Does bcaas break a fast?

BCAAs (branched-chain amino acids) are leucine, isoleucine and valine, three of the building blocks of protein. Protein provides about 4 calories per gram. Counted the same way, a 5 g serving of BCAAs carries about 20 calories, before any flavouring or sweetener.

Autophagy is the goal BCAAs fail most clearly. Leucine is the amino acid at the centre of the research. In cell and animal studies, leucine and other amino acids switch on mTOR, the sensor that tells cells nutrients have arrived and turns autophagy down. Whether a few grams of BCAAs do this in people is not known. The evidence comes only from lab and animal studies.

The other three goals are grey on this site's own four-goal scale. For weight loss, 20 calories sits under the 50-calorie line, which is a common rule of thumb, not an official limit. BCAAs contain no sugar or starch, so a blood-sugar fast treats them as grey, the same as whey protein. They arrive already split into single amino acids, so the gut has less to break down than with a protein shake. That puts gut rest in the grey area too.

BCAAs are marketed to people who train before eating, to protect muscle. For muscle, the protein advice for people who exercise is 1.4 to 2.0 g per kg of body weight a day. You can eat all of that in your eating window, with no supplement during the fast. In a small 4-week trial of 20 young women on the same diet, fasted cardio and fed cardio gave similar changes in body composition.