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Obese persons are often reported to have marked cravings for simple carbohydrate-rich foods. Because of the proposed relationships between protein/carbohydrate selection, plasma tryptophan (TRP) to large neutral amino acids (LNAA) ratios, and brain 5-hydroxytryptamine (5-HT) neurotransmission, we examined the plasma TRP/LNAA ratios in four categories of obese subjects, before and 120 min after oral glucose tolerance test (GTT). Plasma TRP/LNAA ratios were reduced in obese, non-diabetics by 18%, the same extent as for older (approximately 52 yr old) nonobese subjects. In more advanced obesity, ie obesity associated either with glucose intolerance, hyperinsulinemia or hypoinsulinemia, plasma TRP/LNAA ratios were reduced by 25%. One hundred twenty minutes after a 100 g glucose load plasma TRP/LNAA had not been normalized. Based on animal data, these results suggest there may be diminished 5-HT neurotransmission in obese diabetics. The implications of these findings for the cravings of obese for carbohydrate-rich foods is discussed.

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Studies of brain mechanisms controlling food intake and feeding behaviour have often neglected interactions between total food (energy) intake and qualitative aspects of the dietary intake. Generally, experimental animals are presented with a single diet of fixed nutritional composition. Thus, if as a result of a given treatment, an animal has an increased or decreased appetite for a specific nutrient, then this could manifest itself as an increase or decrease in intake from the sole diet offered. As selection of food is a characteristic behaviour of all animals, and their ability to monitor intake of specific nutrients is well known, then, giving experimental animals a choice of dietary constituents could result in a wider understanding of central mechanisms governing food intake. Exploiting the ability of rats to select dietary protein and carbohydrate has suggested that brain 5-hydroxytryptamine (5-HT) is involved in the regulation of protein/carbohydrate intake. Evidence from human studies suggests that appetite disturbances which occur in obese and mood-disturbed individuals may be linked to an impaired functioning of the brain 5-HT system.

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The effect of a carbohydrate, a 20% protein, or a carbohydrate +0.3% tryptophan TRP breakfast on plasma large neutral amino acid ratios was studied in 6 healthy men. The carbohydrate-rich meal produced shifts in plasma amino acid concentrations such that plasma TRP/LNAA ratios increased from 0.13 to 0.15 (p less than 0.04) and the protein meal decreased the ratio from 0.14 to 0.11 (p less than 0.04) after 1 hour. Addition of 0.3% TRP to the carbohydrate-rich meal increased plasma TRP/LNAA ratios more than 2-fold. The TRP containing meal was thus the only one likely to influence brain 5-HT synthesis, although the difference between the plasma TRP/LNAA ratios after carbohydrate and protein breakfasts suggests that the brain may distinguish, by synthesizing more or less 5-HT, the composition of breakfast meals.

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The reaction between primary amines and o-phthaldialdehyde in the presence of a thiol was exploited to measure the concentrations of 21 amino acids in serum by high-performance liquid chromatography with fluorescent detection. The method described here includes an automatic on-line precolumn procedure for derivatizing the amino acids, permitting full automation and avoiding problems due to time-dependent decay of fluorescence of the derivatives. The total analysis time is less than 60 min and limits of sensitivity are about 100 fmol. Proline, hydroxy-proline, and cysteine are not detected. Comparison with data generated by a standard amino acid analyzer shows this technique to be reliable.

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Weanling and adult male rats were offered pairs of diets containing either 0 and 20%, 0 and 40%, 0 and 60%, or 10 and 40% casein. Initially they preferred the lower protein diets, and then after one to ten days abruptly switched to adequate levels of protein choice. After 14 days, the diets were changed so that the first group received 10 and 40%, the second 20 and 60%, the third 10 and 40%, and the fourth 0 and 60% casein. Following the change, all weanling rats showed significant and systematic shifts in percentage of total energy chosen as protein (%P-E); most adults did not. Within each group, the variability in %P-E selected between different rats was higher than the day-to-day variability of individuals. In the adults no significant correlations were observed between protein selection and brain serotonin metabolism. We conclude that protein intake in rats is regulated in the sense that all animals learnt to eat sufficient protein to maintain growth, and most animals ate a constant amount of protein each day. On the other hand, the range of protein intake between individuals, and the shifts in selection among the weanlings when diet choices were changed seem to preclude the existence of a mechanism which precisely regulates protein intake.

