WRITTEN BY Sara G. Miller from Live Science
It seems logical to think that eating a high-fat diet would tip the scale upward, but a new study suggests that might not be the case. What’s more, eating more of certain types of fats may help move the scale in the other direction.
Men and women in the study who followed a high-fat, Mediterranean diet that was rich in either olive oil or nuts lost more weight and reduced their waist circumference more than the people in the study who were simply instructed to reduce their fat intake, according to the study.
The Mediterranean diet, rich in healthy fats and plant proteins, has been linked in previous studies to a wide range of health benefits, including a reduced risk of heart disease and type 2 diabetes — two conditions that are also linked to obesity. [5 Diets That Fight Diseases]
But despite such benefits, “obese people [have] continued to be reluctant to eat vegetable fats such as extra-virgin olive oil and nuts, because they believe these foods lead to weight gain,” said Dr. Ramon Estruch, an internal medicine physician at the University of Barcelona in Spain and the lead author of the study. 
The findings of the new study show, on the other hand, that a diet rich in dietary fats and vegetables, such as the Mediterranean diet, does not promote weight gain, Estruch said.
In the study, the researchers looked at data on people who had participated in the PREDIMED trial, a five-year study in Spain that looked at the effects of the Mediterranean diet on heart health. There were nearly 7,500 older adults in the study, the majority of whom were overweight or obese and all of whom had either type 2 diabetes or at least three risk factors for heart disease.
The people in the study were asked to follow one of three diets: a Mediterranean diet with at least 4 tablespoons of extra-virgin olive oil each day, a Mediterranean diet with at least three servings of nuts each week or a control diet, where the participants were advised to generally avoid fat in their diet.
Both olive oil and nuts contain relatively high amounts of fat, but the fat in them is primarily monounsaturated fat, which is thought to be better for health than the saturated fat found in animal-based foods such as meat and cheese.
The study received funding from both olive oil and nut industry groups. However, these funders had no role in designing the study, in collecting, analyzing and interpreting the data or in writing the report, the researchers wrote in the study, published today (June 6) in the journal The Lancet Diabetes & Endocrinology.
The researchers found that after five years, the people in the olive oil group had lost a small but statistically significant amount of weight, compared to the control group: The people in the olive oil group lost about 1 lb. (0.4 kilograms) more, on average, than those in the control group.
The people in the nut group also lost a small amount of weight as well, compared to the control group. However, the difference between the olive oil group and the nut group was not statistically significant (meaning it could have been due to chance).
In addition, both the olive oil and nut groups experienced slight reductions in their waist circumferences compared to the control group, according to the study.
The key finding is that neither diet, although rich in fats, led to weight gain or increases in waist circumference, Estruch told Live Science.
The researchers noted that although the participants in the olive oil and nut groups were not instructed to limit their calorie intake, the people in both groups did end up consuming fewer calories on average than they had consumed before the study started. This may have been due to the filling effects of fat, the researchers wrote in their study.
Maintaining a certain body weight requires balancing the calories you consume versus the calories you burn, but it seems that calories from vegetable fats have different effects on weight than calories from animal fats, Estruch said. [10 New Ways to Eat Well]
Though the participants in the study were overweight or obese older adults, Estruch said that he believes that the benefits of the Mediterranean diet on weight and waist circumference could extend to people of any age and weight, including young men and women.
This is not the first study to suggest that eating more plant-based fats does not lead to a larger waistline.
The results of this study are consistent with a range of observational studies suggesting that eating more fat is not linked to a change in people’s weights, said Dr. Dariush Mozaffarian, a cardiologist and the dean of the Friedman School of Nutrition Science and Policy at Tufts University, who was not involved in the new study. Mozaffarian wrote an editorial that was published alongside the study in the journal.
People should focus more on eating healthy foods, rather than worrying about dietary fats, Mozaffarian told Live Science.
The new study may in fact have underestimated the health benefits of the Mediterranean diet, Mozaffarian added. Because the study took place in Spain, where people already eat a Mediterranean-style diet, there may not have been as big a change in eating patterns as there would have been if people had shifted from an American-style diet, for example, he said.
