Harvard’s nutrition department rejects WHO updated guidelines on healthy diets citing old research

26 Jul 2023 --- Researchers from the Department of Nutrition at the Harvard T.H. Chan School of Public Health have come out strongly against the World Health Organization’s (WHO) updated guidelines for healthy diets, stating that the recommendation for calories derived from fat “omits decades of research.”

The researchers say the recommendations, mainly on carbohydrates and total fat (saturated and trans fats), are based on “one deeply flawed meta-analysis of weight gain.”

“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,” states Dr. Walter Willett, professor of Epidemiology and Nutrition at the department of nutrition at Harvard T.H. Chan School of Public Health.

“This [the recommendations] 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.”

Implication of a dietary setback 
The guidelines are an addition to the WHO’s previous recommendations on added sugars, sodium and non-sugar sweeteners. The Harvard researchers’ responses were published in The Nutrition Source. 

The WHO advises that people consume a variety of food groups and diverse options within each food group to reduce the risk of vitamin and mineral deficiencies. Evidence suggesting a dose-response is suggested, in that the more fat intake is reduced, the more weight is discarded. Reduced fat intake is associated with a lower calorie intake.

The Harvard researchers disagreed with the WHO guidelines to limit total fat intake to 30%. They cited evidence from dozens of long-term cohort studies and randomized trials showing low-fat diets lack benefits to reduce the risk of chronic conditions, including cancer, diabetes, cardiovascular disease and weight loss.The guidelines exclude the Mediterranean diet, which has been supported by many studies.  

For example, the Predimed trials, which randomly assigned people to a Mediterranean diet with a higher fat intake of up to 42% of total calories (mainly from unsaturated fats) or a low-fat diet, found a lower risk of cardiovascular disease and Type two diabetes on the higher fat diet. 

Citing concerns about lowering fat intake
The Harvard researchers expressed concern that lowering total fat intake could increase carbohydrate intake, especially refined carbohydrates and sugars, which have been shown to increase blood pressure and triglycerides.

The WHO guidelines advise that total fat intake for adults from 20 years old onward should comprise 30% or less of total calorie intake to prevent unhealthy weight gain and the fat consumed should be primarily unsaturated fatty acids. This amount was associated with reduced body weight, body mass index, waist circumference and body fat percentage.

Harvard’s researchers note that the meta-analyses excluded studies that were carefully designed to look at dietary fat and weight changes. In addition, many of the studies that were included provided unequal interventions.

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 monitoring results in minor weight reductions.

“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 adds.WHO's updated guidelines prioritize carbohydrates and saturated fats.  

Meanwhile, in its guidelines for sugar consumption, WHO stated long-term use of non-sugar sweeteners does not have long-term benefits in reducing body weight and may have undesirable effects, such as the risk of Type 2 diabetes, cardiovascular diseases and premature mortality in adults. Several experts also found the supporting evidence for this claim to be insufficient.

On calorie consumption
The WHO guidelines advise 25 g of fiber daily and replacing saturated fatty acids with polyunsaturated fatty acids. According to the guidelines, individuals who can maintain their weight may consume higher levels of fat greater than 30%. 

For example, one may consume 40% of calories from fat, but carbohydrate and protein intake would decrease to maintain energy balance by consuming the same calorie level. 

“On the other hand, the type of dietary fat has major implications for long-term health and well-being, and the recommendation to emphasize unsaturated sources of fat from plants over those high in saturated and trans fat is well-founded.”

In the meta-analyses supporting the WHO guidelines, Harvard’s researchers note the 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 or cardiovascular disease and; therefore, were not considered healthy.

By Inga de Jong