An exception is creatine, a staple of sports nutrition and one of the few supplements with a solid evidence base behind it. One review paper from 2017 concluded that creatine can give athletes a 10-20% performance boost in brief bouts of high-intensity exercise, such as sprinting past a defender or lifting heavy weights. It appears to be safe, too, with no worrying side-effects seen even in people who have been taking the stuff for years. Because there is no test that can distinguish supplementary creatine from the sort naturally produced by the body, or indeed the kind found in meat and fish, most sports do not consider taking it to be doping.

Creatine works mainly by increasing the amount of energy that muscles can produce. Cells use a molecule called adenosine triphosphate (ATP) as a carrier of chemical energy. Aerobic respiration, which uses oxygen to break down fats or sugar, is by far the most efficient way of making ATP. But it is relatively slow. When muscles need a lot of ATP in a hurry most of it is supplied instead by the phosphocreatine system which, as its name suggests, relies on creatine to work. (A third pathway, the glycolytic system, sits between the other two in both power and efficiency.)

When muscles contract, the ATP molecules used to power that contraction lose one of their three phosphate groups, turning into adenosine diphosphate (ADP). Phosphocreatine stored in the muscles can donate a replacement phosphate group, turning ADP back into ATP, which can then power more contractions. But those reserves are sufficient for only a few seconds of maximal effort (this is why it is impossible to run a marathon at the same pace as one would run 100 metres). Creatine supplements boost the amount of phosphocreatine that can be stored, allowing users to squeeze out a couple of extra reps or sprint at full power for a second longer.

That may not be the only benefit. A growing body of research suggests creatine may be good for brains as well as brawn. That makes sense: neurons need ATP just as muscle cells do, and the brain is hungry for energy. Despite accounting for about 2% of the body’s mass, the brain is thought to consume around 20% of its calories.

As summarised in a review published in 2021 in Nutrients, some studies have suggested that creatine might sharpen things like short-term memory or reaction times. Others have reported it may lessen the symptoms of mental-health problems such as depression, and tentative evidence suggests it improves cognition in those with Alzheimer’s disease. Both may be associated with a misallocation of energy within the brain.

In animals creatine seems to protect against the effects of concussions, which likewise seem to play havoc with the way brain cells are supplied with energy. In one study rats given creatine supplements showed a 50% reduction in damage after they were given an artificially induced brain injury. For now, the evidence regarding brains is not nearly as robust as that regarding muscles. But as sport is a common cause of concussions, athletes taking creatine might get two benefits for the price of one. ■