Replacing an unhealthy snack with just half a daily serving of nuts is linked to less weight gain and a lower risk of obesity, a long-term study has found.

Researchers from the US believe the simple intervention could slow down the progressive weight gain that often accompanies ageing.

Nuts are rich in healthy unsaturated fats, vitamins, minerals and fibre, but are not often consumed for weight loss because they are calorie dense.

But there is evidence to suggest that quality of diet as well as counting calories plays a role in weight management.

Researchers from Harvard University and Harvard Medical School analysed information on weight, diet and physical activity in three groups of people: 51,529 male health professionals aged 40 to 75; 121,700 nurses aged 35 to 55 and 116,686 nurses, aged 24 to 44.

Participants were asked every four years to state their weight and how often they had eaten a serving of nuts. They reported their exercise every two years.

Increasing consumption of any type of nut was linked to less long-term weight gain and a lower risk of becoming obese.

Increasing nut consumption by half a serving a day was associated with a lower risk of putting on two or more kilos over any four-year period.

Substituting snacks such as chocolates, pastries, pies and donuts with half a serving of nuts was associated with staving off weight gain of between 0.41 and 0.70kg in any four-year period.

A daily half serving increase of walnuts was linked to a 15 per cent lower risk of obesity.

The findings remained after taking account of changes in diet and lifestyle, such as exercise and alcohol intake.

The researchers say that, as the study is observational, it cannot establish cause.

They suggest that chewing nuts takes more effort than eating fast food, while the high fibre content can make people feel full for longer.

Nut fibre binds well to fat in the gut, meaning more calories are excreted.

The study is published in the online journal BMJ Nutrition, Prevention & Health.

Australian Associated Press