- Researchers report that alternate-day fasting can help reduce body mass and fat mass.
- However, they added that this type of eating plan can also lower a person’s muscle mass.
- Experts say alternate-day fasting can reduce overall calorie intake and help a person lose weight, but they emphasize that daily exercise is important under such a regimen.
A type of intermittent fasting is effective in helping improve body composition, according to a new study.
Researchers report that a small group of study participants who adhered to alternate-day fasting for four weeks showed significant reductions in body mass and fat mass.
However, the fasting did not have a significant impact on blood pressure or blood glucose levels. The regimen also resulted in a loss of muscle mass, even for participants who took protein supplements.
The researchers recently reported their findings in the journal Nutrients.
Dana Hunnes, PhD, a senior clinical dietitian at UCLA Health in Los Angeles, said the results of the study were not surprising.
“A calorie deficit will [almost] always result in weight loss and frequently in muscle loss as well. It is not at all surprising to me that in this study, this occurred,” she told Healthline. Hunnes wasn’t involved in the study.
Mir Ali, MD, a general surgeon, bariatric surgeon, and medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in California, said the study demonstrated the nuances of weight loss programs.
“Alternate-day fasting is a viable weight loss strategy,” Ali, who wasn’t involved in the study, told Healthline. “However, like all weight loss methods, the effectiveness and feasibility will vary greatly.”
To reach their findings, the researchers set up two study groups.
The first contingent consisted of 20 Asian males, ages 21 to 35. The subjects all had a body mass index (BMI) greater than 23. They also didn’t smoke, use tobacco, take any long-term medications, or have a health condition that could be exacerbated by fasting.
These participants, who were studied in February and March 2017, were assigned to a four-week alternate-day fasting protocol. They fasted every other day, drinking only water or zero-calorie beverages while consuming one small meal of 400 to 600 calories. On non-fasting days, they were allowed to eat and drink what they chose.
A second contingent, which was studied from January 2018 to March 2018, consisted of 26 Asian males with similar characteristics to the first group.
They participated in the same alternate-day fasting regimen. However, some subjects in the second group were given a 25-gram whey protein supplement on the days they fasted to determine if this extra protein would help reduce muscle mass loss.
The researchers combined the results of the two groups for their analysis. During the follow-up period, nine participants were excluded for various reasons. That left researchers with a total of 37 subjects with an average age of 25 and an average BMI of 26.
The researchers reported that 35 of the 37 participants showed reductions in body mass by an average of 2.4 kilograms. In addition, 33 participants exhibited losses in fat mass by an average of 1.6 kilograms.
However, 28 participants also showed a decline in muscle mass, averaging 0.8 kilograms. The researchers stated that the ingestion of whey protein did not prevent muscle mass loss in the subjects who took the supplement.
Ali said the results revealed the effectiveness of fasting for weight loss, as well as its implications for muscle mass.
“This study showed that alternate-day fasting is helpful for weight loss, but there is more to preserving muscle mass then just protein intake,” he said. “In order to preserve muscle, adequate protein intake is important, but also regular exercise. A combination of both resistance and aerobic exercise on a consistent basis will help minimize muscle loss.”
Hunnes agreed with this assessment.
“It is not surprising to me that 25 grams of protein on fasting days did not attenuate the muscle loss since in a calorie deficit, it honestly doesn’t matter if you’re consuming protein, because you’re still at an overall energy deficit and the weight loss comes from all areas of the body, including muscle,” she explained.
The basic format is you fast every other day and then eat whatever you want on the other days. A modified version of this plan allows for 500 calories on fasting days.
Some experts have said that alternate-day fasting can be easier for people to follow. However, a 2017 study concluded that alternate-day fasting wasn’t superior to calorie-restriction diet plans.
Nonetheless, that study also reported that alternate-day fasting plans can help a person lose 3–7% of their body weight after 2- to 3-month alternate-day fasting periods.
A 2019 study indicated that alternate-day fasting can help reduce insulin levels and therefore lower the risk of developing type 2 diabetes.
In intermittent fasting, a person restricts their eating to certain parts of the day. A common pattern is to have an 8-hour eating window with 16 hours of fasting.
There is also an “eat-stop-rest” version in which a person fasts for one day once or twice a week and eats normally on other days.
There is also a 5:2 diet where a person consumes 500 to 600 calories on two nonconsecutive days of the week but eats as usual the other five days.
Some early research has shown that intermittent fasting can help with weight loss, insulin resistance, inflammation reduction, and brain health.
Another 2025 study reported that intermittent fasting does not appear to impair mental performance during the periods when a person isn’t eating.
Intermittent fasting is not recommended for people who have diabetes, problems with blood sugar regulation, low blood pressure, or a history of eating disorders. It’s also not recommended for people who are underweight.
There is conflicting research on whether intermittent fasting increases or decreasing the risk of heart disease.
Hunnes has some cautions for people contemplating an intermittent fasting plan.
“In the long term, it does not seem very sustainable as you’re walking or sitting around hungry every other day,” she said. “In fact, this might actually (in uncontrolled environments) lead to binge-eating and/or overcompensation on eating days, depending on the individual.”
“For many people, engaging in physical activity and a mild calorie-deficit, leading to slow but steady weight loss, is often far more sustainable in the long term than this type of fasting dieting,” Hunnes added.
If you want to try an alternate-day fasting routine, there are some suggestions on how to manage it effectively.
For starters, try to consume the allowed 500 calories at one meal with a focus on nutritious, high protein foods and low-calorie vegetables. Suggested foods include
- soup
- eggs
- vegetables
- yogurt with berries
- grilled fish
- lean meat
- salad
“The main concern is that patients maintain good hydration and adequate nutrition during the fasting period,” Ali said.
“I advise people to first consult their physician to make sure they do not have any medical issues that may make fasting for periods of time problematic for them,” Ali said.
“Next, start gradually and then increase the interval of the fasting period to allow the body and mind to adapt to this way of eating. Nutritional intake should emphasize primarily protein and vegetables and minimize carbohydrates and sugars to maximize fat loss. Finally, regular consistent exercise (5 days/week, 30 minutes/day) combining both aerobic and resistance exercises is important to preserve muscle,” Ali concluded.
