A four-week alternate-day fasting protocol produced notable reductions in body mass and fat mass but also led to a loss of muscle mass, according to a new study published in the journal Nutrients.
Study setup and participants
Researchers enrolled two groups of healthy Asian men, ages 21 to 35, all with a body mass index (BMI) above 23 and without smoking, chronic medication use, or medical conditions that could be worsened by fasting. The first group (studied in Feb–Mar 2017) included 20 men who followed an alternate-day fasting plan for four weeks: fasting days consisted of water or zero-calorie beverages and one small meal of 400–600 calories; non-fasting days allowed eating ad libitum. The second group (studied Jan–Mar 2018) included 26 men on the same regimen, with some participants given 25 grams of whey protein on fasting days to test whether extra protein would protect against muscle loss.
After exclusions during follow-up, the combined analysis included 37 participants (average age 25, average BMI 26). The researchers reported that 35 of 37 participants lost body mass (mean loss 2.4 kg) and 33 lost fat mass (mean loss 1.6 kg). However, 28 participants experienced a decline in muscle mass (mean loss 0.8 kg). The whey protein supplement did not prevent muscle mass loss in those who took it. The intervention did not significantly change blood pressure or blood glucose.
Expert perspectives
Dana Hunnes, PhD, a senior clinical dietitian at UCLA Health, noted that these findings are not surprising: a calorie deficit typically leads to weight loss and often to some loss of muscle. Mir Ali, MD, a bariatric and general surgeon, said the study highlights that alternate-day fasting can be an effective weight-loss strategy but that outcomes and feasibility vary between people. Both experts emphasized that preserving muscle during weight loss requires more than occasional protein supplements; regular exercise—especially resistance training—combined with adequate protein intake is important to minimize muscle loss.
How alternate-day fasting works and related evidence
Alternate-day fasting generally alternates fasting days (sometimes allowing ~500 calories in modified versions) with days of normal eating. Some people find it easier to follow than continuous calorie restriction, though prior research (e.g., a 2017 study) found it wasn’t superior to steady calorie restriction. Other studies have shown potential benefits such as 3–7% body-weight loss over 2–3 months and possible improvements in insulin levels, which could reduce type 2 diabetes risk. Recent research has also suggested intermittent fasting can be as effective as daily calorie restriction for weight loss and may not impair cognitive performance during fasting periods.
Risks and cautions
Intermittent and alternate-day fasting are not recommended for people with diabetes or blood sugar regulation problems, those with low blood pressure, individuals with a history of eating disorders, or people who are underweight. Research is mixed on the long-term cardiovascular effects of intermittent fasting. Hunnes also cautioned that alternate-day fasting may be difficult to sustain long-term and could encourage binge-eating or overcompensation on non-fasting days for some individuals. For many, a steady, mild calorie deficit combined with regular physical activity may be more sustainable.
Practical tips if trying alternate-day fasting
– On fasting days, aim to consume allowed calories (around 500) in one meal focused on nutrient-dense, high-protein foods and low-calorie vegetables. Suggested options: soup, eggs, vegetables, yogurt with berries, grilled fish, lean meat, or a salad.
– Maintain hydration and ensure adequate nutrition during fasting periods.
– Ease into fasting gradually, increasing fasting intervals to allow adaptation.
– Prioritize regular exercise—both resistance and aerobic—about 5 days/week for 30 minutes to help preserve muscle mass.
– Consult a physician before starting, to rule out medical issues that could make fasting unsafe.
Conclusion
This short-term trial shows alternate-day fasting can reduce body and fat mass but also carries a risk of muscle loss, even when a modest whey protein supplement is used on fasting days. To minimize muscle loss during calorie-restricted approaches, experts recommend combining adequate protein intake with consistent resistance and aerobic exercise and seeking medical guidance before beginning a fasting regimen.
