Am I losing fat or muscle?

Mostly fat, if three things are true: your lifts are holding or going up, your waist is getting smaller, and you’re losing less than about 1% of your weight a week. If your strength falls week after week while the scale drops fast, some of what you’re losing is muscle.

Why the scale can’t tell you

A home scale’s muscle and lean mass numbers come from the same estimate as its bodyfat number. In a 2023 lab test of 15 devices, the error on a change over 12 to 16 weeks was about 2 to 3 points of bodyfat, which is often the whole change you’re looking for. Your own checks are better.

Your lifts are the receipt. If the numbers on the bar hold, the muscle is still there.

The three checks

1. Strength. Use the same lifts for the same reps each week. Holding steady on a diet is a good result.
2. Waist. Measure once a month at the same spot. A shrinking waist means fat is going.
3. Pace. In a 2011 study of 24 elite athletes who lifted, both groups lost about 5.5% of their weight. The ones losing about 0.7% a week gained lean mass. The ones losing about 1% a week gained none.

The one people forget: sleep

In a 2010 study, 10 adults dieted for two weeks twice, once with 8.5 hours in bed and once with 5.5. They lost about the same weight both times. On short sleep, only about 1.3 pounds (0.6 kg) of it was fat, against about 3.1 pounds (1.4 kg) on full sleep, and they lost more lean mass.

What to do

• Lift two or three days a week and keep the same lifts so you can compare.
• Eat 1.2 to 1.6 g of protein per kg of body weight a day. Find your range.
• Protect your sleep, as seriously as your protein.
• Split your weigh-ins into fat and lean with the fat or muscle calculator, using weigh-ins a month or more apart.