You eat less, the scale drops, and it feels like the plan is working. Some of what comes off is muscle, and the research shows how to keep more of it.
A bathroom scale weighs all of you at once. When the number drops, it does not say what went. The part most of us want gone is the fat. The muscle is the part worth keeping, because it is what you lift, climb and carry with, and it is what strength training builds on.
So the useful question about any way of losing fat is how much of the weight that comes off is fat, and how much is everything else.
Researchers at Washington University School of Medicine in St. Louis pulled the studies on this together in a 2017 review in the journal Advances in Nutrition.1 They looked at what happens to muscle when people lose weight by eating less.
In people with overweight or obesity, about 20 to 30% of the weight lost was fat-free mass. Fat-free mass is everything in the body that is not fat, muscle included, and the review uses it to mean the same as lean mass. In people of normal weight the share was higher, often more than 35%.1
The same review has good news too. In people with obesity, losing weight generally improved physical function, which is how well people move and manage everyday tasks, most likely because they were carrying less fat. The loss of muscle mass did not reduce their muscle strength.1
Two limits are worth knowing. The 20 to 30% is a typical range across many studies, so any one person can land outside it. And because it is fat-free mass, it is not all muscle.
A common story is that metabolism slows down somewhere in adult life. A large study published in Science in 2021 measured it directly.2 An international group of researchers, first author Herman Pontzer, pooled data from 6,421 people in 29 countries, aged from eight days to 95 years, 64% of them women. They used doubly labelled water, a method that works out from isotope measurements how much energy a person burns in normal daily life, outside the lab.
Bigger bodies burn more energy, so the team adjusted for each person's fat-free mass and fat mass. Once they did, the energy people burned in a day, and the energy their bodies used at rest, held steady from 20 to 60. Fat-free mass held steady across those years too. The fall came after about 60, at around 0.7% a year.2
There are limits here as well. The study measured fat-free mass, so it cannot say what happened to muscle on its own. It did not test why people gain weight in midlife. And people varied a lot: even after the adjustment, individuals differed by more than 20% either way.2
Muscle on its own is the subject of a 2004 review from the University of Southern California, which states that muscle mass decreases by about 3 to 8% a decade after the age of 30.3 The review did not measure this itself. It draws the figure from two earlier papers, so it is best read as a rough range. The same review says exercise training and good nutrition can have dramatic effects on muscle mass and strength.
Back to the 2017 review, which looked at what changes how much muscle goes with the fat.1
Strength training came out well. Also called resistance exercise, it is any training where your muscles push or pull against a load. The authors call it an effective way to reduce or even prevent the loss of muscle during weight loss, and it builds strength at the same time. Endurance exercise such as walking or running helped in some studies, though not in all of them.1
Protein helped less than many people expect. The most recent pooled analysis the review cites found that more protein during weight loss kept 400 to 800 g more lean mass, a very small but real effect. Most studies found it did not improve leg strength. One found that extra protein cancelled an improvement weight loss otherwise brings in how muscle responds to insulin, the hormone that helps move sugar out of the blood.1
So the review's advice is a moderate cut in calories, adequate protein without going overboard, and more movement, especially strength training.1 Protein and strength work best as a pair, with the rest of the plate still in view. Squats to a sturdy chair, step-ups and lunges are an easy place to start.
When you eat less, the scale counts fat and lean mass together, and in people with overweight or obesity about 20 to 30% of what comes off is lean mass. To lose fat, keep muscle by pairing a moderate cut in calories with strength training and enough protein. Eighty Thirty works out your daily protein and keeps count for you, so you keep your muscle while the fat comes off. Start with our free macro calculator.
Strength training is the tool the research rates most, with enough protein alongside it. A 2017 review called strength training an effective way to reduce the loss of muscle during weight loss. Its advice was a moderate calorie cut, adequate protein without going overboard, and more movement.1
In people with overweight or obesity, about 20 to 30% of the weight lost by eating less is fat-free mass, which includes muscle. In people of normal weight the share was often more than 35%. Fat-free mass is everything that is not fat, so it is not all muscle.1
Not between 20 and 60, according to a 2021 study of 6,421 people in 29 countries. Once body size and make-up were allowed for, the energy people burned each day held steady across those years. The fall came after about 60, at around 0.7% a year.2
A little. The pooled analysis cited in a 2017 review found that more protein during weight loss kept 400 to 800 g more lean mass, a very small but real effect. Most studies found it did not improve leg strength, so it works best paired with strength training.1
It can help. In a 2017 review, endurance exercise such as walking or running helped keep muscle during weight loss in some studies, though not in all of them. The authors called strength training an effective way to reduce that loss, and it builds strength too.1
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