Mediterranean diet high in healthy fat does not lead to weight gain, according to randomized trial

Eating a non-calorie restricted Mediterranean diet high in vegetable fats such as olive oil or nuts does not lead to significant weight gain compared to a low-fat diet, according to a large randomised trial published in The Lancet Diabetes & Endocrinology journal. The study suggests that current health guidelines that recommend a low-fat, low-calorie diet create unnecessary fear of healthy fats present in a Mediterranean diet, which have known health benefits.

Accumulating scientific evidence suggests that total fat content is not a useful measure of harms or benefits of food, and that fats from nuts, fish and phenolic-rich vegetable oils are healthier than fats from meat and processed foods.

"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," says lead author Dr Ramon Estruch, CIBER OBN-University of Barcelona, Spain. "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."

Obesity is a key risk factor for cardiovascular disease, type 2 diabetes, some cancers and musculoskeletal disorders. The standard recommendation for the prevention and treatment of obesity is a reduced-fat diet and increased physical activity, and many health organisations including WHO recommend a limit of 30% fat for total energy intake.

Perceptions of all fat as unhealthy have resulted in decreased fat consumption in the US population, but the epidemics of obesity and diabetes have continued to grow. Numerous studies have shown that the Mediterranean diet, which includes high levels of vegetable fats (e.g. olive oil, nuts), is linked to reduced mortality, cardiovascular disease and cancer, but fears of eating all fat mean that a low-fat diet continues to be recommended as a means of weight loss.

The study took place in 11 hospitals in Spain during 2003-2010 and included 7447 participants (men and women) aged 55-80 who were randomly assigned to one of three groups -- an unrestricted-calorie Mediterranean diet rich in olive oil (2543), an unrestricted-calorie Mediterranean diet rich in nuts (2454), or a low-fat diet where the advice was to avoid all dietary fat (2450). Trained dieticians gave personalised dietary advice to all participants.

Adherence to the diets was good and monitored by questionnaires for all participants and by taking blood and urine samples in a random subgroup. All participants were at high cardiovascular risk or had type 2 diabetes, and more than 90% were overweight or obese.

After 5 years, total fat intake had decreased in the low-fat diet group (from 40% to 37.4%) and had slightly increased in both Mediterranean diet groups (40% to 41.8% in olive oil; 40.4% to 42.2% in nuts). The percentage of energy intake from protein and carbohydrate decreased in both Mediterranean diet groups.

On average, participants in all three groups lost some weight with the greatest weight loss seen in the Mediterranean diet with olive oil group (0.88 kg weight reduction in the olive oil group, compared to 0.60 kg for the low-fat diet group and 0.40 kg for the nuts group). There was an increase in waist circumference in all three groups with the greatest increase seen in the low-fat diet group (1.2 cm increase for the low-fat diet group, compared to 0.85 cm for the olive oil group and 0.37 cm for the nuts group).

Writing in a linked Comment, Professor Dariush Mozaffarian, Friedman School of Nutrition Science & Policy at Tufts University, Boston, MA, USA says: "Ironically, just as focusing on total fat to prevent heart disease was misguided because it overlooked the different effects of specific fatty acids, the prioritisation of total calories (and by extension, total fat) to prevent weight gain ignores the diverse physiological effects of different foods. Although the rationale has morphed over time, the end result is similar: a proliferation of fat-reduced--and often correspondingly starch-rich and sugar-rich--foods and diets, with paradoxical warnings and caveats about eating healthy, high-fat foods, such as those rich in nuts and vegetable oils."

"Dietary guidelines should be revised to lay to rest the outdated, arbitrary limits on total fat consumption. 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. This illusion leads to paradoxical policies that focus on total calories, rather than food quality, on restaurant menus; ban whole milk but allow sugar-sweetened fat-free milk; compel food manufacturers, retailers, and restaurants to remove healthy vegetable-derived fats from meals and products while heavily marketing fat-reduced products of dubious health value; and mislead consumers to select foods based on total fat and calorie contents rather than actual health effects."

"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. We ignore this evidence-- including these results from the PREDIMED trial--at our own peril."