If you’re not convinced that eating the way we were designed to eat is the only way to stay healthy, a five-year-old study by Wake Forest School of Medicine might change your mind.

In the study, Kylie Kavanagh, D.V.M. said they gave two groups of monkeys the same amount of calories and fat, but one group got monounsaturated fat (healthy plant fats (cashews contain about 68 percent monounsaturated fat) while the other got the human altered trans fats (partially hydrogenated oil). The results were stunning.

Kavanagh said that after over six years, male monkeys fed a western-style diet that contains trans fat had a 7.2 percent increase in body weight, compared to a 1.8 percent increase in monkeys that ate monounsaturated fats, such as olive oil. Remember that these two groups of monkeys were fed the same amount of calories and fat, but the trans fat group gained weight and the others didn’t.

Monounsaturated fats have been part of humans’ diets since the dawn of man, but trans fats (a solid fat created by hydrogenating liquid fats in order to elongate shelf life) are a modern invention.

The study didn’t explain how the animals responded differently to the different fats, but it’s clear, since trans fats are not from nature, our bodies don’t know how to deal with them. They must alter physiology in such a way as to reduce metabolism and thus encourage stored fat.

Regardless of the biology, studies like these make it clear the the natural diet is the only way to go.