Changes in plasma amino Acid levels do not predict satiety and weight loss on diets with modified macronutrient composition.
OBJECTIVE
Serotonin mediates satiety in the central nervous system. Brain serotonin content depends on the plasma ratio of tryptophan (Trp) to large neutral amino acids (LNAA) and may be affected by diet composition. We examined whether high-carbohydrate or high-protein diets induce satiety and weight loss by altering plasma concentrations of these amino acids.


METHODS
In study 1 (n = 16, BMI = 27.0 +/- 2.3), we compared plasma Trp and LNAA concentrations averaged over 24 h after 2 weeks of consuming isocaloric diets containing either 45 or 65% of total energy as carbohydrate. In study 2 (n = 19, BMI = 26.2 +/- 2.1), we made the same measurements following diets containing either 15 or 30% of total energy as protein. To assess satiety in both studies, we recorded caloric intake and weight changes during a subsequent 12-week period of ad libitum consumption of the experimental diets.


RESULTS
Ad libitum caloric intake fell by 222 +/- 81 kcal/day with a 3.7 +/- 0.6 kg weight loss at 12 weeks in study 1. Ad libitum caloric intake fell by 441 +/- 63 kcal/ day with a 4.9 +/- 0.5 kg weight loss at 12 weeks in study 2. The 24-hour averaged plasma concentration of Trp and the Trp:LNAA ratio were unaffected by the isocaloric increase in carbohydrate or protein consumption that preceded the ad libitum administration of the 2 diets.


CONCLUSIONS
An increase in either carbohydrate or protein intake increases satiety and leads to significant weight loss, however, these effects are not mediated by an increase in plasma concentration of Trp or the Trp:LNAA ratio.
Koren
Mikhail S
MS
University of Washington School of Medicine, Seattle, Wash., USA. mkoren@u.washington.edu
Purnell
Jonathan Q
JQ
Breen
Patricia A
PA
Matthys
Colleen C
CC
Callahan
Holly S
HS
Meeuws
Kaatje E
KE
Burden
Verna R
VR
Weigle
David S
DS
D000328:Adult; D000596:Amino Acids / Q000097:blood*; D001066:Appetite / Q000187:drug effects; D004038:Diet, Reducing; D004040:Dietary Carbohydrates / Q000008:administration & dosage*; D004041:Dietary Fats / Q000008:administration & dosage; D004044:Dietary Proteins / Q000008:administration & dosage*; D002149:Energy Intake; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009765:Obesity / Q000097:blood / Q000178:diet therapy*; D012527:Satiation / Q000187:drug effects* / Q000502:physiology; D016896:Treatment Outcome; D014364:Tryptophan / Q000097:blood*; D015431:Weight Loss / Q000187:drug effects / Q000502:physiology
D000596:Amino Acids; D004040:Dietary Carbohydrates; D004041:Dietary Fats; D004044:Dietary Proteins; D014364:Tryptophan