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The biological availability to the weanling rat of two amino acid-enriched plasteins, tryptophan and methionine plastein, was tested. In both experiments rats were fed diets containing graded levels of the amino acid either in the free form or as the amino acid-enriched plastein. The first experiment tested the utilization of tryptophan plastein. The results as indicated by weight gain, food intake and plasma levels of tryptophan showed that the tryptophan in the plastein was utilized to the same extent as the free amino acid. Brain tryptophan levels correlated better with plasma Trp:neutral amino acid ratios (r = 0.83) than with plasma tryptophan itself (r = 0.63) suggesting that the small changes observed in plasma amino acid profiles were responsible for differences in the brain tryptophan concentrations among the L-tryptophan and tryptophan plastein groups. In the second experiment, utilization of methionine plastein was tested. Based on weight gain and food intake data, it was concluded that methionine plastein was also utilized to the same extent as free L-methionine.

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The maintenance of nutrient and energy balance in the body depends on both metabolic and behavioural mechanisms, and is integrated by the brain. The regulatory system was developed by natural selection and not by mechanical engineers. Thus, rather than having unitary mechanisms regulating intake of each nutrient, evolution has incorporated and used a multitude of behavioural traits and metabolic adaptations. The criterion for inclusion was that each one conferred a persisting advantage in the prevailing environment. Behavioural strategies in food choice include: innate preference for sweetness and an aversion towards bitter tastes, a hesitancy towards unknown foods, preference for variety among familiar foods, and a special ability (long delay learning) to acquire information about both positive and negative metabolic consequences of eating different foods. In man, these more basic mechanisms interact with and are complemented by cognitive, social and cultural influences on food choice. In a very few cases, such as regulation of energy, sodium and (perhaps) protein intakes, feeding behaviour is also guided by signals from specific internal receptors. However, for most nutrients, appetites seem to be non-specific and learned. Using studies on the regulation of protein intake from our own and other laboratories as examples, this review illustrates how innate preferences, learning, social interactions, metabolic adaptation and diet-induced changes in brain neurotransmitter metabolism can all play a role in subjective decisions about what to eat.

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The effect of a carbohydrate or a 20% protein (HP) or a carbohydrate + 0.4% tryptophan (TRP) evening meal on plasma amino acids and on plasma neutral amino acid (NAA) ratios was studied in eight healthy men. After consumption of the protein meal, plasma amino acids rose after 1 h and remained at the same level at 2 h. The dietary profile of the essential amino acids, except for TRP, was retained in the plasma. The plasma profile of nonessential amino acids was not related to the dietary pattern. Glutamic acid and aspartic acid increased several-fold less and alanine several-fold more than would be expected from their dietary concentrations. The tyrosine/NAA and phenylalanine/NAA ratios were not altered by any of the meals and the TRP/NAA ratio was increased only by the carbohydrate + 0.4% TRP meal. The leucine/NAA and isoleucine/NAA ratios were decreased by the carbohydrate and carbohydrate + TRP and increased by the 20% protein meals, while valine/NAA was decreased by the carbohydrate + TRP and HP but increased by the carbohydrate meal. From these data it is concluded that if brain neurotransmitter synthesis is controlled by the same mechanisms as in rats, then the evening meal containing added TRP was the only one of the meals likely to affect brain neurotransmitter (serotonin) synthesis.

