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American Journal of Clinical Nutrition, Vol. 77, No. 5, 1112-1118, May 2003
© 2003 American Society for Clinical Nutrition

Original Research Communication

Twenty-four–hour plasma tryptophan concentrations and ratios are below normal in obese subjects and are not normalized by substantial weight reduction1,2,3

Leif Breum, Michael H Rasmussen, Jannik Hilsted and John D Fernstrom

1 From the Department of Internal Medicine and Endocrinology, HS Hvidovre Hospital, University of Copenhagen, Hvidovre, Denmark (LB, MHR, and JH); the Department of Medicine C, Køge Hospital, University of Copenhagen, Køge, Denmark (LB); and the Departments of Psychiatry, Pharmacology, and Neuroscience and the UPMC Health System Weight Management Center, University of Pittsburgh School of Medicine (JDF).

Background: Plasma tryptophan concentrations and the ratio of tryptophan to other large neutral amino acids (plasma tryptophan ratio) are reportedly low in obese subjects. The plasma tryptophan ratio predicts brain tryptophan uptake and serotonin production. If this ratio is low in obese subjects, serotonin function may also be low. Plasma tryptophan concentrations and ratios have been measured only at single time points in obese subjects; it is not known whether low values for these 2 variables persist throughout a 24-h period.

Objective: Our objective was to determine whether plasma tryptophan concentrations and ratios in obese subjects are lower than those in normal-weight subjects throughout a 24-h period and whether they increase when body weight is reduced.

Design: Plasma tryptophan concentrations and ratios were examined in obese subjects before and after weight loss and in nonobese control subjects. Blood samples were drawn frequently throughout the 24-h period. An insulin tolerance test was also used to determine whether weight loss altered the ability of insulin to modify plasma concentrations of tryptophan and of the other large neutral amino acids.

Results: Plasma tryptophan concentrations and ratios in obese subjects were low at all times; these effects persisted after weight reduction. Plasma concentrations of all the large neutral amino acids decreased during insulin infusion in all the groups.

Conclusions: The low 24-h plasma tryptophan ratios in obese and formerly obese subjects suggest that brain tryptophan uptake may be continuously diminished and may remain below normal despite weight reduction.

Key Words: Obesity • weight loss • plasma tryptophan • large neutral amino acids • serotonin • very-low-energy diet • insulin






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