Introduction Diets with altered macronutrient composition have become a popular method of weight loss. Because a frequent claim made about low-carbohydrate diets relates to the beneficial effects on lipid and lipoprotein metabolism, ie lower triglycerides and higher HDL cholesterol, we sought to assess the impact of low-calorie/low-fat vs low-carbohydrate diets on fasting and 24-hour excursions of triglycerides (TGs) and free fatty acids (FFAs).
Methods Thirty-five subjects were randomized to either a low-calorie/low-fat (LCAL; calorie restricted, < 30% fat, 15% protein and > 55% carbohydrate) or low-carbohydrate (LCARB; < 20 g carbohydrate/day, no limitations on fat or protein intake) diet. Subjects underwent two 24-hour feeding studies on the GCRC at baseline and 6 weeks following randomization. During each feeding study, subjects consumed three standardized meals during which TGs, FFAs, insulin, and glucose were measured hourly. During baseline studies, test meals contained 50% carbohydrates, 35% fat, and 15% protein for all subjects. During the 6- week studies, test meal composition reflected each subject's randomized diet.
Results The average weight loss at 6 weeks was not significantly different by diet (−12.92 ± 7.3 and −14.47 ± 10.6 pounds) for the LCAL and LCARB groups, respectively. Twenty-four-hour TG excursions also did not differ between both groups at 6 weeks. In contrast, 24-hour FFAs were nearly twofold higher in the LCARB group (AUC, 9,864.3 ± 1711.9 vs 5,199.6 ± 1,119.1, p < .0001), associated with persistently low insulin and glucose levels on the low-carbohydrate diet. When comparing baseline with 6 weeks within diet groups, fasting and 24-hour TG were significantly lowered in both the LCAL and LCARB groups, despite the dramatic differences in 24-hour FFA.
Conclusion Our data suggest that the effects of weight loss predominate over dietary macronutrient composition in the overall reduction of TGs. Conversely, the persistently elevated 24-hour FFA levels on the low-carbohydrate diet are likely a direct effect of differing dietary macronutrient compositions. The clinical implications of these divergent effects on lipids have not yet been defined.
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