Does Intermittent Fasting Cause Muscle Loss?
Sure, intermittent fasting (IF) may help you lose weight and offer other health benefits. But is there a detrimental side to it? If you don’t lift weights, will IF cause muscle wasting?
The intermittent fasting (IF) lifestyle doesn’t seem to be fading in popularity like other trendy diets. Far too many people have benefitted from restricting their feeding window to a limited number of hours during the day, or limiting their caloric intake to roughly 500 per day a couple days a week, or by completely fasting once a week.
Regardless of which method of IF chosen, the consensus is that it can result in impressive health outcomes. Consider this 2021 conclusion from Annual Review of Nutrition: “In summary, intermittent fasting is a safe diet therapy that can produce clinically significant weight loss (>5%) and improve several markers of metabolic health in individuals with obesity.”
But over the last couple years or so, there have been a few studies which suggest a potential downside to IF…
What Does Research Say About Intermittent Fasting and Muscle Loss?
On a basic, physiological level, any weight loss will result in muscle loss because weight loss is the sum of both a reduction in fat and muscle volume.
But there was a study published last year in Frontiers in Nutrition by Eric Williamson and Daniel Moore, both of whom are on the faculty of kinesiology and physical education at the University of Toronto. The study suggested that there may be a more muscle loss caused by intermittent fasting than other typical weight loss regimens.
The researchers explain that muscle protein is constantly "turning over" through the breakdown of old/damaged proteins and the resynthesis of new functional proteins. But “we propose that infrequent meal feeding and periods of prolonged fasting may be counterproductive to optimizing muscle protein turnover and net muscle protein balance.”
Williamson and Moore add, “It is our position that intermittent fasting likely represents a suboptimal dietary approach to remodel skeletal muscle, which could impact the ability to maintain or enhance muscle mass and quality, especially during periods of reduced energy availability.”
So should you stop doing IF based on the duo’s conclusion? Of course not. One theoretical peer-reviewed article does not make for a definitive conclusion.
Does Intermittent Fasting Cause Muscle Loss If You Lift Weights?
In a 2020 study published in Nutrients titled “The Effects of Intermittent Fasting Combined with Resistance Training on Lean Body Mass,” the researchers noted that IF has been practiced for centuries for religious reasons (example: during the Muslim holy month of Ramadan or Jewish days of fasting such as Yom Kippur).
Regardless of the reasons for engaging in IF, “the impacts on lean body mass (LBM) may be detrimental,” the researchers cautioned. In order to prevent potential muscle wasting caused by fasting, the researchers analyzed studies on various methods of IF combined with resistance training to see what changes in LBM, if any, would present.
The researchers looked for experiments that involved previously sedentary or non-elite athletes. In the 8 studies they selected that fit their criteria, the researchers concluded that “IF paired with resistance training generally maintains LBM, and can also promote fat loss.”
A research study from Beijing Sport University looked at the length of time it took IF to regenerate muscle cells (skeletomuscular autophagy). They found that in the group who did IF, it took only 14 days for autophagy. In comparison, the group that did not do IF but performed aerobic exercise experienced autophagy 28 days after the experiment began.
There was also a research study published in a journal devoted to muscle wasting disorders that examined the effects of 16 men who only consumed 200–250 calories per day from a supplement and performed low-intensity exercise up to 3 hours a day for 10 days.
The researchers concluded that this fasting mimicking protocol “appears safe in healthy humans [and that] protein loss occurs in [the] early fast[ing stage] but decreases as ketogenesis increases.”
Added the researchers, “Fasting combined with physical activity does not negatively impact muscle function.”
Conclusion
Perhaps the researchers from the last study mentioned summed it up best when they concluded, “Future studies will need to confirm these first findings.”
But it may be possible that if you do IF for long-term and don’t do any resistance training and eat an insufficient amount of protein, you may end up losing more muscle mass than if you were not doing IF.
Another takeaway is that all physically-able adults should be performing at least a minimal amount of resistance training exercises, whether it’s pumping iron or doing bodyweight exercises.
To preserve your lean muscle mass, you don’t have to spend hours in the gym. Most experts recommend two or three strength-training sessions per week for just 30 minutes. So pump it up!