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The relationship between plasma and brain tryptophan (TRP) concentrations and brain 5-hydroxytryptamine (5-HT) metabolism was studied in weanling rats fed diets containing either 0.4 g or 1.45 g TRP/100 g casein hydrolysate. Both groups gained weight comparably though food intakes were generally higher in the low-TRP group. Severe depletion of plasma total and free TRP and of brain TRP, 5-HT, and 5-hydroxyindoleacetic acid (5-HIAA) occurred within 1 day of feeding the 0.4% TRP diet. Levels became stable after 7 days. The decreased brain TRP concentration of the rats on the 0.4% TRP diet did not cause a compensatory rise of the tryptophan hydroxylase (TRP OHase) activity in vitro. In the low-TRP group, neither plasma free TRP nor total TRP correlated significantly with brain TRP and although plasma TRP/large neutral amino acid (NAA) ratios (TRP/NAA) correlated significantly (P less than 0.05) with the time course of brain TRP, this statistical relationship depended almost completely on the variation of the TRP values alone. In the higher TRP group none of these correlations were significant. A plot of mean plasma free TRP versus brain TRP gave two distinct regression lines with similar slopes and corresponding to values before and after 7 days on the diet. The time course of brain 5-hydroxyindole concentrations did not parallel those of brain TRP and suggested that changes of TRP OHase activity also had an influence on 5-HT synthesis.

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Weanling rats fed a choice of two diets varying only in protein concentration (10 and 60% casein) select a lower proportion of dietary energy as protein when the dietary minerals are supplied by a mineral mixture high in Na, K, and Cl but low in P and Cu (Hi Cl) than when the diets contain the Bernhart-Tomarelli (BT) mineral mixture. The addition of Na and/or K as acetate salts to the BT mineral mixture, to levels in the Hi Cl mixture, had no effect on regulation of protein intake. However, the addition of Na and/or K as chloride salts to the BT mineral mixture resulted in the rats selecting a protein concentration equivalent to the Hi Cl fed group. Weight gains and energy consumption were similar for all groups. It was therefore concluded that dietary chloride affects protein feeding behavior.

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Three experiments were conducted to test the ability of weanling rats to utilize the oxidized forms of the sulfur amino acids methionine and cysteine for growth. In the first two experiments, diets were fed which contained graded levels of methionine, methionine sulfoxide and methionine sulfone. The third experiment included a comparison of two dietary levels of cysteine and cysteic acid. The 2 week weight gain and food consumption data indicated that methionine sulfoxide was utilized for growth with only 60% of the efficiency of that achieved by rats fed methionine. Methionine sulfone was not utilized for growth. Analysis of plasma sulfur amino acids showed that the rat has a limited capacity to utilize methionine sulfoxide by effecting its reduction to methionine. Cysteic acid did not support weight gain. This amino acid appeared to be rapidly catabolized to taurine. It was concluded that methionine sulfone and cysteic acid cannot be utilized by the weanling rat. Methionine sulfoxide cannot fully meet the dietary requirement of the rat methionine because of its limited capacity to reduce this amino acid.

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The effects of altering the quality of the dietary protein source on the self-selection of protein and energy by the weanling rat simultaneously offered a choice of two diets differing only in protein concentration were tested. The protein-energy selected was measured when the first limiting amino acid lysine was added to gluten; when lysine, methionine, or the first four limiting amino acids were added to gluten or to casein; or when the nutritional quality of zein was altered by manipulation of the content of tryptophan, lysine, or the four most limiting amino acids. The additions of lysine to gluten caused a decrease in the protein-energy selected and an increase in growth rate of the weanling rat. However, improving the amino acid balance of casein or zein did not have this effect. It was concluded that the selection of protein and energy by weanling rats is not related to the nutritional quality of the protein fed.

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The relationship between changes in blood plasma amino acids and the quantity of protein and energy self-selected by the weanling rat, simultaneously offered two diets varying only in protein concentration, was examined. The changes in the quantity of protein and energy consumed by the rats, which were brought about by the addition of the essential limiting amino acids and groups of essential amino acids to gluten, casein, and zein, were not linearly related to alterations in the total plasma amino acid concentrations or to the accumulation of the added amino acids in the blood plasma. However a consistent relationship between food intake and plasma acids was identified when the plasma tryptophan to neutral amino acid ratio was correlated with the protein intake selected. It is postulated that the ratio of tryptophan to neutral amino acids in the plasma is involved in, or reflects, a mechanism regulating protein intake.

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