A soy, whey and caseinate blend extends postprandial skeletal muscle protein synthesis in rats

Background & aims: Blends of dairy and soy protein are used in commercial sports nutrition products; however, no studies have systematically compared blends to isolated protein sources and their effects on muscle protein synthesis (MPS). Dairy whey protein (WP), soy protein isolate (SP), and two blends (Blend 1 and Blend 2) consisting of ratios of 50:25:25 and 25:50:25 for whey:caseinate:soy, respectively, were evaluated for their ability to affect MPS. Methods: Male Sprague-Dawley rats were trained to eat 3 meals/day: a 4g meal at 0700-0720hours followed by ad lib feeding at 1300-1400hours and 1800-1900hours. After ~5 days of training, fasted rats were administered their respective 4g meal at 0700-0720hours and an intravenous flooding dose of ²H₅-phenylalanine 10min prior to euthanasia. Individual rats were euthanized at designated postprandial time points. Blood and gastrocnemius samples were collected and the latter was used to measure mixed muscle protein fractional synthetic rates (FSR). Results: Plasma leucine concentrations peaked in all groups at 90min and were still above baseline at 300min post-meal. FSR tended to increase in all groups post-meal but initial peaks of FSR were different times (45, 90 and 135min for WP or SP, Blend 1 and Blend 2, respectively). Blend 2 had a significantly higher FSR compared to WP alone at 135min (P<0.05). Conclusions: Single source proteins and protein blends all enhance skeletal MPS after a meal, however, Blend 2 had a delayed FSR peak which was significantly higher than whey protein at 135min.