How do I keep my muscle while losing fat in a calorie deficit?

The body doesn't pick fat or muscle at random in a deficit. What it breaks down is shaped largely by two signals you set yourself: how much protein you eat and whether the muscles are still being used.

Longland and colleagues had young men eat in a steep deficit for four weeks alongside strength and interval training. The group eating 2.4 grams of protein per kilogram of body weight added 1.2 kilos of lean mass and lost 4.8 kilos of fat. The group on 1.2 grams roughly held onto their muscle (up 0.1 kilo) but lost less fat, 3.5 kilos. More protein improved both muscle retention and fat loss.

The pattern holds in larger data. A meta-analysis by Wycherley and colleagues (24 randomized trials, 1,063 participants) found that higher-protein reducing diets preserved on average 0.43 kilos more fat-free mass and removed 0.87 kilos more fat than diets with ordinary protein.

Training is the other half. A network meta-analysis by Deller and colleagues (34 trials, 1,455 participants) showed that exercise during a diet prevented nearly half of the muscle loss. The effect was largest for mixed training (+1.20 kilos), followed by strength training (+0.83 kilos), though the difference between training types was not statistically significant. For someone already lean who wants to get truly defined, Helms and colleagues recommend up to 2.3–3.1 grams of protein per kilo of lean body mass, a level most people never need.

Muscle feels hard-won, so the idea that it disappears first stirs more worry than fat ever does. The scale feeds this: it shows a single number and can't say what you lost, so a quick early drop on a diet, mostly water and glycogen, is easily read as lost muscle. The fear is also a market. Supplements like BCAAs, amino acids and anti-catabolic powders are sold on the promise of saving muscle while you diet, even though enough regular food and continued training do the same job. Gym culture's image that any deficit is catabolic keeps the story alive, because it suits those with something to sell against it.

You don't have to choose between dieting and keeping your muscle. Eat plenty of protein, around 1.6 to 2.0 grams per kilo of body weight, and move toward the higher end the leaner you get. Keep lifting two to three times a week as you lose; that is the signal telling the body to hold on to muscle. Keep a moderate pace, roughly half to one percent of body weight per week, rather than starving out a fast drop. Work out your own protein target with the protein calculator.