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 facilitate endocrine system dysfunction, resulting in a suppression of reproductive and metabolic hormones and unfavorable alterations in markers of bone metabolism.[6] For these reasons, a hypocaloric diet would be expected to negatively affect muscle strength.
However, a 2021 meta-analysis that compared the effects of performing resistance training in an energy deficit or without an energy deficit reported that, while an energy deficit impaired lean mass gains, it did not impair strength gains;[7] interventions that prescribed resistance training in an energy deficit and without an energy deficit both resulted in a large increase in muscle strength. Furthermore, the average energy deficit in the included studies was 567 kcal per day.
A caveat to these findings is that the studies included in this meta-analysis were between 3 and 28 weeks long, with an average intervention duration of about 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. This stems from the fact that, as outlined in “Which other factors affect muscle strength?,” differences in muscle mass largely explain differences in muscle strength. Because an energy surplus enhances muscle gain (while an energy deficit impairs gains in muscle mass and can cause muscle loss), and a larger muscle has greater force-generating capacity, individuals interested in maximizing strength gains should spend a notable amount of time performing resistance training in an energy surplus.