Researchers found a Mediterranean diet high in vegetable fats - such as those from olive oil and nuts - had little impact on body weight and weight circumference, compared with a low-fat diet.
Lead author Dr. Ramon Estruch, of the CIBER OBN-University of Barcelona, Spain, and colleagues say their findings challenge current dietary guidelines, which have long hailed the benefits of a low-fat diet.
"More than 40 years of nutritional policy has advocated for a low-fat diet but we're seeing little impact on rising levels of obesity," notes Dr. Estruch.
"Our study shows that a Mediterranean diet rich in vegetable fats such as olive oil and nuts had little effect on body weight or waist circumference compared to people on a low-fat diet."
Rates of obesity have more than doubled worldwide since 1980, according to the World Health Organization (WHO), increasing people's risk for heart disease, stroke, type 2 diabetes, and some types of cancer.
Reducing total fat intake is considered a key strategy for reducing obesity, and the WHO recommend that to avoid weight gain, total daily fat intake should not exceed 30 percent of total daily calorie intake.
The study authors say such guidelines have fueled the perception that all fats are unhealthy, and in the United States, this perception has led to a reduction in fat consumption.
However, they point out that this decrease in fat intake has failed to reduce rates of overweight and obesity.
On the other hand, a Mediterranean diet - which is high in vegetable fats, such as those in olive oil and nuts - has been associated with reduced risk of cardiovascular disease, cancer, and mortality.
But according to the authors, people may be failing to reap such benefits out of fear of consuming too much fat with a Mediterranean diet and gaining weight. Instead, they turn to the recommended low-fat diet.
Mediterranean diet vs. low-fat diet
For their study, Dr. Estruch and colleagues set out to investigate how a low-fat diet and Mediterranean diet affect weight and waist circumference.
The researchers analyzed the data of 7,447 men (aged 55-80) and women (aged 60-80) who were part of the PREDIMED trial - a study in Spain that assessed the health effects of a Mediterranean diet.
More than 90 percent of the participants were overweight or obese, and they had either type 2 diabetes or at least three risk factors for cardiovascular disease.
As part of the trial, participants were randomized to one of three groups:
- A non-calorie-restricted Mediterranean diet rich in olive oil
- A non-calorie-restricted Mediterranean diet rich in nuts
- A low-fat diet, in which subjects were asked to avoid all dietary fat.
All participants received dietary advice from trained dietitians and were followed-up for around 5 years.
The body weight and waist circumference of each participant were measured at study baseline and each year thereafter, and adherence to each diet was assessed through dietary questionnaires and urine and blood samples.
Weight loss, waist circumference increased for all three groups
At the end of the 5-year follow-up, researchers found total fat intake had reduced for participants who followed the low-fat diet, from 40 percent to 37.4 percent.
In the Mediterranean diet groups, however, total fat intake slightly increased, rising from 40 percent to 41.8 percent in the olive oil group, and from 40.4 percent to 42.2 percent in the nut group.
The researchers found that after 5 years, participants in all three groups had lost weight.
Subjects who followed the Mediterranean diet rich in olive oil lost the most weight - an average of 0.88 kilograms, compared with 0.60 kilograms for those who followed the Mediterranean diet rich in nuts, and 0.40 kilograms for those who followed the low-fat diet.
All three groups also experienced an increase in waist circumference, with the largest increase found among subjects in the low-fat diet group - an average increase of 1.2 centimeters.
Subjects who followed the Mediterranean diet rich in olive oil experienced an average 0.85-centimeter increase in waste circumference, while those who followed the Mediterranean diet rich in nuts had an average waist circumference increase of 0.37 centimeters.
Total fat, calorie content 'not a useful metric' to judge harms, risks
Dr. Estruch and colleagues say their results indicate that increasing intake of vegetable fats has little effect on body weight and waist circumference, compared with a low-fat diet, adding:
"Instead, small but significant reductions in weight and lesser increases in waist circumference were seen in participants given the Mediterranean diet interventions compared with the control group.
Even small changes in bodyweight have implications for long-term adiposity-related conditions, including diabetes and cardiovascular disease."
In an editorial accompanying the study, Prof. Dariush Mozaffarian, of the Friedman School of Nutrition Science & Policy at Tufts University in Boston, MA, says the current dietary guidelines should be revised to put the "outdated, arbitrary limits on total fat consumption" to rest.
"Calorie-obsessed caveats and warnings about healthier, higher-fat choices such as nuts, phenolic-rich vegetable oils, yogurt, and even perhaps cheese, should also be dropped," he adds.
Prof. Mozaffarian notes that focusing on the fat content of foods and diet is "simply not a useful metric" to assess the long-term health risks or benefits, nor is total calorie content.
"Rather, modern scientific evidence supports an emphasis on eating more calories from fruits, nuts, vegetables, beans, fish, yogurt, phenolic-rich vegetable oils, and minimally processed whole grains," he adds, "and fewer calories from highly processed foods rich in starch, sugar, salt, or trans-fat. We ignore this evidence - including these results from the PREDIMED trial - at our own peril."
The study received funding from the California Walnut Commission, Morella Nuts, Patrimonio Comunal Olivarero, Borges SA, CIBERobn, Instituto de Salud Carlos III, Hojiblanca, and the Spanish Government.