Do you lose less muscle when you do strength training during a diet?
Strength training during a diet preserves on average almost a kilogram of extra muscle compared with dieting alone. Combine it with enough protein, at least 2 grams per kilogram of body weight per day, for the best result.
Strength training during a diet clearly helps preserve muscle mass. On average, you hold on to around 0.87 kg more lean mass than with dieting alone. That is close to half the muscle loss you would otherwise experience. People who combine strength and cardio training do best, retaining on average 1.20 kg more muscle. Strength training alone comes in just below that at 0.83 kg. Cardio training on its own achieves this effect less reliably.
Building muscle and losing fat at the same time does not always work. Once your calorie deficit reaches roughly 500 kcal per day, further muscle growth stops for most people. Strength training can then prevent muscle loss, but the scales do not tip towards new muscle until the cuts are less drastic. On the positive side, muscle strength does increase, even when you are running a calorie deficit.
The combination of enough protein and strength training makes all the difference. In a smaller study of trained people eating 40% less than usual, muscle mass held up well as long as they ate at least 2.3 grams of protein per kilogram of body weight every day. In another small study, a moderate deficit combined with 2.5 grams of protein per kilogram led to simultaneous muscle gain and fat loss (body recomposition). These are small, short-term studies, but their direction is consistent with what larger studies show.
The amount of training (many or few sets per week) seems to matter less for preserving muscle mass than you might expect, as long as protein intake is sufficient. More sets per week did, however, produce greater gains in strength. For people with poorer insulin sensitivity there is good news: once they exercise, that disadvantage for muscle preservation disappears entirely.
In women, time-restricted eating combined with exercise appears slightly more favourable for muscle preservation than standard calorie reduction, though the differences between approaches are statistically small. A ketogenic diet combined with exercise scored lowest for fat reduction and muscle preservation. If you are unwell or taking medication, discuss with your doctor which type of training suits your situation.
This answer draws on a combined summary of 34 studies and a network comparison of 78 studies, supplemented by smaller studies in trained individuals and specific populations. The large summaries provide solid support for the main finding, that exercise preserves muscle during a diet. The sub-questions about training volume, protein strategy, and specific dietary approaches are based on small, short-term studies involving tens of participants; those findings are preliminary.