Findings published today in the British Medical Journal show that exchanging fatty foods for lower fat alternatives will help people shift around three-and-a-half pounds - without dieting. People taking part in trials also saw their waist-lines 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 report 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 waist line was measured after at least six months.
The results show that eating less fat reduces body weight by 1.6kg, BMI by 0.56kg/m² 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 research was led by Dr Lee Hooper from UEA's Norwich Medical School. She said: "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.
"The effect isn't dramatic, like going on a diet. The research specifically looked at people who were cutting down on fat, but didn't aim to lose weight - so they were continuing to consume a normal amount of food. What surprised us was that they did lose weight, their BMI decreased and their waists became slimmer. On top of this, they kept their weight down over at least seven years. There isn't a specific goal, the more fat you cut down, the more your weight falls.
"We didn't consider different types of fat in this study," said Dr Hooper. "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.
"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.
"Keeping healthy is not just about fat and weight - but cutting down on fat, especially saturated fat, is a great start. Being physically active, not smoking, drinking alcohol in moderation, eating plenty of fruit and vegetables, and drinking plenty of fluid also help to keep us healthy. We just need to get in the habit of doing these things," 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."
The small amount of data available for children in the same analysis confirmed a relationship between total fat intake and subsequent weight change. Further trials are needed to examine the effect of reducing fat intake on body weight in developing countries as well as in children.
'Effect of reduction total fat intake on body weight: systematic review and meta-analysis of randomised controlled trials and cohort studies' is published in British Medical Journal (BMJ).