Creatine

What it isA nitrogenous acid made from glycine, arginine and methionine that, as phosphocreatine, buffers ATP regeneration in muscle and brain.

Why this oneIt is among the best-supported sports supplements for short-burst power output, and it is under study in neurologic and psychiatric conditions where cerebral energetics are implicated.

What limits itIt raises serum creatinine — which looks like renal impairment on a basic metabolic panel and is not — and causes weight gain from intracellular water; benefit outside high-intensity exercise is unsettled.