When lean men reduce their dietary protein intake, their blood levels of a signaling molecule called FGF21 rise significantly within five weeks, and this rise is closely tied to the increased calorie intake needed to keep their body weight stable, indicating FGF21 may play a role in connecting protein consumption to energy use.

If young guys eat more protein while working out hard and cutting calories for a month, they’ll gain more muscle and lose more fat compared to eating less protein.

When lean, healthy men consume a diet with very low protein for five weeks while maintaining the same total calorie intake, they naturally eat more food to keep their weight stable, suggesting that low protein intake increases the body’s energy needs for weight maintenance.

If you keep fasting or cutting calories for a long time without giving your body a break, your body might stop responding to a helpful hormone called FGF-21, and you won’t get as many health benefits from it anymore.

If young guys lift weights hard and do intense workouts while eating less for a month, chugging 2.4 grams of protein per pound of muscle each day helps them gain more muscle and lose more fat than eating half that much protein.

If you're an untrained adult with extra weight, doing resistance training for 12 weeks while eating less—either through intermittent fasting or cutting calories every day—and getting enough protein, gives you similar gains in muscle, strength, and fat loss, no matter which diet approach you pick.