Accumulating scientific evidence suggests that total fat content is not a useful measure of harms or benefits of food.
According to a large randomised trial published in The Lancet Diabetes & Endocrinology journal: “Effect of a high-fat Mediterranean diet on bodyweight and waist circumference: a prespecified secondary outcomes analysis of the PREDIMED randomised controlled 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.
The 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 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.
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.
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).
Authors suggests that the myth that lower-fat, lower-calorie products lead to less weight gain must be abandoned. 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.
Estruch, Ramon et al. (June 2016). Effect of a high-fat Mediterranean diet on bodyweight and waist circumference: a prespecified secondary outcomes analysis of the PREDIMED randomised controlled trial. The Lancet Diabetes & Endocrinology , Volume 4 , Issue 8 , 666 – 676. Available at: http://www.thelancet.com/pdfs/journals/landia/PIIS2213-8587(16)30085-7.pdf