Patricia A. Breen

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. Conclusion: 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.

Exchanging carbohydrate or protein for fat improves lipid-related cardiovascular risk profile in overweight men and women when consumed ad libitum

Background The impact of low-fat diets on the plasma lipoprotein profile is incompletely understood. Methods We conducted two 16-week dietary studies to compare the effects of a moderate-fat (mod-FAT) baseline diet with isocaloric and ad libitum low-fat diets rich in either carbohydrates (high-CHO, n = 16) or protein (high-PRO, n = 19) on plasma lipids, post-heparin lipase activities, cholesteryl ester transfer protein, and phospholipid transfer protein. Results Switching from the mod-FAT to the isocaloric high-CHO diet lowered plasma high-density lipoprotein cholesterol concentrations (P < 0.001) and tended to increase triglyceride levels (P = 0.087). Cholesterol content in the larger, buoyant low-density lipoprotein (LDL) fractions decreased, whereas those of the very-low-density lipoprotein, intermediate-density lipoprotein, and smaller, denser LDL fractions tended to increase. These changes were largely reversed when subjects lost weight by consuming this high-CHO diet ad libitum. Switching from the mod-FAT diet to the isocaloric high-PRO diet did not increase cholesterol content in the small-dense LDL fraction and led to decreases in both LDL and high-density lipoprotein cholesterol in plasma (P < 0.001 for both). Consumption of the high-protein ad libitum diet accompanied by weight loss did not change plasma lipids further, except for a shift of cholesterol from dense low-density lipoprotein fractions to more buoyant low-density lipoprotein fractions. Cholesteryl ester transfer protein concentrations decreased with high-cholesterol feeding, whereas cholesteryl ester transfer protein concentrations and hepatic lipase and phospholipid transfer protein activities all decreased during high-protein feeding. Conclusions Both high-CHO and high-PRO diets improve plasma lipid-related risk of cardiovascular disease when consumed ad libitum.

319 C-REACTIVE PROTEIN CONCENTRATION IS NOT AFFECTED BY ISOCALORIC DIETARY FAT REDUCTION

Background Consumption of a high-fat diet is associated with the development of insulin resistance and obesity. Both conditions are pro-inflammatory states characterized by increased concentrations of plasma C-reactive protein (CRP), interleukins, and TNF-alpha. However, it is unclear whether dietary fat content by itself has an effect independent from weight change on markers of inflammation. We examined plasma CRP levels in healthy volunteers who sequentially consumed a weight-maintaining moderate fat diet, an isocaloric low fat diet, and an ad libitum low fat diet. Subjects and Methods We studied 16 subjects, 45±13 years old, 2M/14F, who were weight-stable for at least 3 months before enrollment. The mean weight of subjects was 74.9 ± 10.2 kg, BMI 27.1 ± 2.3 kg/m2. The weight-maintaining moderate fat diet consisted of 35% fat, 45% carbohydrate, and 20% of energy as protein. After consuming this diet for 2 weeks, subjects were switched to an isocaloric low fat diet consisting of 15% fat, 65% carbohydrate, and 20% protein for another 2 weeks. For the final 12 weeks of the study, subjects consumed the low fat diet ad libitum. All meals were prepared in the clinical research center and consisted of typical food items. Subjects maintained the same level of physical activity throughout the study. At the end of each diet phase, two blood samples were collected at 0800h 24 hours apart and the average CRP was measured by a high sensitivity CardioPhase hsCRP assay (Dade Behring Inc., Deerfield, IL, USA). The study had 80% power to detect a 30% change in CRP concentration between the isocaloric moderate fat and low fat diets with type I error of 0.05. Data analysis was performed with repeated measures ANOVA using SPSS software. Results The weight of the subjects remained unchanged during the first 4 weeks of the study. The plasma CRP concentrations after 2 weeks on the weight-maintaining 35% fat diet and 2 weeks on the isocaloric 15% fat diet were not significantly different (mean ± SD were 2.49±2.79mg/L and 2.97±3.65 mg/L, respectively). Three months of ad libitum low fat diet resulted in a 4±2 kg weight loss associated with a downward trend in CRP concentration to 2.15±2.47 mg/L (p=0.11). Conclusion Without concurrent diet-induced weight loss, weight-maintaining isocaloric 35% fat diet and 15% fat diet for 2 weeks had no significant effect on plasma CRP concentration of modestly overweight subjects.