Diet and weight loss through surgery left people fitter, even with some muscle lost

Here is a relief if you are working on your diet and weight: losing some muscle along the way does not stop you from getting fitter. In a study of 1022 people after weight-loss surgery, fitness improved in every group, men, women before menopause and women after menopause.

When the scale goes down, part of the loss is not fat. It is what scientists call fat-free mass, which includes muscle. Many people worry that means getting weaker. This study shows the opposite can happen: the body still became fitter.

How the study worked

Researchers followed 1022 people who had a first weight-loss operation, also called metabolic bariatric surgery, for at least 18 months. There were 221 men, 614 women before menopause and 187 women after menopause. The team measured weight and fat-free mass before surgery and again at check-ups for up to 72 months, and tested fitness with a bicycle test. It was an observational study with no comparison group, and the bike test estimates fitness rather than pushing people to their limit. It was published in 2026 in The American Journal of Clinical Nutrition, on September 4.

What they found

Everyone lost some fat-free mass, but the share differed. Looking at all the weight each group lost by 24 months, here is how much of it was fat-free mass:

So in every group, roughly a quarter of the weight lost was not fat. Women before menopause lost the smallest share. Men lost the most fat-free mass in total.

Now the good part. Maximal oxygen consumption, a key measure of how well your heart, lungs and muscles work together, improved significantly in all three groups. That happened even with the muscle loss. The researchers also found that changes in fat-free mass after surgery were not independently linked to changes in fitness. In plain words, losing some muscle did not hold fitness back.

The study also showed that people differ a lot. Who lost more or less muscle was not fully explained by sex, menopause or starting body fat. Your own result is your own.

What does fat-free mass actually mean?

It is everything in your body that is not fat: muscle, bone, organs and water. Muscle is the part you can most easily influence with how you eat and move. That is why it gets so much attention in any plan for diet and weight.

What you can do today

You do not need surgery to use this. Whenever you are losing weight, you can give your muscles a reason to stay. Start with two simple habits. At your next meal, include a protein food you enjoy, like eggs, beans, yogurt, chicken or fish. Then pick two days this week for a short strength session: stand up from a chair ten times, push against the kitchen counter, or carry some grocery bags for a lap around the block. A local YMCA or a free online video can show you more moves. If you have had or are planning weight-loss surgery, ask your care team to build this into your plan. Tonight, add protein to tomorrow’s breakfast.