Energy restriction only slightly influences protein metabolism in obese rats, whatever the level of protein and its source in the diet

BACKGROUND: High protein (HP) diets during energy restriction have been studied extensively regarding their ability to reduce body fat and preserve lean body mass, but little is known about their effects on protein metabolism in lean tissues. OBJECTIVE: To determine the effects of energy restriction and protein intake on protein anabolism and catabolism in rats. METHODS: For 5 weeks, 56 male Wistar rats were fed an obesity induction (OI) diet. They were then subjected to a 40% energy restriction using the OI diet or a balanced HP diet for 3 weeks, whereas a control group was fed the OI diet ad libitum (n = 8 per group). HP-restricted rats were divided into five groups differing only in terms of their protein source: total milk proteins, casein (C), whey (W), a mix of 50% C and W, and soy (n = 8). The animals were then killed in the postprandial state and their body composition was determined. Protein synthesis rates were determined in the liver, gastrocnemius and kidney using a subcutaneous C-13 valine flooding dose. mRNA levels were measured for key enzymes involved in the three proteolysis pathways. RESULTS: Energy restriction, but not diet composition, impacted weight loss and adiposity, whereas lean tissue mass (except in the kidney) was not influenced by diet composition. Levels of neoglucogenic amino acids tended to fall under energy restriction (P<0.06) but this was reversed by a high level of protein. The postprandial protein synthesis rates in different organs were similar in all groups. By contrast, mRNA levels encoding proteolytic enzymes rose under energy restriction in the muscle and kidney, but this was counteracted by a HP level. CONCLUSIONS: In adult obese rats, energy restriction but not diet composition affected fat pads and had little impact on protein metabolism, despite marked effects on proteolysis in the kidney and muscle. International Journal of Obesity (2013) 37, 263-271; doi:10.1038/ijo.2012.19; published online 21 February 2012