German scientists have discovered why male mice fed a high calorie diet with a daily dose of Viagra don't get fat. Now the question is whether the sexual potency drug could have the same effect on humans. Some 30 million men around the world have taken sildenafil (known by its trade name Viagra) as a treatment for erectile dysfunction and pulmonary arterial hypertension, according to Pfizer - the American pharmaceutical company that has produced the diamond-shaped baby blue pills since 1998.

Now scientists, led by a team at the University Clinic in Bonn, found that laboratory mice on a high-fat diet, which were fed Viagra, were able to metabolize the excess fat so fast that they maintained their weight. And the mice didn't put on weight because of increased sexual activity.

"We found that the white fat of the mice which get sildenafil, is becoming more brownish. Brown fat is the type of fat that burns fat," Alexander Pfeifer, director of Bonn University's Institute for Pharmacology and Toxicology, told DW.

Pfeifer's team looked at how sildenafil increases the production of a messenger molecule called cyclic GMP. This is the molecule that improves the sex life of men by regulating the flow of blood to the penis.



Viagra stimulates conversion of undesirable fat

The Bonn researchers discovered that cyclic GMP also stimulates and speeds up the conversion of undesirable white fat into healthy brown fat, which can burn it. This is what kept the mice on Viagra slim.

"You have a group which is fed only with a high fat diet, then the group fed a high fat diet with sildenafil. This group that has just a high fat diet is gaining a lot of weight and the group which is getting sildenafil is not gaining weight, so there's a 15 percent difference in body weight, which is a lot if you think in human terms," Ana Kilic, who is part of the research team, told DW.

"If you have a human being who is 70 kilograms and the other one who's the same height weighs 10 to 15 percent less, which is around 60 kilograms - that is a big difference," she added.

White fat is the type of fat that's responsible for the flabby spare tire look around the waistline in both mice and men. "We all have this fat around our waist, abdominal fat which is actually not a good fat," said Kilic.



White fat isn't necessarily bad

Like the mice that remained fit, slim people have more brown fat than heavier ones. But that doesn't make white fat bad - its task is to store energy. The problem is when there's too much white fat.

"When a certain level of lipid or fat concentration is reached, it is then stressful for the cell and it cries for help," said Pfeifer, adding that when fat cells become overloaded, they spread inflammation throughout the body. "People can get Type 2 diabetes and are prone to myocardial infarction and atherosclerosis. So this is the big drawback of obesity," he noted.

Sildenafil helps mice remain slender by inducing the browning of white fat cells. The question is whether the drug would have the same effect on humans.

Lab mice got viagra daily, men take it when they need it

One of the differences between the mice in the experiment and the men on viagra is that the latter don't take it everyday, said Guido Fechner, a urologist who is familiar with the study and treats men suffering from erectile dysfunction.

"There are almost no patients who take Viagra daily. Viagra is usually taken on demand, 99% of men use it on demand only," he told DW.



The study by Pfeifer and his colleagues looked at mice loaded up with sildenafil for seven consecutive days on a dosage three times higher than the amount that men take for erectile problems.

"People should not calculate 'How much Viagra do I need to get slimmer?' from these animal or cell studies. One would have to make people aware that a high dosage of sildenafil over a long period of time could induce hypertension or other serious side effects," Alexander Pfeifer warned.

Whereas sildenafil helps mice burn fat, further research is needed to determine whether Viagra produces the same effect in humans. In the meantime, urologist Fechner is advising his patients to shed the extra weight by eating healthier foods, consuming less alcohol and exercising more.

"The study is promising as a means of understanding the mechanisms of obesity. It's not very promising for therapeutic possibilities. I don't see sildenafil as an anti-obesity drug," he said.