The Balance of Dietary Protein and Branched-Chain Amino Acids Regulates Metabolism and Tissue Inflammation During Aging

Aging is associated with deterioration of multiple body organ functions, and development of metabolic and degenerative diseases including obesity, type 2 diabetes, cancer, and Alzheimer’s disease. Caloric restriction has been reported to be the most effective approach to promote longevity across species. However, there are limitations and side effects of caloric restriction such as low compliance, malnutrition, bone loss, anemia, and impaired host defense against infections. Hence, alternative dietary approaches to promote healthy aging are needed. Dietary protein restriction has been shown to increase the lifespan of rodents. In particular, ad libitum-fed C57BL/6 mice with low intake of protein and high intake of carbohydrate had an extended lifespan and exhibited an improved metabolic phenotype. Protein restriction has also been reported to induce Fibroblast growth factor 21 (Fgf21), a hormone that regulates metabolism and prolongs lifespan. Interestingly, branched-chain amino acids (BCAA), leucine, isoleucine, and valine, have been shown to act as nutrient signals to positively modulate body composition, glucose metabolism, and alleviate tissue inflammation. Moreover, prolonged supplementation of the BCAA-enriched mixture increased the lifespan of mice. Although protein restriction and BCAA supplementation appear to be the two potential dietary approaches to promote healthy aging and longevity, it remains unclear whether the combination of protein restriction and BCAA supplementation leads to a better metabolic and immunological profile during aging. To answer this question, we used a 2x2 factorial design to investigate how dietary protein levels and BCAA supplementation impact metabolism and immune profile during a 12-month intervention in adult C57BL/6J male mice. There were four test diets used in this study, including the AIN-93M diet as the control diet, the low protein diet containing half of the casein in the AIN-93M diet, and the control+BCAA diet and the low protein+BCAA diet that are the diets supplemented with BCAA at the level present in the AIN-93M diet. We found that protein restriction increased hepatic Fgf21 mRNA, circulating interleukin (IL)-5 concentration, stimulated browning of subcutaneous white fat, and improved insulin tolerance. On the other hand, BCAA conditionally increased body weight, lean mass, fat mass, and deteriorated insulin intolerance during protein restriction, but not during protein sufficiency. BCAA also induced the mRNA expression of pro-inflammatory genes in the visceral adipose tissue under both protein sufficient and restricted conditions. These results suggest that dietary protein levels and BCAA supplementation induced a complex regulation of metabolism and tissue inflammation, and that BCAA specifically increased body lean and fat mass, and insulin intolerance during prolonged protein restriction