Harvard experts say most recommendations are well-supported, but guidance on total fat intake omits decades of evidence
The World Health Organization (WHO) has released updated guidelines for defining healthy diets, with particular attention to carbohydrates, total fat, and specific types of fat such as saturated and trans fats. The guidelines are an addition to their previous recommendations on added sugars, sodium, and non-sugar sweeteners. With the exception of total fat intake, the recommendations below are geared toward everyone ages 2 and older:
- A healthy dietary pattern—a combination of foods that is influenced by availability, affordability, preferences, culture, traditions, and other factors—is encouraged with the core tenets of adequacy, diversity, balance, and moderation.
- A variety of food groups and a variety of foods within each food group are encouraged to reduce the risk of vitamin and mineral deficiencies, and to provide health-promoting components found in a diverse diet.
- Carbohydrate intake should comprise 40-70% of total calorie intake and come mainly from minimally processed whole grains, vegetables, fruits, and legumes, with research finding that these foods are associated with a reduced risk of deaths from any cause and diet-related non-communicable diseases (e.g., diabetes, cardiovascular disease, cancer). They advised 400 grams of fruits and vegetables daily (equivalent to about 2 servings each), and 25 grams fiber daily.
- Saturated fatty acid intake should be reduced to 10% or less of total calorie intake, and trans fat intake to 1% of total calorie intake, which is associated with reduced risk of deaths from any cause, reduced cardiovascular disease, and lower LDL levels. They advise replacing saturated fatty acids with polyunsaturated fatty acids.
- Total fat intake for adults ages 20 and older should comprise 30% or less of total calorie intake for the prevention of unhealthy weight gain, and the type of fat consumed should be mostly unsaturated fatty acids. This amount was associated with reduced body weight, body mass index, waist circumference, and body fat percentage. They cited evidence that suggested a dose response, in that the more fat intake was reduced, the greater the reduction in body weight. A reduced-fat intake was associated with a lower calorie intake, leading to decreased weight. The guidelines also cited that individuals who can maintain their weight may be able to consume higher levels of fat greater than 30%; for example, one may consume 40% of calories from fat but intakes of carbohydrate and protein would decrease in order to maintain energy balance by consuming the same calorie level. [1]
Experts in the Department of Nutrition at the Harvard T.H. Chan School of Public Health disagreed with the WHO guidelines to limit total fat intake to 30% or less of total calorie intake, citing evidence from dozens of long-term cohort studies and randomized trials that showed a lack of benefit of low-fat diets for lowering risk of chronic conditions including cancer, diabetes, cardiovascular disease, and weight loss. For example, the PREDIMED trials—which randomly assigned people to a Mediterranean diet with a higher fat intake of 39-42% of total calories (mostly from unsaturated fats) or a low-fat diet—found a lower risk of cardiovascular disease and type 2 diabetes on the higher fat diet. [2,3] The Harvard researchers also expressed concern that lowering total fat intake could mean increasing carbohydrate intake, especially refined carbohydrates and sugars, which has been shown to increase blood pressure and triglycerides.
“The new WHO recommendation that intake of total fat be limited to 30% of calories is narrowly based on one deeply flawed meta-analysis of weight gain,” said Dr. Walter Willett, Professor of Epidemiology and Nutrition. “This ignores the last several decades of research on dietary fat and excludes the traditional Mediterranean diet, which has been widely recognized as a healthy model for eating, based on a massive body of evidence. Although other aspects of the WHO dietary recommendations are well-supported, the limit on total fat is best ignored.”
In the meta-analyses supporting the WHO guidelines, Willett and colleagues noted that the WHO report did not include a comprehensive assembly of randomized controlled trials but rather selective studies in which weight change was not the primary outcome, and many participants had chronic conditions like cancer, diabetes, and cardiovascular disease, and therefore were not considered healthy.
They also noted that the meta-analyses excluded studies that were carefully designed to look at dietary fat and weight changes, and that many of the included studies provided an unequal intervention. For example, in many studies, the low-fat diet group received intensive guidance and monitoring of fat reduction, whereas the control group received no advice or monitoring. This is meaningful as close dietary guidance and monitoring itself results in small reductions in weight.
“Even if the result of the meta-analysis were to be believed, the difference between the low- and high-total fat groups was only about two pounds (0.9 kg after accounting for sample size), hardly sufficient to be setting global dietary recommendations,” Willett added. “On the other hand, the type of dietary fat has major implications for long term health and wellbeing, and the recommendation to emphasize unsaturated sources of fat from plants over those high in saturated and trans fat is well-founded.”
Last updated July 18.