Cutting down on fat will help people lose weight, without dieting, according to new research from the University of East Anglia.
People taking part in trials also saw their waistlines become slimmer, and levels of bad cholesterol decrease.
The results prove for the first time that weight loss can happen without trying to lose weight - simply by choosing foods lower in fat.
The research was commissioned by the World Health Organisation (WHO) Nutrition Guidance Expert Advisory Group (NUGAG) Subgroup on Diet and Health following a request to update their guidelines on total fat intake. The results will be crucial in making global recommendations.
The research is particularly important because being overweight or obese increases the risk of many cancers, coronary heart disease and stroke. Reductions in total fat were also associated with small but statistically significant reductions in cholesterols and blood pressure, suggesting a beneficial effect on other major cardiovascular risk factors.
The systematic review included results from 33 randomised controlled trials, in North America, Europe and New Zealand, involving 73,589 men, women and children.
Those taking part had varying states of health. Comparisons were made between those eating less fat than usual (intervention group) and those eating their usual amount of fat (control group). The effect on weight and waistline was measured after at least six months.
The results show that eating less fat reduces body weight by 1.6kg, BMI by 0.56kg/m and waist circumference by 0.5cm. All these effects were in trials in which weight loss was not the intended outcome, suggesting that they occur in people with normal diets. The weight loss happened quickly and was maintained over at least seven years.
"The weight reduction that we found 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 Dr Lee Hooper from UEA's Norwich Medical School, who led the research.
"We didn't consider different types of fat in this study. But cutting down on saturated fat reduces our risk of heart disease and strokes, so the healthiest way to cut down on fat is to cut down on saturated fats," said Dr Hooper.
"This means having low fat milk and yogurt, cutting down on butter and cheese, and cutting the fat off meat. Most importantly have fruit instead of fatty snacks like biscuits, cake and crisps. And remember, this isn't a diet, so don't take it to extremes, but work out a way of eating that you can stick to permanently," she added.
Co-author Prof Carolyn Summerbell, from Durham University, said: "A healthy diet is a way of eating that people can sustain over time. That's the trick, to find a comfortable way to eat that you can stick to for life which helps you maintain your weight. Cutting down on fat will help.
"Doing exercise and being physically active is good for maintaining weight and also has other health benefits, but it's not a replacement for a healthy diet," she added.
The findings published today in the British Medical Journal. (ANI)