A recent study indicates that following an alternate-day fasting plan for four weeks can reduce body weight and fat levels, but it may also cause a decrease in muscle mass.
According to experts, alternate-day fasting can lower overall calorie consumption and support weight loss. However, they stress that regular physical activity—particularly strength training—is crucial to help protect muscle when using this type of eating pattern. The findings contribute to ongoing research showing that some forms of intermittent fasting can influence body composition.
In the study, a small number of participants who practiced alternate-day fasting for one month experienced meaningful drops in total body weight and fat mass. The researchers found no significant changes in blood pressure or blood sugar levels. Muscle mass, however, declined during the fasting period, even among participants who supplemented with protein.
The study results were recently published in the journal Nutrients.
Dana Hunnes, PhD, a senior clinical dietitian at UCLA Health, said the findings were consistent with what is typically seen during calorie restriction.
“When people are in a calorie deficit, weight loss is almost inevitable, and muscle loss often comes with it,” Hunnes told Healthline. She was not involved in the research.
Mir Ali, MD, a bariatric and general surgeon and medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in California, said the study illustrates the trade-offs common to many weight loss approaches.
“Alternate-day fasting can work for weight loss,” Ali said. “But, as with any method, results and sustainability vary widely between individuals.” Ali also did not participate in the study.
Study Findings on Alternate-Day Fasting
To conduct the research, investigators divided participants into two groups.
The first group included 20 Asian men between 21 and 35 years old, all with a body mass index (BMI) above 23. None of the participants smoked, used tobacco, took long-term medications, or had medical conditions that could be negatively affected by fasting.
These participants followed a four-week alternate-day fasting program between February and March 2017. On fasting days, they consumed only water or zero-calorie beverages and ate one small meal containing 400 to 600 calories. On non-fasting days, there were no dietary restrictions.
The second group, studied from January to March 2018, included 26 Asian men with similar characteristics. They followed the same fasting schedule, but some were given a 25-gram whey protein supplement on fasting days to determine whether extra protein could help prevent muscle loss.
After combining data from both groups and excluding nine participants, the final analysis included 37 individuals with an average age of 25 and an average BMI of 26.
The researchers found that 35 participants lost weight, averaging a reduction of 2.4 kilograms. Fat mass decreased in 33 participants by an average of 1.6 kilograms.
At the same time, muscle mass declined in 28 participants, with an average loss of 0.8 kilograms. Protein supplementation did not prevent this muscle loss. Blood pressure and blood glucose levels showed little change, and participants’ physical activity decreased during weeks two and three of the program.
Ali said the results highlight both the benefits and limitations of fasting-based diets.
“The study shows that fasting can be effective for weight loss, but muscle preservation requires more than protein alone,” he said. “Consistent resistance and aerobic exercise are essential.”
Hunnes agreed, noting that protein intake cannot fully counteract muscle loss during a calorie deficit.
“When overall energy intake is low, the body draws weight from multiple sources, including muscle,” she explained.
Different Approaches to Fasting
Alternate-day fasting is one form of intermittent fasting (IF). In its standard version, individuals fast every other day and eat freely on non-fasting days. Modified versions allow up to 500 calories on fasting days.
Some experts suggest alternate-day fasting may be easier for certain people to follow. However, a 2017 study found it was no more effective than traditional calorie-restricted diets. That research also showed that people following alternate-day fasting could lose 3% to 7% of their body weight over two to three months.
A 2019 study reported that alternate-day fasting may reduce insulin levels, potentially lowering the risk of type 2 diabetes.
Intermittent fasting more broadly includes patterns such as eating within a daily time window, commonly 16 hours of fasting followed by an 8-hour eating period. Other methods include fasting for a full day once or twice per week (“eat-stop-rest”) and the 5:2 diet, which limits calories on two nonconsecutive days each week.
Early studies suggest intermittent fasting may aid weight loss, improve insulin sensitivity, reduce inflammation, and support brain health. Research published in 2025 found intermittent fasting to be as effective as daily calorie restriction for weight loss, and another 2025 study reported no negative effects on cognitive performance during fasting periods.
Intermittent fasting is not recommended for individuals who are underweight or who have diabetes, blood sugar regulation issues, low blood pressure, or a history of eating disorders. Evidence remains mixed regarding its effects on heart disease risk.
Hunnes cautioned that alternate-day fasting may be difficult to maintain long term.
“Feeling hungry every other day isn’t sustainable for many people,” she said. “It can also lead to overeating on non-fasting days in some cases.”
She added that gradual weight loss achieved through regular exercise and a modest calorie deficit is often easier to maintain over time.
Practical Advice for Alternate-Day Fasting
For those considering alternate-day fasting, experts recommend a thoughtful approach.
On fasting days, consuming the allowed 500 calories in a single meal focused on high-protein, nutrient-dense foods and low-calorie vegetables may be helpful. Suggested options include:
Soups
Eggs
Vegetables
Yogurt with berries
Grilled fish
Lean meats
Salads
“The key concern is maintaining hydration and adequate nutrition during fasting days,” Ali said.
He also advises easing into fasting gradually, prioritizing sleep, and maintaining a consistent exercise routine.
“People should consult their doctor before starting,” Ali said. “Begin slowly, allow time for adaptation, focus meals on protein and vegetables, and limit sugars and refined carbohydrates.”
Ali emphasized that exercising about 30 minutes a day, five days a week—using both resistance and aerobic training—can help reduce muscle loss while pursuing weight loss.