HMB Attenuates A Loss of Myofiber Cross‐Sectional Area during Prolonged Exercise with Calorie Restriction by Enhancing Regenerative Capacity

Abstract

β‐hydroxy‐β‐methylbutyrate (HMB) has shown to attenuate loss of muscle mass and function. However, the efficacy of HMB during catabolism generated by prolonged exercise with calorie restriction has yet been addressed. Therefore, the present study investigated the impact of HMB on myofiber cross sectional area (CSA) and molecular regulators during a 6‐wk normal or catabolic condition. C57BL/6 male mice (Age= 6 wks) were divided into 4 groups (10/group): ALT=ad libitum+trained (1h/d for 3d/wk), ALTH=ALT+HMB (0.5 g/kg BW/day), C=caloric restricted (−30%)+trained (6hrs/d, 6d/wk) and CH=C+HMB. After the experimental period, the gastrocnemius muscles were isolated and analyzed histologically for myofiber number and CSA. Protein levels of atrogin‐1, myogenin, and phosphorylated Akt (pAkt) were determined by western blot. One‐way ANOVA was used and significance was set at p<0.05. Mean myofiber CSA (μm2) of the gastrocnemius in C was significantly lower than the ALT and ALTH (−38%), and CH (−35%), while the fiber number per field CSA was 35% higher in C than CH. Myogenin levels were significantly lower only in C compared to ALT and ALTH, while no group differences were detected for atrogin‐1 and pAkt. Our findings demonstrate the potential mechanisms of HMB action in enhancing regenerative capacity and attenuating the loss of myofiber CSA during prolonged exercise with low calorie intake.