Scientists have pinpointed an unlikely potential weapon in the war against obesity - seaweed.
Brown seaweed is an ingredient of miso soup
They found rats given fucoxanthin - a pigment in brown kelp - lost up to 10% of their body weight, mainly from around the gut.
They hope fucoxanthin can be developed into a slimming supplement or a drug that targets harmful fat.
The Hokkaido University research was presented to an American Chemical Society meeting in San Francisco.
Brown kelp, Undaria pinnatifida, is a key ingredient of Japanese miso soup.
But the researchers said drinking large quantities of the soup in an effort to lose weight would have little effect.
Fucoxanthin is tightly bound to proteins in the seaweed and not easily absorbed in its natural form.
The researchers, led by Dr Kazuo Miyashita, said it might take another three to five years before a slimming pill based on fucoxanthin was available to the public.
The compound is found at high levels in several different types of brown seaweed. But it is absent from green and red seaweeds, which are also used in Asian cooking.
Dr Miyashita's team studied the effects of fucoxanthin on more than 200 rats and mice.
They found it fought flab on two fronts.
In obese animals, the compound appeared to stimulate a protein called UCP1 which causes fat to be broken down.
The protein is found in a type of fat called white adipose tissue, which is responsible for the thickening of the girth dubbed "middle-age spread".
Research has shown that excess amounts of fat around the midriff are particularly linked to heart disease and diabetes.
The pigment also caused the liver to produce a compound called docosahexaenoic acid (DHA), which can help cut levels of "bad" cholesterol associated with obesity and heart disease.
No adverse side effects were seen in the animals used in the study.
Dr Ian Campbell, medical director of the charity Weight Concern, said a 5% to 10% weight loss was comparable with existing anti-obesity drugs.
But he added: "We don't know in how many humans this would be achieved, nor for how long.
"The possibility of it being side-effect free is attractive but it is a long way away from being anything resembling an anti-obesity pill.
"The problem remains that medication, however good, will only ever plaster over the cracks.
"The hard fact is that only a significant change in lifestyle will ever achieve long-term meaningful weight loss and it is here that we should be focusing our efforts."