Abstract
Caffeine is a well-known stimulant that is widely used to increase alertness and performance. However, there is little information on the effect of chronic caffeine consumption on exercise adaptations. We first sought to characterize the effect of caffeine on protein synthesis in vitro using immortalized C2C12 and TTD6 muscle and tendon cells. Total protein synthesis, as measured by puromycin incorporation, decreased 31% in TT-D6 cells with 4mM caffeine (p < 0.05) and 41% in C2C12 cells with 0.5 mM caffeine (p <0.01). The structure and function of in vitro engineered ligaments were also reduced with caffeine, including a 45% reduction in maximal tensile load (MTL) (p = 0.0128) and 30% reduction in collagen content (p = 0.0038) with 1 mM caffeine. To assess the effect of caffeine in vivo, sedentary and running mice were provided with a moderately high dose of caffeine (0.22 mg/mL, roughly equivalent to 5.7 mg/kg for a human) or water as a placebo control. Free access to a running wheel was effective in increasing gastrocnemius, soleus, and heart muscle mass relative to body weight, oxidative phosphorylation proteins, and Achilles tendon collagen concentration and Col1a1 gene expression. Mice that consumed caffeine while exercising did not gain skeletal muscle mass (gastrocnemius, soleus) to the same extent as the non-caffeinated exercising mice. Together, these data suggest that high caffeine consumption can dampen the molecular signals associated with protein synthesis and in excess may limit exercise-induced skeletal muscle adaptations.
