Lots of people, even including those wanting to build muscle, are doing intermittent fasting (IF) these days. The goal for these people is to lose body fat and, hopefully, retain muscle or build muscle at the same time.
Others practice IF as an anti-aging method. This has solid science behind it, and it may even be more beneficial for anti-aging purposes than calorie restriction, heretofore the most effective strategy for inhibiting or reversing aging.
Many of those in the first camp (lose body fat, retain muscle) use branched-chain amino acids (BCAAs), or leucine (one of the BCAAs) or perhaps whey (which is rich in BCAAs) during their fast. BCAAs are powerful signals that tell cells to ramp up muscle protein anabolism and to decrease muscle protein catabolism. So the use of BCAAs while fasting is the practice that will give one the best shot at retaining or even building muscle, even while doing regular, periodic IF.
However, one thing that BCAAs, whey, or leucine do is to stop autophagy, the cellular self-cleaning process. This process is radically enhanced by fasting, and it is responsible for the beneficial effects of calorie restriction and fasting. No autophagy, no anti-aging benefit from fasting. One of the ways (not the only one) that BCAAs increase muscle protein synthesis is through increased insulin levels. Insulin suppresses autophagy.
So, if you practice IF for the purpose of anti-aging, you must not take any BCAAs, nor, if you are in the habit of taking small amounts of food during a fast, must you ingest any protein or carbohydrates, which raise insulin and stop autophagy, which is the central component of the anti-aging effect of IF.
However, fat, or in any case small amounts of it, do not raise insulin and do not signal the metabolic machinery to stop autophagy. Therefore you won’t sabotage an anti-aging fast with small amounts of fat. The best thing to do here, if needed, is to drink coffee or tea with cream; the fat in the cream will help satisfy the appetite, and the caffeine in coffee or tea will also decrease hunger.
That’s why you should be clear on what your intermittent fasting purpose is. If it is for fat loss, by all means use BCAAs to decrease muscle breakdown. If your purpose is anti-aging, you should avoid BCAAs.