How does a low-fat diet affect muscle mass in adults with obesity?
When you lose weight on a low-fat diet, some of that weight loss comes from muscle, not just fat. A study of adults with obesity found that after 6 months on a healthy low-fat or low-carb diet, people lost a small amount of muscle in their arms and legs — about 0.8 to 1.0 kg 5. However, research shows that adding resistance exercise (like lifting weights) to your weight loss plan can help keep your muscle strong while you lose fat 9.
What the research says
A 2019 study looked at 374 adults with obesity who followed either a healthy low-fat or low-carb diet for 6 months. On average, women lost about 0.8 kg of muscle in their arms and legs, and men lost about 1.0 kg 5. This muscle loss is considered modest, but it is a real concern because losing muscle can slow your metabolism and make it harder to stay active.
A meta-analysis of 15 studies on people with sarcopenic obesity (low muscle plus high body fat) found that diet alone — even a low-calorie, high-protein diet — did not improve muscle mass or grip strength 9. However, when people added resistance exercise to their routine, they were able to lose fat and keep or even improve their muscle strength 9.
Other research confirms that weight loss from different diets, including low-fat and low-carb plans, tends to reduce both fat and lean tissue 58. For example, a study on wholegrain rye diets found similar weight loss to refined wheat diets, with no significant difference in fat loss between groups 8. The key takeaway is that the type of diet matters less for muscle preservation than whether you include exercise — especially resistance training 9.
What to ask your doctor
- How much muscle loss is typical with a low-fat diet, and should I be concerned?
- What type of exercise (like resistance training) would you recommend to help preserve my muscle during weight loss?
- Should I adjust my protein intake while on a low-fat diet to protect my muscles?
- Can you refer me to a dietitian or exercise specialist to create a personalized plan?
This question is drawn from common patient questions about this topic and answered using cited medical research. We do not provide individualized advice.