Objective: The recent literature suggests that high-protein, low-fat diets promote a greater degree of weight loss compared to high-carbohydrate, low-fat diets, but the mechanism of this enhanced weight loss is unclear. This study compared the acute, energy-cost of meal-inducedthermogenesis on a high-protein, low-fat diet versus a high-carbohydrate, low-fat diet. Methods: Ten healthy, normal weight, non-smoking female volunteers aged 19-22 years were recruited from a campus population. Usingarandomized, cross-overdesign, subjects consumed the high-protein and the high-carbohydrate diets for one day each, and testing was separated by a 28-or 56-day interval. Control diets were consumed for two days prior to each test day. On test day, the resting energy expenditure, the non-protein respiratory quotient and body temperature were measured followinga 10-hour fast and at 2.5-hour post breakfast, lunch and dinner. Fasting blood samples were collected test day and the next morning, and complete 24-hour urine samples were collected the day of testing. Results: Postprandialthermogenesis at 2.5 hours post-meal averaged about twofold higher on the high protein diet versus the high carbohydrate diet, and differences were significant after the breakfast and the dinner meals ( p 0.05). Body temperature was slightly higher on the high protein diet ( p 0.08 afterthedinner meal). Changes in the respiratory quotient post-meals did not differ by diet, and there was no difference in 24-hour glomerular filtration rates by diet. Nitrogen balance was significantly greater on the high-protein diet compared to the high-carbohydrate diet (7.60.9 and 0.4 0.5 gN/day, p 0.05), and at 24-hour post-intervention, fasting plasma urea nitrogen concentrations were raised on the high protein diet versus the high-carbohydrate diet (13.90.9 and11.21.0 mg/dLrespectively, p 0.05). Conclusions: These data indicate an added energy-cost associated with high-protein, low-fat diets and may help explain the efficacy of such diets for weight loss. Counter to the current U.S. Dietary Guidelines which promote diets high in complex carbohydrates (58%of total daily energy) [1], recent clinical investigations support the efficacy of high-protein, reduced fat diets for weight loss, as well as for improved insulin sensitivity and blood lipid profiles. In aran- domizedtrial, 65 healthy, overweight and obese subjects consumed ad libitum a high-protein (HP), reduced fat diet (24% energy from protein, 29%energy from fat) or a high-carbohydrate (HC), reduced fat diet (59%energy from carbohydrate, 29%energy from fat) for six months[2]. Subjects consuming theHPdietlost more weight compared to subjects consuming theHCdiet (7.5 kg vs. 5.0 kgafterthree months and 8.7 kg vs. 5.0 kgaftersix months)…

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