In a caloric deficit the body breaks down some muscle protein for fuel while anabolic signals weaken, so muscle protein breakdown outpaces synthesis.In a caloric deficit, the body needs energy. It uses glycogen and fat first, but also breaks down some muscle protein for fuel (gluconeogenesis). Meanwhile, anabolic signals weaken (insulin, and activity drop).
Net result: muscle protein breakdown (MPB) > muscle protein synthesis (MPS). Without resistance training and with little protein, a considerable share of the weight lost is lean mass (including muscle).
This is why weight loss ≠ fat loss. The scale can mislead — some of what you lose may be muscle and water, not only fat.
2 · MPS · the switch for muscle protein synthesis
Leucine from dietary protein and resistance training both activate and switch on muscle protein synthesis, and together they beat either alone.Two signals can switch on MPS:
1. Dietary protein (especially leucine): leucine binds Sestrin2 → releases GATOR2 inhibition → activation → MPS ↑. It is a switch with a threshold: a meal needs enough leucine to flip it. A commonly quoted rule of thumb is about 2.5-3 g; Volpi 2013 estimated about 3 g for older adults, equal to 25-30 g of high-quality protein. These figures come from short-term metabolic studies that measure synthesis rates, not muscle gained months later.
2. Resistance training: mechanical tension → phosphatidic acid + the small G-protein Rheb → mTORC1 activation. After training, muscle synthesis stays raised for a day or two.
The two together > either alone: eating protein around training gives a bigger synthesis response than training or eating alone. But there is no 30-minute window to catch: Aragon & Schoenfeld 2013 advise keeping the pre- and post-training meals no more than about 3-4 hours apart, and the day's total matters more than timing.
3 · Per-meal threshold · ~20 g / 0.24 g/kg in the young
Once a meal reaches its plateau, muscle synthesis over the next few hours stops rising much, and older adults reach that plateau at a higher dose than the young.Moore 2009: in young men after resistance exercise, whole-egg protein brought muscle synthesis close to its maximum at 20 g, and 40 g did not push it higher; more of the extra was oxidized and turned into urea.
Moore 2015: a breakpoint analysis of pooled dose-response data put the plateau at about 0.24 g/kg body weight in younger men and about 0.40 g/kg in older men (0.25 vs 0.60 g/kg of lean mass). Older adults need more protein per meal.
These are measurements over a few hours: eating more in one meal no longer raises synthesis much over a few hours; whether a big serving buys some extra synthesis over half a day or longer is still debated. So the conclusion is no need to chase a huge meal, not every extra bite is wasted.
How to eat in a deficit: A common recommendation is 1.6-2.4 g/kg a day, toward the upper end the bigger the deficitSpread it over 3-4 meals so each one clears the threshold, roughly 0.3-0.4 g/kg per meal as a rule of thumb For 70 kg: about 112-168 g of protein a day, over 3-4 meals of roughly 30-40 g. Especially important on training days.
4 · Protein leverage + practical application
The protein leverage hypothesis holds that people eat until they get enough protein, so low-protein food leads to passive overeating and a higher protein share may curb appetite.The protein leverage hypothesis (Simpson & Raubenheimer): people need an absolute amount of protein, not a proportion. When food has little protein per calorie (true of many ultra-processed foods), the body keeps eating until it has enough protein → passive overeating. It is still a hypothesis, not a proven main cause of obesity.
Used the other way: raise the share of protein in meals and appetite may rein itself in — you reach your protein sooner and stay full longer.
In practice: Start each meal with a serving of protein (eggs, chicken breast, fish, tofu, whey), then add vegetables and carbohydrateEat a normal protein-containing meal 1-3 hours before training and another 1-2 hours after; there is no need to rush a shake the moment you finishIn a deficit, do not cut carbohydrate so far that training suffers — carbohydrate fuels training intensity, and training itself protects muscle
Mechanism
MPS · protein strategy to preserve muscle in deficit