Eating a diet consisting largely of fast food could cause your waistline to bulge more than eating the same amount of fat from healthier sources.

Monkeys fed a diet rich in trans-fats--commonly found in fast foods--grew bigger bellies than those fed a diet rich in unsaturated fats, but containing the same overall number of calories. They also developed signs of insulin resistance, which is an early indicator of diabetes.

After six years on the diet, the trans-fat-fed monkeys had gained 7.2% of their body weight, compared to just 1.8% in the unsaturated group. CT scans also revealed that the trans-fat monkeys carried 30% more abdominal fat, which is risk factor for diabetes and heart disease.

"We were shocked. Despite all our enormous efforts to make sure they didn't gain weight, they still did. And most of that weight ended up on their tummies," says Kylie Kavanagh, who presented her findings at the American Diabetes Association meeting in Washington DC, on Monday. "This is walking them straight down the path to diabetes."

Read the full Why fast foods are bad, even in moderation article at New Scientist.