A review of 33 trials involving 73,589 men, women and children in America, Europe and New Zealand found that choosing low fat foods helped people lose around 3.5 pounds, slim their waist-lines and cut bad cholesterol - all without dieting.

Researchers who led the study said its results prove for the first time that people can lose weight without trying to.

"The weight reduction..when people ate less fat was remarkably consistent - we saw it in almost every trial. Those who cut down more on fat lost more weight," said Lee Hooper from the University of East Anglia medical school, who led the work.

"The effect isn't dramatic, like going on a diet," she said, adding that the research specifically looked at people who were cutting down on fat, but didn't aim to lose weight - so were continuing to consume a normal amount of food.

"What surprised us was that they did lose weight, their BMI (body mass index) decreased and their waists became slimmer," Hooper said. The lower fat eaters also kept their weight down over at least seven years.

The review - commissioned by the WHO's Nutrition Guidance Expert Advisory Group (NUGAG) after a request to update their guidelines on fat intake - will now form a crucial part of global recommendations, the researchers said.

Being overweight or obese increases the risk of many illnesses such as cancer, heart disease and stroke. Together, strokes, heart attacks and other cardiovascular diseases are the biggest killers worldwide and claim more than 17 million lives a year, according to the WHO.

More than half of Europeans are obese or overweight, and in America more than 35 percent of adults and almost 17 percent of children qualify as obese.

People are defined as overweight if their body mass index or BMI - a ratio of weight to height - is more than 25 kg per metre squared (kg/m2) and obese if it is more than 30 kg/m2.

Among the 73,500 people taking part in the studies analysed by Hooper's team, there were varying ages and states of health. The researchers compared those eating less fat than usual and those eating their usual amount of fat, and measured the effect on weight and waistline after at least six months.

The results, published in the British Medical Journal, showed that eating less fat reduces body weight by 1.6 kg, cuts BMI by 0.56 kg/m² and reduces waist circumference by 0.5 cm.

Hooper's team found that reductions in total fat intake were also linked with small but statistically significant reductions in cholesterol and blood pressure, suggesting a lower fat diet could have a beneficial effect on this heart risk factors.

Carolyn Summerbell of Durham University, who co-led the research, said the trick to slimming down and staying that way was to find a way to eat what you can stick to for life.

"Cutting down on fat will help," she said, adding that this meant opting for low-fat yoghurts, skimmed milk and reducing intake of butter, cheese and fatty snacks like crisps and cakes.

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