Plasma amino acids and insulin levels in obesity: response to carbohydrate intake and tryptophan supplements.
We assessed the plasma amino acids, glucose, and insulin responses of obese and lean control subjects to midafternoon carbohydrate snacks. After a standard 400 kcal lunch, eight lean and nine obese subjects received, at 2PM, a 30 g sucrose snack; blood samples were obtained at hourly intervals until 6 PM. Each subject participated in four similar studies in which the carbohydrate snack was consumed alone or with 250, 500, or 1,000 mg of L-tryptophan (Trp), offered as a capsule. The obese group exhibited elevated plasma levels of the branched-chain amino acids, phenylalanine and tyrosine, and the levels of these amino acids declined much less in response to carbohydrate intake than in lean controls. As a consequence, the plasma ratio of Trp:large neutral amino acids (Trp/LNAA ratio), which normally rises after carbohydrate consumption, showed virtually no change in the obese group. The plasma Trp/LNAA response of this group did not reach control values even when carbohydrate intake was increased to 50 or 75 g. Peak plasma Trp concentrations and Trp/LNAA ratios after 250, 500, and 1,000 mg Trp doses were also significantly lower in the obese. Since brain Trp uptake is strongly correlated with the plasma Trp/LNAA ratio, which in turn determines the rate of brain serotonin synthesis, the blunted Trp/LNAA response to carbohydrate intake in the obese could contribute to alterations in the serotonin-mediated regulation of food intake.
Caballero
B
B
Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge 02142.
Finer
N
N
Wurtman
R J
RJ
D000328:Adult; D000596:Amino Acids / Q000097:blood*; D004040:Dietary Carbohydrates / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D007328:Insulin / Q000097:blood*; D008297:Male; D009765:Obesity / Q000097:blood*; D013395:Sucrose / Q000494:pharmacology; D014364:Tryptophan / Q000494:pharmacology*
D000596:Amino Acids; D004040:Dietary Carbohydrates; D007328:Insulin; D013395:Sucrose; D014364:Tryptophan