l-Phenylalanine (Phe), is a potent releaser of the satiety hormone, cholecystokinin (CCK) and previous studies, conducted primarily in men, show that ingestion of Phe reduces energy intake. The objective of the current study was to test the effects of Phe on energy intake in overweight and obese women. Subjects (n = 32) received three treatments (high-dose (10 g Phe), low-dose (5 g Phe and 5 g glucose) or control (10 g glucose)) 20 min before an ad libitum lunch and dinner meal in a within-subjects’, counterbalanced, double-blind study. No effect of Phe was found, however, interactions with dietary restraint status were detected in post-hoc analyses. Energy intake over the day was 11% lower following high-dose Phe versus control for women classified in the lower tertile of rigid restraint, a subscale of the dietary restraint scale, whereas no effects were noted for women in the middle and upper tertiles. High-dose Phe increased ratings of nausea, however, reduced energy intake in the high-dose condition was noted only for subjects with low nausea ratings. These results suggest that the satiety response to Phe is modulated by rigid restraint status and that reductions in food intake occur independently of Phe's effects on nausea.
The prevalence of obesity continues to rise in the United States and has increased by more than 60% since 1990 (Mokdad et al., 2001). Recent research has focused on the effects of dietary factors that enhance satiety and potentially influence energy intake and body weight. There is evidence that dietary macronutrients differentially influence satiety with previous studies showing that protein may be more satiating than carbohydrate or fat (Barkeling, Rossner, & Bjorvell, 1990; Butler, Davies, Gehling, & Grant, 1981; de Castro, 1987; Stubbs, van Wyk, Johnstone, & Harbron, 1996; see reviews by Westerterp-Plantenga & Lejeune, 2005 and Halton & Hu, 2004). Protein is thought to affect satiety through the release of biologically active peptides that engage a number of central and peripheral mechanisms. A key mechanism thought to mediate the effect of dietary protein on satiety is the release of gut hormones, such as CCK. l-phenylalanine (Phe), an essential amino acid, has been shown to enhance satiety in humans, rodents and primates (Gibbs & Smith, 1977; Mathur & Manchanda, 1991; Muurahainen, Kissileff, & Pi-Sunyer, 1988) and to be a more potent releaser of CCK than other amino acids (Konturek, Radecki, Thor, & Dembinski, 1973). In human studies, 10 g of Phe has been shown to elicit a five-fold increase in plasma CCK within 20 min (Ballinger & Clark, 1994) and reduce energy intake by 16–30% 20 min, or one to two hours after consumption in a dose-dependent manner (Ballinger & Clark, 1994; Muurahainen et al., 1988; Rogers & Blundell, 1994; Ryan-Harshman, Leiter, & Anderson, 1987). In the four studies examining effects of Phe on energy intake, three used only male subjects and one included two women. Therefore, the purpose of the current study was to examine the effects of Phe on energy intake and satiety in women. We focused our investigation on overweight and obese women as this population potentially may benefit most from the development of dietary approaches that enhance satiety and reduce food intake.
Recent results from our laboratory show that in women, dietary restraint, and in particular, classification based on scores for the rigid restraint subscale of the dietary restraint instrument, modulated the satiety effects of a fiber beverage in one study (Pelkman, Navia, Miller, & Pohle, 2007) and the effects of Phe in another (Pohle-Krauza, Carey, & Pelkman, in press). Other investigators also showed that behavioral characteristics related to eating, such as dietary restraint (Burton-Freeman, 2005; Ogden & Wardle, 1990; Rolls et al., 1994) and binge eating (Geliebter, Gluck, & Hashim, 2005) affected satiety responses. Thus, a secondary objective of our investigation was to determine if dietary restraint, as well as other baseline behavioral characteristics of the subjects such as binge eating, modulated the satiety effects of Phe.
