Intermittent fasting: do you lose muscle?

Skipping breakfast, eating within an 8-hour window, fasting two days a week: intermittent fasting promises to melt away fat, and many lifters fear it will melt away muscle with it. The studies provide a fairly clear answer. If you do resistance training and eat enough protein, your muscle mass holds up. Without resistance training, part of the weight lost is muscle. And at equal calories, fasting does no better than a conventional diet.

With resistance training, muscle holds up

In 34 already trained men, eating all their calories over an 8-hour window, from afternoon to evening, during 8 weeks of resistance training lowered fat mass more than three meals spread from morning to evening, at equal calories and protein. Fat-free mass, arm and thigh muscle cross-sectional area, and maximal strength in the bench press and leg press were maintained in both groups.

In 40 trained women, 8 weeks of supervised resistance training with a window of about 7h30 per day yielded the same fat-free mass gain (+2 to 3%) and the same hypertrophy as eating over a 13-hour window, with the same protein intake (1.6 g/kg per day). Performance progressed with no difference between groups. In 26 active men in a 25% caloric deficit with 1.8 g/kg of protein per day, four weeks with or without an 8-hour window gave the same results: fat-free mass maintained in both groups, strength and muscle cross-sectional area up in both.

The "5:2" fast was tested the same way. In 34 beginners, 12 weeks of resistance training with either 2 days a week at about 70% restriction, or 20% restriction every day, and at least 1.4 g/kg of protein: lean mass up by 3.7%, fat down by 24.1%, strength up, with no difference between the two diets.

Both reviews point in the same direction. A systematic review of eight studies concludes that lean mass is generally maintained when intermittent fasting is combined with resistance training, and one study even found an increase. A meta-analysis of eight studies on 221 people finds no significant effect on fat-free mass (−0.27 kg, compatible with zero), but a greater loss of fat mass (−1.36 kg) than with a non-fasting diet.

Without resistance training, part of the weight lost is muscle

A trial on 54 overweight young adults separated the two effects over 8 weeks. The 10-hour window alone caused a loss of 2.3 kg of fat-free mass; resistance training alone gained 1.6 kg; the two together maintained muscle mass while causing the greatest fat loss (3.2 kg).

The other trials without exercise say the same thing. In the TREAT trial, on 116 overweight adults, eating only within an 8-hour window starting at noon, for 12 weeks, did not cause greater weight loss than three structured meals; in the in-person measured subgroup, the lean mass index of the arms and legs evolved less favorably under fasting than in the other group (difference of −0.16 kg/m²). In a small feasibility study on 20 people, the 8-hour window led to a 3.7% weight loss, but also 3.0% of lean mass, for 4% of fat mass.

In the long term, a meta-analysis of 24 trials of at least six months on 2,032 people finds, compared to a control diet, 3.06 kg less fat and 0.81 kg less fat-free mass. Another, on 35 trials and 1,266 people, adds fasting or caloric restriction to exercise: 2.86 kg less fat and 1.65 kg less fat-free mass than with exercise alone. A 2024 meta-analysis confirms that exercise plus fasting leads to greater weight loss (−3.03 kg) and fat loss than exercise alone, but no more than fasting alone. Fasting therefore does not protect muscle by itself: in trials where it holds up, there is resistance training and protein.

No better than a conventional diet at equal calories

The promise of fasting is to lose weight "better". Compared to continuous caloric restriction, the 2025 meta-analysis finds only a small advantage on fat mass (−0.70 kg) and no difference on fat-free mass; its authors conclude that the two approaches are comparable. When the deficit is the same, the 8-hour window does not cause greater fat loss. Over 24 weeks in 100 overweight adults, alternate-day fasting and daily restriction improved the proportion of fat-free mass in weight in the same way. In 35 overweight active people followed for 14 weeks, the 16:8 window and a macronutrient-controlled diet gave the same results, but fasting was easier to follow.

This ease is undoubtedly its real benefit. In young men who ate over 4 hours, 4 days a week, intake dropped by about 650 kcal on those days, without being asked to count. But in this same trial, the non-window group gained slightly more lean mass (+2.3 kg, small effect), and in the 5:2 trial, the continuous diet increased muscle cross-sectional area more. If your goal is to build muscle, fasting does not help, and could hinder it slightly.

Strength and performance: nothing lost

The meta-analysis on 35 trials finds no effect of fasting or caloric restriction, added to exercise, on the bench press, leg press, jump, or VO₂max; only grip strength increases slightly (+1.7 kg). In 18 trained men, four weeks with or without an 8-hour window did not change fat mass or fat-free mass differently; fasting gave better upper body strength values, but poorer jumps. In 40 men aged 20 and 50, 12 weeks of fasting did not change muscle power. In 16 elite cyclists, four weeks with an 8-hour window led to a 2% weight loss without loss of fat-free mass or performance.

A signal recurs in two trials: testosterone and IGF-1 dropped under fasting, in trained men as well as in cyclists, without strength or muscle suffering over the duration of the trials (4 to 8 weeks).

Key takeaways

In trials where muscle is maintained, we find supervised weight training two to three times a week and, when intake is given, 1.4 to 1.8 g/kg of protein per day.

Every figure in this article comes from a study you can open: the underlined mentions lead to the summary of the work it is taken from, written solely from the abstract. Most trials last 4 to 12 weeks and involve 16 to 54 people; the reviews combine different forms of fasting (daily window, 5:2, alternate-day). Informative content, which does not replace the advice of a healthcare professional.