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Acute nutrient consumption attenuates intramuscular 26S proteasome activity during energy deficit

The effects of dietary protein and feeding during energy deficit (ED) on cellular regulators of muscle proteolysis are not well documented. This study examined intramuscular caspase‐3 and 26S proteasome responses to feeding during sustained ED in adults consuming varying levels of dietary protein. Using a randomized*block design, 32 men and 7 women were assigned to either standard (0.8 g·kg−1·d−1; n =13), moderate (1.6 g·kg−1·d−1; n =14), or high (2.4 g·kg−1·d−1; n =12) protein diets for 31 days. A 10 day weight maintenance period was followed by 21 days of ED, during which energy intake was restricted to 60% of total daily energy expenditure. Enzyme activities were assessed using fluorescence‐based assays and muscle samples obtained on days 10 and 31 following an overnight fast (FAST) and 120 min after consuming a mixed meal (480 kcals, 20 g protein; FED). The activity of the 26S proteasome's β1, β2, and β5 subunits were 28%, 14% and 22% lower (P < 0.05), respectively, for FED relative to FAST regardless of energy status and dietary protein. Caspase‐3 activity remained steady in response to feeding and was not influenced by energy and protein manipulations. These data demonstrate that feeding attenuates ubiquitin‐mediated proteolysis, suggesting a novel cellular mechanism for preservation of muscle mass independent of habitual dietary protein intake and decrements in energy balance. Supported by USAMRMC, USDA ARS GFHNRC and EMU CHHS FRF