A hypocaloric diet impairs muscle protein synthesis[1][2][3] and increases muscle protein breakdown.[4] It can also unfavorably alter the anabolic hormone response to resistance exercise.[5] Furthermore, prolonged consumption of a hypocaloric diet can disrupt endocrine system function, suppressing levels of reproductive and metabolic hormones and causing unfavorable changes in bone metabolism biomarkers.[6] For these reasons, a hypocaloric diet would be expected to negatively affect muscle size and strength.
A 2021 meta-analysis that compared the effects of performing resistance training in an energy deficit or without an energy deficit reported that an energy deficit impaired lean mass gains;[7] more specifically, it was estimated that an energy deficit of 500 kcal per day could prevent gains in lean mass. However, an energy deficit did not impair strength gains; interventions that prescribed resistance training with and without an energy deficit both resulted in a significant increase in muscle strength. The average energy deficit in these studies was 567 kcal per day.
A caveat to these findings is that the studies included in the above meta-analysis were between 3 and 28 weeks long, with an average intervention duration of 16 weeks. While the results suggest that increases in muscle strength can occur in an energy deficit in the short term, spending too much time in an energy deficit will undoubtedly compromise long-term strength gains. Because larger muscles have greater force-generating capacity,[8] and an energy surplus enhances muscle gain,[9] individuals interested in maximizing strength gains should spend a notable amount of time consuming a hypercaloric diet.