In a study so surprising that the research team repeated their experiment, a simple potato extract put a cap on weight gain even when the laboratory mice were allowed a diet high in fat and refined carbohydrates.

Mice were fed what was reportedly an obesity-inducing diet for 10 weeks, soon gaining more than half their body weight.

They had weighed an average of 25 grams at baseline and packed on an average of 16 grams, but a group that consumed the same diet with the addition of a potato extract gained only seven extra grams.

"We were astonished by the results," says co-author Dr. Luis Agellon of McGill University. "We thought this can't be right -- in fact, we ran the experiment again using a different batch of extract prepared from potatoes grown in another season, just to be certain."

The extract, say the researchers, contains a high concentration of polyphenols, well known to be beneficial.

Human trials are the next step for the project, for although the mouse metabolism is representative there are factors to be considered on the human side that include pinpointing the optimum dose for different metabolisms.

"The daily dose of extract comes from 30 potatoes, but of course we don't advise anyone to eat 30 potatoes a day," says Kubow, "as that would be an enormous number of calories."

The extract could become available in the form of a dietary supplement or cooking ingredient, according to the researchers, who are currently seeking partners to fund clinical trials.

The study was published in the journal Molecular Nutrition & Food Research.