Before a marathon or a triathlon, or a long-distance swim, elite athletes usually "carbo-load" with huge plates of pasta, bread, fruit, a sugary drink and perhaps, at the starting line, a chocolate bar. By the time the race begins, their muscles are charged with fast-burning carbohydrates that will enhance their performance and endurance.

Meanwhile, your typical couch potato embarks on a training program after his doctor informs him that he is overweight, unfit and showing early signs of non-insulin dependent diabetes. After just two weeks, his sugar metabolism is greatly improved, and his cells have regained their responsiveness to insulin.

According to Professor Hargreaves of Deakin University's School of Health Sciences, the underlying mechanism in both resposes involves a protein called glut-4, found at high levels in our muscles, which regulates the way our muscles burn energy. Glut-4 appears to be a major player in carbohydrate metabolism.

Carbohydrates are the body's fast foods - a source of quick-burning fuel, particularly for hard-working tissues like muscle, brain and heart, where the body's metabolic fires burn hottest. When carbohydrates run short the body turns to its fat reserves or, as a last resort, precious protein.

Professor Hargreaves says that, through its primary role in muscle tissue, glut-4 indirectly wields a strong influence on whole-body metabolism - exercise your muscles, and your entire body benefits.

"My research interest is how muscles obtain energy during exercise, particularly how they obtain energy from carbohydrate, because high-intensity athletic performance depends on the availability of carbohydrates," he said.

He says exercise enhances the capacity of muscle tissue to generate energy by a process called oxidative metabolism. Exercise also modulates the body's alternative metabolic pathways - exercise regularly, and your body will soon switch from storing surplus energy as fat, to burning it as glucose.

After carbo-loading before the big event, marathon runners and triathletes will also recharge their carbohydrate reserves during the race with a cocktail of glucose and other sugars, mixed replacement electrolytes to replace the salts lost in sweat.

Professor Hargreaves is investigating how the pasta gets turned into glucose, the primary fuel for muscle cells, and how the glucose is delivered to muscle cells.

Glut-4 belongs to a large family of molecules called transporter proteins, and appears to be a vital cog in the system that delivers glucose to the muscles. Glut-4 molecules concentrate in the intracellular spaces within muscle tissues, collecting glucose and ferrying it to the muscle cell membrane. The glut-4 molecules then "dock" with special receptors on the cell surface and release their energy-rich cargo, which is conveyed into the cell's interior.

Professor Hargreaves says a more detailed understanding of how physical exercise switches on the glut-4 glucose-delivery system could be helping athletes maximise their fast-burn carbohydrate reserves. And at the other end of the spectrum, it could throw light on why couch potatoes develop inefficient energy metabolism and begin to store energy as fat.

"We still don't know exactly how exercise induces the movement of glucose transport proteins to the muscle cell membrane, or how different nutritional and exercise regimes influence this process," Professor Hargreave said.

"We'd like to develop a better understanding of the process. If it proved to be a separate mechanism to the energy-release process mediated by insulin, we might be able to treat obesity by bypassing the insulin signalling pathway."

The hormone insulin, by converting excess glucose into glycogen and storing it in the liver, regulates the balance between readily available and stored energy.

In overweight or obese individuals, a malfunction in the insulin-signalling pathway renders cells insensitive to insulin; excess glucose, instead of being converted into glycogen, is converted into fat and placed in long-term storage, worsening the individual's weight problem. Untreated, insulin resistance can result in non-insulin dependent diabetes (IDDM).

Exercise has long been recognised as a useful adjunct to managing IDDM. Professor Hargraeve wonders if exercise, by inducing the glut-4 glucose transport system, bypasses the insulin pathway.

For the endurance athlete, whose performance is determined by how fast he or she can pump glucose into muscle cells, the key question is: if glut-4 is part of a pathway that bypasses the insulin-mediated energy supply line, is there some choke-point in the pathway that limits the athlete's rate of glucose uptake. If so, is there a way around it?

Endurance athletes have higher levels of glut-4 protein in their muscles. Glut-4 has also been found to enhance the action of insulin, which might allow endurance athletes to store more glycogen in their muscle cells, then release it progressively as glucose during sustained exercise.

"However, it's unlikely that one would try to identify elite athletes on the basis of their glut-4 levels. Other factors, such as the proportion of slow-twitch muscle fibres, high oxidative capacity and a well-developed heart lung system are probably better indicators of elite performance," he said.

Although Professor Hargreaves and his colleagues have not explored the limits of the system's adaptability in different individuals, they have made some intriguing findings about how rapidly a person's fitness level changes in response to exercise.

"In previously untrained, sedentary individuals who exercise at 75 per cent of their aerobic capacity, we see a 30 to 50 per cent their oxidative capacity after only 10 days, and a significant rise in glut-4 levels in their muscles.

"In contrast, in elite athletes who stop training, we see a 40 to 50 per cent decrease in oxidative capacity and a sharp decline in glut-4 levels after only 10 days.

"So there are significant gains to be obtained fairly quickly when unfit people undertake regular exercise. People who are already fit can also get an extra benefit quite rapidly by increasing their exercise, but the downside is that the improvement is rapidly lost when people stop exercising."

Athletes who stop training rapidly lose their adaptation, but still remain at a higher level than untrained individuals - years of sustained training, plus genetic factors, probably account for the difference.

With a joint study with researchers at the University of Copenhagen, Professor Hargreaves and his colleagues are studying short-term interactions between diet and training, to determine how diet influences glut-4 and glycogen levels in muscles.

"We want to know what effect exercise intensity has on adaptation. For example, is it better to exercise for a long time at low intensity, or for a short time at high intensity, or to use some combination of both?