Scientists have identified two substances which tell the body to stop eating - and say they could be used to create drugs treat obesity.
The hormone regulates appetite
UK researchers say "topping-up" levels of a hormone produced by the gut could reduce the appetite of overweight people.
A second study found a fatty acid is also involved in regulating hunger.
But experts said the best way to lose weight was still to eat sensibly and exercise regularly.
The UK study found the amount of a particular hormone in obese people's bodies was around a third lower than in lean people.
Boosting levels of the PYY3-36 hormone was found to reduce appetite and calorie consumption in both lean and overweight people.
Researchers, writing in the New England Journal of Medicine, say their work offers more evidence the hormone could form the basis of a treatment for obesity.
The PYY3-36 hormone is produced by endocrine cells in the gastro-intestine.
It is released after a meal to and sends signals to the hypothalamus in the brain to say that enough food has been eaten.
Researchers from Imperial College London and Hammersmith Hospital studied hormone levels in 12 obese and 12 lean patients.
After fasting overnight, all were given an intravenous drip either containing the hormone or an ineffective saline solution for 90 minutes.
Neither the doctors nor the patients knew who had been given which solution.
Two hours later, the patients were given a buffet meal and allowed to eat as much as they wanted.
The process was repeated on a second occasion, with participants being given the other solution.
At the end of the study, funded by the Wellcome Trust and the Medical Research Council, it was found all 24 ate less when they were given the hormone solution than when they were given the saline infusion.
Overall, calorific intake was reduced by around a third.
Professor Steve Bloom, one of the study authors, said: "The discovery that obese people have lower levels of PYY3-36, an important factor limiting appetite, suggests a possible new treatment for the millions suffering from obesity.
"Our previous research has shown that it is possible to reduce calorific consumption in lean volunteers by giving PYY3-36.
"These new findings suggest boosting PYY3-36 offers a novel approach towards treating the epidemic of obesity in our society."
Dr Rachel Batterham, a Wellcome Trust researcher who also worked on the paper, said: "If this passes clinical trials, it will not be a wonder treatment.
"People will not be able to continue eating high fat foods and drinking beer, hoping that this hormone would help them to lose weight.
"PYY3-36 would only be useful if used in addition to lifestyle changes."
In the US research published in Nature, scientists found by increasing levels of a natural fat compound called OEA (oleylethanolamide), which activates receptors in the body to regulate hunger and metabolism, in rats reduced appetite and weight rats.
Dr Daniele Piomelli, of the University of California, Irvine, who led the study, said: "This gives us hope for a new class of anti-obesity drugs based on natural chemicals."
Blood fat levels were also reduced.
Dr Ian Campbell, chairman of the National Obesity Forum, gave a cautious welcome to the research findings.
He told BBC News Online: "We have been here before, with other hormones such as leptin.
"Initial research promises a lot, but that's not borne out by further studies.
"If this leads to better drugs for overweight people, that's a good thing."
But he said people could take action themselves, by eating a healthy diet and exercising regularly.
"People have to modify their lifestyle. That's got to be the most important thing to do."