Can you build muscle and lose fat at the same time? What a controlled trial found

Scroll through any fitness forum’s FAQ and one question sits at the top, over and over: should I bulk or cut — and is it actually possible to build muscle and lose fat at the same time? The textbook answer for decades was no: bulking and cutting were treated as separate phases you had to alternate between. A 2016 randomized trial in the American Journal of Clinical Nutrition put that assumption to a direct test, and the results say the textbook answer is incomplete.

The experiment

Tyler Longland and colleagues at McMaster University put young men into a genuinely aggressive energy deficit — about 40% below what they needed — for four weeks, while all of them performed six days a week of resistance training combined with high-intensity interval training. The only difference between the two groups was protein intake: one group ate a lower-protein diet (1.2 g per kg of body weight per day, close to a typical intake), the other ate more than double that (2.4 g/kg/day).

Both groups lost weight. Only one gained muscle while doing it

The lower-protein group barely moved the needle on lean mass, gaining just 0.1 kg on average, while losing 3.5 kg of fat. The higher-protein group gained 1.2 kg of lean mass — a statistically significant increase — while losing more fat, 4.8 kg. In other words: during the exact same brutal calorie deficit, with the exact same training, doubling protein intake was the difference between treading water on muscle and measurably building it, while losing more fat in the process.

A diet containing 2.4 g protein·kg⁻¹·d⁻¹ was more effective than 1.2 g·kg⁻¹·d⁻¹ in promoting increases in lean body mass and losses of fat mass when combined with a high volume of resistance and anaerobic exercise.

Why this doesn’t contradict "bulk or cut"

This trial doesn’t mean the deficit-vs-surplus framework is wrong — it means recomposition has specific ingredients, and most people trying it are missing at least one. The conditions that made it work here were a real, structured resistance-training stimulus every single session, a genuinely high protein intake (more than most people eat even when they’re trying), and a population with real room to adapt: young, reasonably fit but not yet close to their genetic ceiling. Recomposition is generally easiest for people newer to lifting, returning after time off, or carrying more body fat to begin with — exactly the people with the most "newbie gains" headroom left. Very lean, highly trained lifters chasing a new deadlift PR still get more from a deliberate bulk or cut phase, because their margin for simultaneous gain and loss is much smaller.

The practical takeaway

If you’re asking whether you have to choose between losing fat and building muscle, the honest answer is: not entirely, and protein is the lever that decides how much of both you get. A structured resistance-training plan combined with a genuinely high protein target, even inside a real calorie deficit, is a legitimate way to move both numbers in the right direction at once — it just won’t be as fast at either as a dedicated phase would be.

This is exactly why Misi sets protein floors independent of your calorie target rather than as a simple percentage of intake, and pairs every deficit with a resistance-training plan rather than treating cardio alone as the fat-loss tool — the combination this trial tested is the combination Misi builds by default.

Misi is a coaching and wellness platform, not a medical service, and this article is educational and not a substitute for personalised medical or nutritional advice.