What it is: Though it is not a protein itself, creatine is a compound made from three amino acids, which are building blocks of protein. Its found naturally in meat, poultry, and fish. Creatine is also made in the body, mostly in the liver, and stored, mostly in muscle. Creatine plays a role in energy production by helping replenish a compound called adenosine triphosphate (ATP), which supplies quick energy. Though regimens vary, many athletes today take a single 5-gram daily dose.
Who may benefit: Studies, overall, have found that creatine benefits athletes involved in repeated bouts of high-intensity exercise that last less than 30 seconds (anaerobic), such as sprinting, jumping, and weight-lifting. It provides a burst of energy that allows you to run a little faster, jump a little higher, or lift a heavier weight. Taking creatine during training can thus result in greater increases in muscle mass, muscle strength, and power.
Who won't benefit: Creatine doesn't seem to help in endurance (aerobic) exercise - that is, activities performed at low-to-medium intensity for longer periods of time, such as distance running and cycling. A study in the European Journal of Applied Physiology, for example, found no effect in young endurance cyclists. Similarly, in a study in the British Journal of Sports Medicine, five weeks of creatine supplementation did not enhance performance in tennis players. (Though tennis involves bouts of high-intensity action, it is considered more of an endurance sport.) This is a drawback, since most sports call for both aerobic and anaerobic energy.
Creatine caveats: There are still plenty of unknowns about creatine. Many studies have been small and short (one to several weeks) and have tended to include young, highly trained athletes. There are fewer studies in older people and women. It's unclear whether creatine's benefits last if you take it for more than a couple of months. And if your creatine stores are already high, more is not necessarily better - any extra would be broken down and excreted.
Creatine is thought to be safe - up to 5 grams a day - in healthy people, and past reports of cramps, strains, and dehydration have been questioned. But there are lingering concerns about its effects on the kidneys, especially in older people and people with existing kidney problems. It's unclear whether weight gain, sometimes reported, is from muscle or water. Besides creatine monohydrate, other forms of creatine have not been tested, and supplements often come with other untested ingredients.
Bottom line: There's no reason for most people to take creatine. It may give some competitive athletes a small edge, but this is meaningless for recreational athletes and exercisers; and it wont have much, if any, benefit in endurance sports. People with kidney impairment or other diseases that affect the kidneys should be especially cautious with creatine.