Does 16:8 fasting cost you muscle if you keep lifting?

Short answer

No. In 8 trials where people lifted while doing 16:8, lean mass was no different from normal eating, and they lost 1.25 kg more fat.

Does 16:8 fasting cost you muscle if you keep lifting?
'Longer fasting → more muscle breakdown → less muscle' — does this formula hold?
16:8 fasting, where you skip food for 16 hours and eat only within 8, is a popular way to lose weight. It comes with a worry: fast too long and your body breaks down muscle. We checked studies where people also did strength training.
Same calories: muscle stayed, only fat dropped
Fat fell 16.4% vs 2.8% after 8 weeks
34 people did the same strength training for 8 weeks while eating about 2,800-3,000 kcal a day. Half ate within 8 hours, half spread meals from breakfast on. The 8-hour group lost 16.4% of their fat versus 2.8%, and lean mass (muscle and everything except fat), arm and thigh muscle size and strength held steady in both.
No worse when building muscle
67 kg vs 1.82 kg gained in a 17-person trial
When 40 people trained for 8 weeks on the same calories and protein, both those eating within about 7.5 hours and those eating over 13 hours gained 2-3% lean mass. In a trial where 17 people trained while eating 10% extra calories, the 16:8 group gained 2.67 kg and the normal group 1.82 kg.
Pooling the trials: no difference in muscle
And 1.25 kg more fat lost
An analysis that pooled 8 trials of people lifting while eating within 8-10 hours found lean mass no different from normal eating, and 1.25 kg more fat lost. Another analysis that pooled 15 studies with 338 people also found no drop in lean mass.
Take away the exercise and it changed
Fasting alone cost more arm and leg muscle
In a 12-week trial of 16:8 with no exercise, the 50 people measured in person (25 per group) showed a bigger drop in an arm-and-leg muscle index in the 16:8 group than in those eating all day. Body weight was no different between groups. What protected muscle was strength training, not the fasting window.
The real variable is eating less
The shorter the window, the fewer calories
23 people tried to eat in a surplus on 16:8 for 12 weeks but fell short of their target, yet muscle gain was no different at 1.07 kg vs 1.13 kg. In a trial with a 4-hour eating window, people ate about 650 kcal less per day, and it was the normal-eating group whose lean mass rose 2.3 kg.
Bottom line: keep lifting and 16:8 does not cost muscle
The formula breaks at the 'fasting → muscle breakdown' link
In trials where people lifted and ate within 8 hours, lean mass was no different from normal eating, and more fat came off. Muscle dropped more only when people fasted without exercising. Keeping up strength training and eating your usual amount within the 8 hours is enough.