Effects of HMB and beta‐alanine co‐supplementation and running wheel activity on skeletal muscle morphometry and metabolomics in aged rats

Dietary intervention with either beta‐hydroxy‐beta‐methylbutyrate (HMB) or beta‐alanine (BA) has been shown to improve muscle strength and physical working capacity, respectively, in elderly subjects. We examined the effects of 8 weeks HMB (343 mg/kg bw) and BA (274 mg/kg bw) co‐supplementation (HMB+BA) on skeletal muscle morphometry and metabolomic profiles in aged, male Sprague Dawley rats (22 months), versus AIN‐93M control diet (CTL). Rats were also assigned either continuous voluntary running wheel access (RW+) or cage‐matched without running wheel access (RW‐). Thus, we examined a total of 4 experimental groups (9–10 rats per group). RW activity across the 8 weeks of study was very low across both diet groups, and the HMB+BA diet had no significant effect on RW activity compared to rats fed CTL diet (84 ± 17 m/day vs. 73 ± 16 m/day). The HMB+BA diet also had no significant effects on food intake or body weight in either RW‐ or RW+ rats. Across both RW+ groups, average % body weight change decreased compared to the RW‐ groups (0.2% vs. 6.1%, P<0.00001). RW activity also decreased average weekly food intake by 6.1% (125 ± 2 g vs. 133 ± 2 g, P=0.023). Neither HMB+BA supplementation nor RW activity had significant effects on forelimb grip strength or absolute hindlimb muscle weights (i.e., gastrocnemius, soleus, plantaris). HMB+BA/RW‐ rats showed a trend for increased gastrocnemius myofiber cross‐sectional area (3430 ± 134 μm 2 vs. 3802 ± 152 μm 2 , P=0.084), however with minimal effect on myofiber Feret diameter, relative to CTL/RW‐ rats. Metabolomic profiling highlighted effects of RW activity and HMB+BA diet on carbohydrate and lipid metabolism. In gastrocnemius, RW activity was associated with a decrease in the glycolytic intermediate, fructose‐6‐phosphate, and this effect was more pronounced in combination with the HMB+BA diet. The HMB+BA/RW+ group also showed a significant reduction in pyruvate and citrate, relative to all other groups. Adenine and 2′‐ deoxycytidine levels were also less abundant with the HMB+BA diet. Metabolomic profiling was also carried out on rectus femoris muscle from the RW‐ groups only, and the HMB+BA diet was associated with increased long chain fatty acids (LCFAs), relative to CTL diet. Increased LCFAs may indicate enhanced beta‐oxidation of lipids, and the aforementioned decreases in glycolytic intermediates suggest improved carbohydrate metabolism, as we have previously shown that these biochemicals are more abundant with age in muscle. Altogether, these data suggest that both low‐volume physical activity and HMB+BA co‐supplementation contribute to improved bioenergetics in aging skeletal muscle. Support or Funding Information Funding for this study was provided by Abbott Nutrition.