Effects of L-phenylalanine on satiety in women: Interactions with dietary restraint status and phase of the menstrual cycle
Pohle, Rachael J.
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The essential amino acid, L-Phenylalanine (Phe), has been shown to be a potent releaser of the satiety hormone, cholecystokinin (CCK) and previous studies, conducted primarily in men, show that ingestion of Phe reduces food intake. Two studies were conducted to test the effect of Phe on energy intake in women. In the first study, 32 overweight or obese women received three treatments [10 g Phe (High-dose), 5 g Phe and 5 g dextrose (Low-dose) and 10 g dextrose (Control)] in capsule form for one day (20 min before lunch and dinner) in a within-subjects', double-blind study. Treatment order was counterbalanced with a one-week washout between treatments. Subjective satiety over the course of the day and ad libitum food intake at lunch, dinner and evening snack were measured. No effect of condition was found on energy intake, however an interaction of condition and restraint status was found. Energy intake over the day was 11% lower following high-dose Phe versus control for women classified in the low tertile of rigid restraint, a subscale of the dietary restraint scale, whereas no effects were noted for women in the middle and upper tertiles. Our results are consistent with other studies that have demonstrated that highly-restrained individuals are insensitive to physiologic satiety signals and rely on cognitive strategies to modulate food consumption. Animal studies demonstrate potentiation of the satiety effects of CCK by estradiol (E 2 ). The aim of our second study, therefore, was to compare the effects of Phe versus placebo on food intake and subjective satiety in the follicular phase of the menstrual cycle, when E 2 concentrations are elevated, versus the luteal phase, when the effects of E 2 are antagonized by progesterone. Twenty non-obese women completed test sessions which were scheduled to coincide with the follicular and luteal phases of two consecutive menstrual cycles. Subjects received encapsulated Phe (10 g) or dextrose placebo (10 g) for the two phases within a menstrual cycle. Subjective satiety over the course of the day and ad libitum food intake at meals and evening snack were measured. Daily energy intake was suppressed by 9% in the Phe compared to placebo condition. The effects of condition and phase differed as a function of dietary restraint status. Phe suppressed follicular-phase energy intake in low-rigid restraint women, and luteal-phase energy intake in high-rigid restraint women. The results show that Phe suppressed energy intake in unrestrained women, and that intake was suppressed in the follicular phase of the menstrual cycle, but the interaction of Phe and menstrual phase was modulated by rigid restraint status. Further studies are needed to elucidate the effect of dietary restraint on reproductive hormones and how this impacts the satiety response to protein.