Should I take BCAAs before exercise if I work out while fasted?

Supplements are probably not absolutely necessary, although there is a lack of direct research to look to when guiding this decision. One study^[1] found that supplementation with BCAA enhanced exercise capacity and lipid oxidation during endurance exercise after muscle glycogen depletion. However, an extremely important point to remember is that the only way to substantially deplete muscle glycogen is through exercise. That means whatever muscle glycogen you have when you go to sleep at night is mostly all still there in the morning when you’re at the gym, so your muscles may have a full tank of gas for the workout. Perhaps surprising to some, one study^[2] showed that resting muscle glycogen levels were at the same level after an overnight fast and an 84-hour fast! It has also been shown that BCAA intake affects protein metabolism in muscle after but not during exercise.