When a calorie isn't just a calorie: isocaloric, isonitrogenous diets containing different protein sources produce differential body composition outcomes in rats

Abstract

Peak activation of muscle protein synthesis and associated initiation factors is proportional to the leucine (LEU) content of a meal. Long‐term effects of LEU in complete meals on body composition remain controversial. This study evaluated the impact of two different protein sources fed in isocaloric, isonitrogenous meals on long term body composition and tissue weights. WHEAT (supplemented with lysine; 6.8% LEU) and WHEY (10.9% LEU) diets provided 16/54/30% of energy from protein/carbohydrate/fat. Male SD rats ~275g were fed 3 meals/d for 75d to mimic human meal patterns. Energy was restricted to ~80% of ad libitum intake to increase efficiency of energy utilization. After 11 wk, body composition was determined by DXA (n = 12), and muscles and fat pads were harvested and weighed. Body fat % was significantly lower in WHEY than WHEAT (9.7% vs 12.4%). Lean body mass was not different, but gastrocnemius weight was 6% higher in WHEY. Fat mass was 26% greater in WHEAT and epididymal fat weight was 18% higher. Using DXA values, WHEAT deposited 62% more energy, of which 55% was partitioned to fat and 44% to lean tissue, while WHEY partitioned 30% to fat and 70% to lean tissue. These data indicate that protein quality and specifically LEU content of proteins fed in isocaloric conditions can impact energy partitioning and body composition. Support: National Dairy Council and Egg Nutrition Center.