When Jennifer Aniston was spotted with a shopping trolley full of coconut oil, the manufacturers of this little-used fat must have jumped for joy. Coconut oil has had a bad press because of its high saturated-fat content, but devotees claim it is misunderstood. It is heart-healthy and fantastic for weight loss, they say, because it speeds up the metabolism. It is also cholesterol-free and - according to some of the wilder proponents - can cure anything from candida to cancer. The coconut is being touted as the health food of 2008, but a closer look at the science behind the claims highlights the hyperbole that is rife in the "superfoods" industry. Can the answer to all modern ills really be found up a palm tree?

Coconut oil - which is derived from the flesh of the fruit, and not to be confused with coconut milk - is certainly catching on among British athletes. It was included in the diet of the England rugby squad in 2007, who were an average of 2kg of muscle heavier than the 2003 team, and the new squad is already using it. According to rugby union nutritionist Matt Lovell, virgin coconut oil can raise the metabolic rate and therefore help the body to burn fat more effectively. "It is the most misunderstood of all fats," he says. "It is what we call a 'functional food' because it provides many health benefits beyond its nutritional or calorie content."

Garry Stivens, director of Coconoil, the company that supplies the RFU, Premiership rugby clubs such as the Leicester Tigers, and football's Bolton Wanderers, is equally enthusiastic. "As part of an overall healthy lifestyle and diet, it can certainly enhance performance and energy."

At first glance, the so-called "fat that makes you thin" argument looks distinctly suspect. Like butter, coconut oil is extremely high in saturated fat. However, it differs from fats such as butter because it contains a lot of medium-chain triglycerides (MCT). "[These are] absorbed directly by the liver so they burn very much like carbohydrate," says Lovell. "This means they provide instant energy and are suitable for endurance-based events." Other fats, such as butter, contain long-chain fatty acids, which are deposited in fat cells and burn off more slowly.

Even though coconut oil is calorie-dense (about nine calories a gram), coconut enthusiasts maintain that because of the MCTs, the body burns off these calories far more quickly than it would calories from other fat sources. One 2003 study published in Obesity Research seemed to confirm this when it found that coconut oil could help overweight men to burn more calories and lose weight.

"This is interesting research," says Dr Toni Steer, nutrition scientist with the Medical Research Council's Human Nutrition Research Unit in Cambridge. "But the study was of a small group over a short period of time so the results are not highly significant. If coconut oil has any effect, it possibly raises metabolic rate very temporarily and very marginally."

She is less convinced by the calorie claims. "Like any fat, it is calorie-dense. If you thought all you had to do was glug coconut oil and your excess pounds would melt away, you'd need to think again."

"There is a small amount of evidence that MCTs are metabolised and processed differently from LCTs," agrees Bridget Aisbitt, a nutrition scientist at the British Nutrition Foundation. "But ultimately it is the balance of calories in versus calories out that determines weight loss or gain, and one of the simplest ways to reduce calories in is to reduce the amount of fat in the diet. Gram for gram, fat has more than double the calories of carbohydrate or protein."

As it is a saturated fat, you would assume that coconut oil is bad for the heart. However, Lovell points out that "virgin coconut oil, unlike other saturated fats, actually contains no cholesterol, so it does not increase blood serum cholesterol level." Furthermore, he says, "It actually possesses healing properties far beyond that of any other culinary oil and is extensively used in traditional medicine among Asian and Pacific populations."

Others are less enthusiastic. "Of course coconut oil doesn't contain cholesterol," says Professor Peter Emery, head of the department of nutrition and dietetics at King's College London. "The same applies to all plant-derived oils. As always, there is a grain of truth behind the exaggeration. Yes, it does contain a lot of medium-chain triglycerides, which are absorbed straight into the liver [from the small intestine]. And there is some evidence that MCTs do increase the metabolic rate slightly. But the amount you could realistically consume in place of other oils would have a minimal effect on energy balance or weight loss."

When it comes to the health-giving qualiities, the evidence is even sketchier. Many proponents claim that it can fight heart disease because it has antibacterial properties. There is, says the British Heart Foundation, no evidence for this. "We have yet to find conclusive evidence of any one food that in isolation will improve people's heart health," says a spokesperson.

Dr Christine Tomlinson, director of the National Candida Society, believes in the antibacterial properties of coconut oil, and advises candida sufferers (those with a yeast overgrowth) to include it in their diet. Elsewhere, there are claims that it can help with hyperthyroidism, diabetes, pancreatic and thyroid function, vitamin absorption, the immune system and digestive problems.

Solid, peer-reviewed scientific evidence for such theories is scarce. And occasionally the claims are downright irresponsible. "Virgin coconut oil can save the life of cancer patients," raves one website. It can also, the website says, be used to cure HIV and herpes. Thankfully, not all purveyors of coconut oil take this line. "I would never make such cruel claims," says Stivens. "They are nonsense."

"There is absolutely no evidence to support the claim that coconut oil can prevent or cure cancer," says Dr Joanna Owens, senior science information officer at Cancer Research UK. "A small number of studies suggest coconut and other vegetable oils might have anti-cancer properties, but other reports have shown that coconut oil has no anti-cancer effect or that it might even promote cancer. Much more research is needed. "

Ultimately, of course, we should all be ditching saturated fats (such as butter) in our diet in favour of plant-based ones. "The evidence on coconut oil is minimal, certainly when you compare it to the substantial amount of scientific evidence to show that polyunsaturates (vegetable-based oils and spreads) and monounsaturates (such as olive oil or rapeseed oil) help to lower cholesterol levels," says Steer. "Used sparingly, as you would a normal cooking oil, coconut oil will certainly do you no harm."

There is, then, absolutely nothing wrong with coconut oil. Indeed, there may be some good reasons to give it a go. Stivens, a former field director for Save the Children, set up Coconoil as a relief project when he visited Sri Lanka in the immediate aftermath of the tsunami in December 2004, and says it provides local farmers with a crucial income. His virgin coconut oil, at least, is organic, tastes surprisingly good in a stir-fry, and can be used as a body lotion or hair conditioner - or even to grease your bike chain.

· Information and stockists: coconoil.co.uk