Eating saturated fats from butter, cream and meat, as well as trans fats found in hydrogenated oils can boost our risk of cardiovascular disease, while consuming mono-unsaturated fat can be good for our heart.

Yet what's the effect of all these fats on our weight? Are some better than others?

"Research on animals and some clinical trials show that not all fats have the same effect on weight," says Nadiah Moussavi, a Master's student from the Université de Montréal Department of Nutrition. "Few epidemiological studies exist on the subject and the results of those are contradictory."

The goal of the work is to see whether a connection could be made between the prevalence of obesity and the various forms of fat found in the 168 countries around the world. 

Using statistics from the United Nations' World Health Organization, Moussavi studied the prevalence of obesity in women aged 15 and older. For each year between 1998 and 2002, the total amount of calories from fat was calculated for each person. What she found was that in the countries where mono-unsaturated fats, found in olive oil, formed an integral part of the diet, no or few people were found to be overweight.

Countries where the obesity rate was high and the consumption of mono-unsaturated fat low included Saudi Arabia, Egypt and Peru. Countries such as France, Denmark and Italy showed an ideal portrait – low rates of obesity with a high consumption of mono-unsaturated fats. 

The situation in Canada and the United States appears more complex. In Canada, 22.2 percent of women aged 15 and over were obese. They consumed 147 grams of fat a day, of which 59.2 grams were mono-unsaturated. Worse, 37.8 percent of American women were obese, and ate 152.2 grams of fat daily, of which 45.9 grams were mono-unsaturated.

Other factors, admits Moussavi, must also be taken into consideration, such as family history, amount of exercise, consumption of fruit and vegetables. "Our study shows that the consumption of mono-unsaturated fats and maybe other fats also play a role," she says.

In her thesis, under the direction of professors Olivier Receveur and Victor Gavino, Moussavi showed that not only did trans fats lead to heart disease, they also could be associated with a higher risk of obesity.