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.
Trans-fats, or partially hydrogenated oils, are found in many fast foods and also in baked goods and processed snacks. They dramatically increase the risk of heart disease - even more than saturated fats found in animal products.
Kylie Kavanagh, at Wake Forest University Baptist Medical Center in Winston-Salem, North Carolina, US, wondered how this "killer fat" would affect the risk of diabetes in 51 vervet monkeys.
She fed one group of monkeys a diet where 8% of their daily calories came from trans-fats and another 27% came from other fats. This is comparable to people who eat a lot of fried food, says Kavanagh. A different group of monkeys was fed the same diet, but the trans-fats were substituted for mono-unsaturated fats, found in olive oil, for example.
Both groups ate the same total calories, which were carefully metered to be just enough for subsistence.