Something you may find surprising: If you’re reducing your calorie intake with something like a time-restricted eating form of intermittent fasting (think: 16:8 fasting where you eat all your food within an 8-hour window), that doesn’t mean your protein intake should go down proportionally—unless you were eating too much protein to begin with.
“Unlike fat and carbs, the recommendation for protein is more of an absolute number,” says Layman. “It’s grams per kilogram of body weight. So even if you eat less calories, you still need the same protein,” says Layman. Paddon-Jones agrees: “You still have to hit that gram amount per meal to maintain lean mass.” And remember, that lean muscle mass helps you burn more calories, even while you’re at rest.
Maintaining adequate protein on keto is also crucial. Miller says people may not be eating enough protein when they shift to a low-carb, high-fat keto diet. “If you’re following a classic ketogenic diet, which was developed more for epilepsy and neurological disease management, it’s often too protein-restricted,” says Miller. “What happens, often with women, is that their appetite is regulated and they don’t have organic hunger. So they under-eat—and they under-eat protein pretty dramatically.” This, in turn, can lead to symptoms like fatigue, brain fog, and even hair loss.
All of this might have you wondering: How do more intense fasting plans (think: a day or longer) affect your ability to maintain muscle mass? Most of the experts we spoke with said there’s just not enough research to be sure. While, in theory, it makes sense that going a day or longer without protein would cause a decrease in muscle protein synthesis and an increase in muscle breakdown, “there seem to be some muscle-sparing effects that come into play,” says Miller.
But, the extent to which these muscle-sparing effects would be able to offset muscle breakdown over the long-term isn’t clear. “I think a 25- or 30-year-old might be just fine, but I would never recommend fasting for a 55- or 60-year-old,” says Layman. “The older you get, any muscle loss tends to be permanent, but a younger person tends to recover pretty quickly.”