Topping up levels of a hormone found naturally in the stomach could be a new way to treat obesity.
Boosting oxyntomodulin limits appetite and raises activity levels at the same time - leading to speedy but healthy weight loss rates, a UK study suggests.
The hormone tells us we are full after a meal, but the obese have less of it.
The fact dieting tends to lead to reductions in activity often makes weight loss harder, the International Journal of Obesity study says.
Professor Steve Bloom head of the Division of Investigative Science at Imperial College London said earlier studies had shown oxyntomodulin decreased appetite.
But this was the first time it had been shown to increase physical activity levels.
The fact that oxyntomodulin was naturally found in the body was also an advantage as it was unlikely to have unpleasant side-effects, he said.
"It's not like one of those nasty drugs where you have to take some horrible chemical for years.
"It is naturally a occurring hormone. We are using the body's own method of limiting appetite."
There are some conditions where people have high levels of the hormone after certain types of injuries to the gut, for example, and they tend to loose a lot of weight and stay very thin.
And there do not appear to be any harmful effects of having high levels of oxyntomodulin, he said pointing to some medical conditions that produce this.
The only side-effect appears to be sustained weight loss, he said
Fatter people tend to have lower levels of the hormone and therefore cannot recognise when they are full.
So treatment would restore obese and overweight people's levels of the hormone to their correct levels.
He envisages people giving themselves daily injections like diabetes patients do with insulin.
"If people strip half naked in the street, exposing themselves to the public to undertake this exercise called jogging then I can't see that they would have a problem with taking this hormone," he said.
The study looked at 15 healthy overweight male and female volunteers, aged between 23 and 49.
They were divided into two groups, with one being given doses of oxyntomodulin three times a day for four days and others saline.
On the fourth day, volunteers returned to have their energy expenditure and food intake measured.
After the first meal volunteers ate on average 128 kcal or 17.4% less, while energy expenditure increased by 143 kcals or more than a quarter.
The team also found body weight was reduced by 0.5% over the four days.
This would translate to a weight loss of around a pound or half a kilo a week, Professor Bloom said.
"The discovery could provide doctors with a whole new way to treat the obesity epidemic. We need to get away from the focus on food and start to think about how to increase exercise," he added.
Dr Colin Waine, Chairman of the National Obesity Forum, said the research was very exciting and opened up a potentially exciting new way of tackling a major health issue.
He said it does not negate the need for people to eat a balanced diet and exercise regularly, around 30 minutes daily, he said.
"But it really increases our physiological understanding of the problem and opens up a new therapeutic approach."