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Yet chocolate's potential role in exercise performance has only just gone under the microscope.

Scientists at the University of California in San Diego gave middle-aged, sedentary male mice a purified form of cacao's primary nutritional ingredient, known as epicatechin, and had the mice work out.

Epicatechin is a flavonol, a class of molecules which are thought to have widespread positive effects on the body.

The mice were given small liquid doses of epicatechin twice a day. Meanwhile, a separate control group of mice drank equal amounts of water.

The groups were divided into two. Half of the animals in each group began a light exercise routine, which consisted of strolling on a treadmill for a short period each day.

The regime wasn't meant to get the animals into Olympic shape, only to get them moving. After 15 days, all of the animals completed a treadmill test, during which they ran to exhaustion.

The researchers also biopsied the animals' back leg muscles to see if consuming the chocolate component was having any effect on muscle.

Overall, the mice that had been drinking water were the first to give out during the treadmill test.

They became exhausted quicker than the animals that had received epicatechin. Even the mice on water that had lightly exercised grew tired more quickly than the non-exercising mice that had been given epicatechin.

For those who adore dark chocolate, however, there is a catch, as this isn't an invitation to indulge.

"A very small amount is probably enough," said Dr Francisco Villarreal, one of the authors of the study which appeared in The Journal of Physiology.

It remains to be seen whether the fitness-boosting effects of epicatechin will be identical in humans. For starters, highly processed chocolate should be avoided as it destroys epicatechin.

Dr Villarreal suggests eating five grams of dark chocolate daily, or just a sixth of an ounce -- about half of one square of a chocolate bar -- if your aim is to intensify the effects of a workout.

Meanwhile, the muscle biopsies of the mice offered some explanation as to why those getting the epicatechin had done better.

The muscles of all of the mice that had been given epicatechin contained brand new capillaries, as well as biochemical markers indicating that their cells were making new mitochondria. (Mitochondria are structures in cells that produce energy.)

The more functioning mitochondria a muscle has, the more fatigue-resistant it is.

Even the mice that had taken epicatechin and not exercised contained markers of increased mitochondrial health -- suggesting that it prompts a physiological reaction even among the sedentary. While the response in muscle strength is greatly heightened by exercise, no matter how slight.

Good news for chocolate lovers, but you probably won't be able to ascribe your daily fix to medical grounds just yet.