A new study published in Nature looked into the effects of a prolonged fast on human performance.
13 healthy adults of unclear training status fasted for a week and then had their strength, endurance, and various other muscle-related metrics tested.
After a week of fasting, subjects lost an average of 5.7kg. Of this, 1.7kg was presumed muscle mass in their arms and legs, 1.4 kg of fat, and the rest was other “lean mass”, which was likely water, bone, organ, and other “non-fat” tissue.
Muscle strength on isokinetic (constant speed) testing remained unchanged from baseline despite the decreased leg mass. Muscle glycogen levels were also halved, from 408 mmol/kg to 191 mmol/kg on average. While reduced muscle glycogen may be concerning from a mechanism standpoint, previous research has suggested that muscle glycogen levels below ~70 mmol/kg represents a threshold where muscular force production decreases, contributing to fatigue and limiting performance.
Absolute VO2peak went down by 13%, which is expected due to reduced lean mass and no real training. Less lean mass means less oxygen-hungry tissue, potentially smaller left ventricle, and other changes that reduce absolute vo2max. Relative vo2peak, e.g. vo2peak adjusted for bodyweight, went down by 7%, which is likely a more accurate representation of performance change due to short-term fasting.
Overall, this study adds to the body of evidence showing that human performance persists even in the face of low energy availability (LEA). LEA is not a monolith however, as it can refer to a number of different scenarios. For example, LEA can also be local, e.g. low protein intake, or systemic, e.g. low Calorie intake.
Additionally, any Calorie deficit big or small is technically a form of LEA. Modest deficits show virtually no impact on strength performance, as one recent meta-analysis reports:
“Strength gains were unaffected by the presence or absence of an energy deficit as well as its estimated severity. That subjects gained strength despite impaired gains,or even losses, of lean mass suggests these strength gains may be independent of hypertrophy and instead due to neural adaptations or microarchitectural changes typically preceding detectable gains in LM at the onset of a resistance training program.”
On the other hand, a large deficit, such as complete fasting is also a form of LEA. The present study showed that people of unknown training history preserved their strength in the short-term in response to fasting, though we should be cautious about extending these findings to longer periods of LEA.
With respect to energy intake and strength, I feel comfortable saying that in the short- to moderate-term (~6-12 months), strength gain in non-elite strength athletes is mostly related to programming, not whether someone is in a surplus, maintenance, or modest deficit. Larger and/or longer deficits are likely worse for strength gain when it comes to long-term strength outcomes due to the compromise in muscle mass gain. Muscle mass takes a long time to gain and doesn’t have as strong of a correlation to strength as most people think. However, I believe the body will leverage (pun intended) any and all possible mechanisms to match muscle performance to the imposed demands. Because people gain less muscle mass on a deficit or maintenance compared to an energy surplus, sustained LEA is highly likely to result in reduced strength gain compared to maintenance, which is somewhat likely to result in reduced strength gain compared to an energy surplus.
Yes, muscle strength and mass are both important for health and performance. Weight management decisions should integrate health, performance, and personal preferences together, adjusting over time based on results.