Using data from the PREDIMED study, which involved almost 7,500 Spanish participants aged between 55 and 80, the researchers looked at the links between body weight and waist circumference, and how this changed depending on consumption of nuts or olive oil, or a low-fat diet.
Lead author Dr Ramon Estruch of the University of Barcelona said the findings meant a "radical reassessment" of the low-fat approach that has dominated nutrition advice was needed.
"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. 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.
"The Mediterranean diet has well-known health benefits and includes healthy fats such as vegetable oils, fish and nuts."
However, he said the findings did not imply diets high in unhealthy fats such as butter and processed meat or sweetened beverages, desserts or fast-foods were beneficial.
However experts have urged caution when interpreting the results.
Susan Jebb, professor of diet and population health at the University of Oxford, pointed out that those in the low-fat group actually only managed to reduce their fat intake from 40% to 37%.
“[This was] a long way from the 30% the investigators aimed to achieve and still higher than the fat content of the UK diet (35%). This was not a low fat diet,” she said.
“It is impossible from this study to draw any conclusion about the impact of the low fat diets on body weight since each group consumed more than the UK average (35% fat) and way more than the World Health Organisation recommendation (30% fat).”
Tom Sanders, professor emeritus of nutrition and dietetics at King’s College London, said the context of the Mediterranean diet is important and it would be wrong to interpret the study as showing that eating lots of healthy fats does not result in weight gain.
"It remains sensible to use oils in moderation and get most of the fat from plant and marine sources. It is worth reflecting that both Greece and Spain have some of the highest rates of obesity in Europe, and Greece consumes the highest proportion of fat in Europe (46% energy).
"Although the Mediterranean diet is perceived to be a diet rich in fruit and vegetables with lots of olive oil, in the past most of the food energy was derived from plant based foods such as cereals, legumes, nuts and pulses with only small amounts of meat, fish and dairy produce.”
Participants were randomly assigned to one of three groups - an unrestricted-calorie Mediterranean diet rich in olive oil; an unrestricted-calorie Mediterranean diet rich in nuts or a low-fat diet where they were advised to avoid all dietary fats for five years.
Fat intake fell from 40% to 37.4% for those in the low-fat group while it slightly increased from 40% to 41% for the olive oil group and rose from 40.4% to 42.2% for the nut group.
Scientists monitored adherence to the assigned diets through questionnaires and urine and blood samples.
Source: The Lancet Diabetes & Endocrinology
Published online ahead of print, DOI: http://dx.doi.org/10.1016/S2213-8587(16)30085-7
“Effect of a high-fat Mediterranean diet on bodyweight and waist circumference: a prespecified secondary outcomes analysis of the PREDIMED randomised controlled trial”
Authors: R. Estruch et al.