Now a trial is under way to test the effect in humans; a group of men will be given either Viagra or a placebo to take three times a day for a week and the effects on their body-fat measured.

Two-thirds of UK adults and a quarter of children aged between two and ten years old are overweight or obese, according to Public Health England. The dangers are well documented, and include a raised risk of heart disease, stroke and cancer. The extra weight also puts pressure on joints, which can cause pain and lead to osteoarthritis.

Fat builds up in the body when energy intake consistently exceeds energy expenditure: in other words, when we eat more calories than we burn.

But scientists now know that not all fat is created equal. The majority of fatty tissue in an adult’s body is white fat, which is how excess calories are stored. Our bodies also have small amounts of brown fat which, rather than storing calories, burns them to generate heat, in order to keep the body warm.

Brown fat is found in its highest levels in newborn babies, who are less able to regulate their temperature and so need it to help keep them warm.

As we grow into adults, we lose most of our brown fat. But small amounts – teaspoonfuls – remain, particularly around the neck and shoulders, where it is needed to help maintain a constant core body temperature.

While white fat can amount to 20 to 25 per cent of bodyweight, brown fat only makes up between 3 and 7.5 per cent.

Recently, scientists have begun investigating ways to harness the power of brown fat – or even grow more of it.

They have found that exposure to cold temperatures can trigger the conversion of unhealthy, energy-storing white fat into energy-burning, heat-producing brown fat, a process known as browning – and research now suggests that Viagra can cause this browning.

Viagra – the brand name of the generic drug sildenafil – treats erectile dysfunction by relaxing the smooth muscles of the penis and increasing blood flow. But studies have shown that mice given the drug over longer periods of time were less likely to become obese, even when they were fed a high-fat diet.

Working on this understanding, researchers at the University of Bonn in Germany gave mice Viagra for seven days, and looked at the effects on fat cells.

They found that Viagra converted white fat cells into ‘beige’ fat – which, like brown fat, burns the energy from food and converts it to heat. Writing in the Journal of the Federation of American Societies for Experimental Biology, the researchers say this suggests the drug is able to effectively ‘melt’ unhealthy fat away.

Exactly how the drug does this is not yet fully understood.

One theory is that sildenafil effectively boosts levels of a compound, cGMP, that transmits signals between cells and is thought to be involved in fat browning.

It’s thought that the higher levels of this compound create more brown fat and so speeds up metabolism.

The researchers reported that the converted beige fat cells contain a protein that’s also found in brown fat and which is involved in heat production and the burning of fat.

Other research has shown that levels of this protein are low in people with a body mass index of more than 30.

The next step is to see if the drug works as safely and effectively in humans.

In a trial at Wuhan General Hospital in China, 40 healthy men take either a sildenafil or placebo pill three times a day for a week, and the effects on their fat will be measured.

They will also undergo other tests, such as blood pressure checks, and their levels of cGMP will be measured before and for up to three months after taking the pills. The trial should be complete in October this year.

‘These studies represent very complex, sophisticated science, and the trials are at an early stage,’ says Professor David Haslam, chairman of the National Obesity Forum.

‘The mechanisms of brown and white fat are being re-evaluated by researchers, which could ultimately lead to exciting and novel ways of tackling obesity.

‘But for now there is little alternative for most of us to old-fashioned physical activity and improved nutrition.

‘Obese middle-aged men shouldn’t all rush down to their GP surgery for a shopping basket full of Viagra just yet.’