A no-calorie limit Mediterranean diet rich in olive oil is not a recipe for weight gain - and may actually be a better strategy than avoiding all fatty foods, research has shown.
The finding adds to the current controversy surrounding health advice which recommends low-fat diets.
Current guidelines are misguided because they do not differentiate between "good" and "bad" fats, it is claimed.
A total of 7447 men and women took part in the new research in Spain between 2003 and 2010. All had Type 2 diabetes or were considered at high risk of heart disease, and 90 per cent were overweight or obese.
Participants were randomly assigned to one of three diets - an unrestricted Mediterranean diet especially rich in olive oil, a similar diet with the focus on nuts, and a diet that generally avoided all kinds of fat.
After five years, all three groups were found to have shed a small amount of weight, but people on the olive oil-rich diet lost the most - an average of 0.88kg.
Members of the low-fat group lost 0.60kg while those on the nut-rich diet were 0.40kg lighter.
Curbing fat consumption was the least effective way to trim "spare tyres". Waist circumference increased by 1.2cm in the low-fat group compared with 0.85cm in the olive oil group and 0.37cm in the nuts group.
The percentage of energy intake from protein and carbohydrates decreased in both Mediterranean diet groups.
Lead researcher Dr Ramon Estruch, from the University of Barcelona in Spain, said: "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 bodyweight 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. Our findings certainly do not imply that unrestricted diets with high levels of unhealthy fats such as butter, processed meat, sweetened beverages, deserts or fast-foods are beneficial."
The results are published in the journal The Lancet Diabetes & Endocrinology.
Commenting in the same journal, US nutrition expert Professor Dariush Mozaffarian, from Tufts University in Boston, said it was time "outdated" guidelines on fat consumption were swept away.
He maintained that current guidelines aimed at tackling obesity were as misguided as attempts to prevent heart disease by reducing total fat which overlook the effects of beneficial fatty acids.
"Dietary guidelines should be revised to lay to rest the outdated, arbitrary limits on total fat consumption," said Prof Mozaffarian.
"Calorie-obsessed caveats and warnings about healthier, higher-fat choices such as nuts, phenolic-rich vegetable oils, yoghurt, and even perhaps cheese, should also be dropped.
"We must abandon the myth that lower-fat, lower-calorie products lead to less weight gain."
Prof Mozaffarian added: "The fat content of foods and diets is simply not a useful metric to judge long-term harms or benefits. Energy density and total caloric contents can be similarly misleading.
"Rather, modern scientific evidence supports an emphasis on eating more calories from fruits, nuts, vegetables, beans, fish, yoghurt, phenolic-rich vegetable oils, and minimally processed whole grains; and fewer calories from highly processed foods rich in starch, sugar, salt, or trans-fat."
