Alternate-day fasting (ADF) is often promoted as a quick way to reduce weight and improve metabolic health.
A new trial shows it can indeed lower body fat in four weeks – but it also trims away muscle, and a modest protein shake on fasting days did not stop that.
What is alternate-day fasting
ADF involves:
- “Fasting” days: only water or zero-calorie drinks plus one small meal of about 400–600 kcal, usually at lunchtime
- “Feeding” days: eating freely, without set restrictions
In this study, fasting and feeding days alternated every 24 hours for four weeks.
Who took part
Two linked studies recruited young Asian men aged 21–35 with:
- Body mass index (BMI) above 23 kg/m²
- No smoking or tobacco use
- No long-term medication or major health conditions that could be worsened by fasting
Study 1 tested basic ADF. Study 2 tested ADF with or without a 25 g whey protein shake (about 125 kcal) taken on fasting days.
What happened to weight, fat and muscle
In total, 37 men completed the programme. On fasting days:
- The basic ADF group ate around 525 kcal
- The protein group ate about 495 kcal
- The control group in Study 2 ate about 533 kcal
Across the combined data sets:
- Body weight dropped
- Fat mass decreased
- Fat-free mass also fell – which includes muscle
So the weight loss was not pure fat; some muscle was lost too.
Physical activity levels also went down during the middle of the programme, which may have added to the muscle loss.
Did extra protein help
The short answer is no, at least in the way it was used here.
The 25 g whey protein shake on fasting days:
- Raised the proportion of energy from protein on those days
- Did not prevent a fall in fat-free mass
The authors point out that:
- Total protein intake on fasting days was still below typical recommendations
- Differences in overall protein intake between groups were small
- Protein intake on feeding days was not controlled or closely measured
All of this makes it hard to protect muscle.
- Popular 5:2 fasting diet beneficial for people with type 2 diabetes and obesity
- Fasting triggers changes in the gut and brain that could help delay dementia
Effect on blood pressure and blood sugars
The researchers measured:
- Resting blood pressure
- Fasting blood glucose
They saw small, short-lived changes:
- Diastolic blood pressure dipped at week 3 then returned to baseline
- Fasting glucose dropped at week 1 but was not significantly different by the end
- Systolic blood pressure barely changed
So the main consistent changes were in body composition, not cardiovascular or glucose markers over four weeks.
What this means if you are thinking about fasting
Key points:
- Four weeks of alternate-day fasting can reduce body weight and fat
- It also reduces muscle mass, which is not ideal for long-term health, glucose control or physical function
- A small protein shake on fasting days was not enough to prevent muscle loss
Protecting muscle during weight loss usually requires:
- Adequate protein across the whole week, not just on fasting days
- Resistance or strength training
- Enough total energy intake to avoid extreme deficits over long periods
For people with diabetes, fasting changes how and when medicines should be taken and can increase the risk of hypos or swings in blood sugar.
Any fasting plan should be discussed with a diabetes team first.
Study: Pang BW et al. Nutrients 2025.
