivation. If starvation used by the liver for fuel and enter the circulation continues, glycogen stores are exhausted and gluco- for oxidation elsewhere - for example, the brain in neogenesis becomes the sole source of glucose. prolonged starvation. The main importance of fat as Utilisation of tissue glucose falls as a result of fuels is their role in preserving glucose in the starved inhibition by fatty acids and ketone bodies.4 The state for use by obligatory tissues. This indirectly brain adapts to ketone body utilisation, and glucose spares protein. In catabolic states, however, this on September 25, 2021 by guest. Protected copyright. requirements decrease further. Amino acids are protein sparing capacity is lost. conserved with lactate pyruvate and glycerol con- tributing carbon fragments. The serum glucose ENERGY METABOLISM AND THERMOREGULATION concentration falls slightly. Many of the clinical features of severe malnutrition, such as physical inactivity, unresponsive emotional PROTEIN METABOLISM state, bradycardia and decreased body temperature, Unlike carbohydrate, which is in limited reserve as are consistent with decreased metabolic rate. glycogen, and fat, which has large reserves, there is Recent studies have confirmed this position and no reserve of protein. Every molecule is performing even indicated particular organ contributions to a vital function, whether it is actin or myosin, part of basal metabolic rate. The picture is changed con- the somatic protein compartment, or any one of the siderably by the presence of injury or sepsis when hundreds of visceral proteins. This includes the hypermetabolism is operative. The starvation proteins of liver, kidney, and gut, as well as studies conducted by Keys'( show that when body antibodies, enzymes, leucocytes, blood proteins, weight decreased by 25% the basal metabolic rate and other essential tissues. Tissue proteins are fell by about 40%. After surgery or major infection, continually being synthesised and degraded. In a such as peritonitis, the expenditure may increase by normal adult obligatory nitrogen loss of 50-60 30-50% above predicted levels. The largest Gut: first published as 10.1136/gut.27.Suppl_1.9 on 1 November 1986. Downloaded from Metabolic response to malnutrition 11 increases are seen in the previously well nourished, response to glucose loads and in promoting glycogen heavily muscled young adult male. Currently there deposition. The differences in response to parent- is much debate as to the desirability or even the eral and enteral calorie delivery may be extremely possibility of nutritional support to replace these important, particularly in situations demanding high deficits.