Creatine is a naturally occurring substance which we ingest by eating meat, fish and other animal products. It is an amino acid, a compound which is one of the basic building blocks of the human cellular system.

A typical diet includes 1 to 2gms of creatine daily, although vegetarians consume much less. Ninety-five per cent of the body's total creatine content is in muscle. Much of it exists in the form of phospho creatine.

During brief periods of intense exercise the demand for energy increases rapidly and phospho creatine plays a central role in providing this extra energy boost.

The practice of taking creatine supplements began with professional athletes, notably American football players. Research has shown that creatine supplements can increase the total muscle creatine content by up to 20 per cent. This extends the duration of high-intensity exercise, with a possible performance gain for the sportsman.

There have been dozens of medical trials which have examined the effects of creatine supplements on exercise performance. The results are inconsistent. Some trials have shown no benefit; this has particularly been the case when actual athletic performance, outside of laboratory conditions, is tested.

The side-effects of creatine supplements are well documented. Consumers report cramping of muscles, dehydration, diarrhoea and dizziness. Weight gain occurs as a result of water retention. This reflects the central role of the kidneys in breaking down and getting rid of creatine from the body.

Medical experts warn that not everybody can process excess amounts of creatine and kidney function could be damaged as a result of taking supplements.

A case report in the British medical journal, the Lancet, in 1998 described kidney problems in a 25-year-old taking a drug called cyclosporine as well as creatine supplements. His kidney function returned to normal when he stopped taking creatine.

The issue of drug interactions with creatine has not been widely studied, but represents a potentially serious health risk for athletes who self-medicate with creatine. Another area of concern is the fact that dietary supplements such as creatine are not manufactured to the same standard as drugs and there is a definite risk of contamination with impurities.

Sports people who are taking creatine should seek immediate medical attention if they experience flank pain, blood in the urine, nausea or vomiting.