Alternate-day fasting cuts body fat but also reduces muscle

New research shows that while four weeks of alternate-day fasting can quickly reduce body weight and fat, it also leads to a measurable loss of muscle, and adding a whey protein shake during fasting days isn’t enough to stop it.

In a recent study published in Nutrients, researchers investigated the impact of short-term alternate-day fasting (ADF) with or without protein supplementation on health and body composition.

Around 44 % and 16 % of adults worldwide are overweight and obese, respectively. In Singapore, the corresponding estimates are approximately 41 % and 14 %. Several weight loss strategies have been introduced to combat obesity, with intermittent fasting gaining significant interest due to its potential benefits on metabolic and cardiovascular health, as well as lipid profiles. It involves alternating between fasting and feeding periods, with fasting periods lasting 16 to 20 hours/day.

ADF, a type of intermittent fasting, involves alternating between feeding and fasting days every 24 hours. However, ADF includes a single small meal of ~400 to 600 kcal during fasting days rather than extended daily fasting windows. Some evidence suggests that ADF decreases body mass, fat mass, body mass index (BMI), cholesterol levels, blood pressure (BP), and triglyceride levels. Despite the benefits of ADF, muscle loss remains a particular concern, and whether protein supplementation prevents muscle loss remains unclear.

Study design compares ADF with and without protein intake

In the present study, researchers investigated whether short-term ADF has beneficial effects on body composition and whether higher protein intake preserves muscle during weight loss. Asian males aged 21 to 35 years with a BMI > 23 kg/m² who did not smoke or use tobacco were enrolled in Study 1. Individuals taking long-term medications, those with a health condition that might be aggravated by fasting, or those unable to meet the diet and physical activity requirements were excluded.

Participants were subjected to a four-week ADF protocol that began at 12 AM, alternating between fasting and feeding days in 24-hour cycles. On fasting days, only water or zero-calorie beverages and a small meal of 400 to 600 kcal were allowed between 12:00 and 2:00 PM. On feeding days, free intake of foods and beverages was permitted. Body composition was assessed before and after intervention using bioelectrical impedance analysis.

Resting BP and fasting blood glucose (FBG) were measured weekly. Physical activity levels were assessed using the Global Physical Activity Questionnaire, administered every week. A dietary record was also maintained on any two days of fasting each week. Furthermore, the team conducted a randomized controlled trial (Study 2) to investigate whether increasing protein intake would help preserve muscle during weight loss.

Study 2 also enrolled young Asian males aged 21 to 35 with a BMI > 23 kg/m² for a four-week ADF, but participants were assigned to a control or protein group. The protein group consumed a 25 g whey protein supplement (125 kcal) during the fasting days. Besides the protein supplementation on fasting days for the protein group, the rest of the trial was identical to Study 1, including dietary records, GPAQ, health assessments, and laboratory measurements.

ADF lowers weight and fat but reduces muscle too

Study 1 enrolled 20 participants in February and March 2017, while Study 2 recruited 26 subjects from January to March 2018. Seven individuals from Study 1 and two from the protein group were lost to follow-up. As such, the final analytic sample consisted of 37 participants, with an average age of 25 years and a BMI of 26.7 kg/m². During days of fasting, Study 1 subjects consumed an average of 525 kcal, of which 38 %, 37 %, and 25 % were from carbohydrates, fat, and protein, respectively.

The protein group consumed 495 kcal on fasting days, of which 34 %, 26 %, and 40 % were from carbohydrates, fat, and protein, respectively. The control group consumed 533 kcal, with corresponding estimates of 46 %, 33 %, and 22 %, respectively. Hydration status before and after intervention remained consistent in both studies. Since no interaction effects were observed between the protein and control groups, data from Study 1 and Study 2 were combined for analysis.

All body composition measures post-intervention showed significant reductions, including body mass, fat mass, and fat-free mass (muscle). Physical activity levels were also significantly lower at the end of weeks 2 and 3 compared to baseline. Diastolic BP and FBG showed statistically significant time effects, but these changes were short-lived. DBP decreased only at week 3, and FBG decreased only at week 1; neither marker showed a significant difference between baseline and post-intervention. No changes were noted for systolic BP.

Low-dose protein proves insufficient for muscle preservation

Taken together, the findings illustrated that short-term ADF resulted in significant reductions in body mass, fat mass, and fat-free mass. Increasing protein intake through a low-dose whey protein supplementation did not mitigate muscle loss.

The study notes that fasting-day protein intake remained below the recommended daily levels and that the differences in total protein intake between groups were small, while feeding-day protein intake was not monitored. These factors may explain why supplementation failed to preserve muscle.

Further studies may investigate the efficacy of protein or leucine supplementation during both fasting and feeding days, combined with concomitant resistance training, on the preservation or improvement of muscle during weight loss.

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Journal reference:

• Pang BW, Yang Y, Rashiqah N, Huang CB, Sim DW (2025). Effects of Four Weeks of Alternate-Day Fasting with or Without Protein Supplementation - A Randomized Controlled Trial. Nutrients, 17(23), 3691. DOI: 10.3390/nu17233691. https://www.mdpi.com/2072-6643/17/23/3691