The meta-analysis published Feb. 9 found no evidence to support the recommendation to reduce dietary fat intake to 30% of total energy and saturated fat to 10% of total energy as a way of lowering the number of deaths from heart disease, prompting the researchers to agree with the recently voiced sentiment that the advice needs to be reviewed.
But critics of the study note that it included only randomized controlled trails available at the time the public health recommendations were made in 1977 and 1983, and therefore does not take into account the full picture. In addition, they note, the RCTs in the study have already been rigorously reviewed and add nothing new to the ongoing debate. (Read more about the controversial research on saturated fat HERE and HERE.)
The study disregards “an enormous body of observational and experiential evidence” linking high-fat diets to plasma lipid levels, which have been linked to hardening arteries, said Thomas Sanders, a professor of nutrition and dietetics at King’s College London.
Are RCTs the gold standard for long-term behavior changes?
The researchers, led by Zoe Harcombe of the University of the West of Scotland, acknowledged some of the observational work, but reasoned that such studies could reveal only relationships, not causation. For that, “RCTs provide the best evidence,” they note.
Working on this premise, they reviewed six RCTs and found overall the number of deaths from all causes to be identical between the participants who lowered their fat intake and those who did not. They acknowledged slightly more participants died from heart disease in the group that did not lower their fat intake, but said the difference from the control group was insignificant.
From this they concluded “the available RCTs did not support the introduction of the dietary fat recommendations in order to reduce CHD risk or related mortality,” and therefore the recommendation should be reconsidered.
The authors’ conclusion, however, is open to debate, suggests Rahul Bahl, an associate editor for Open Heart with the University College London. He explains in an editorial that accompanies the study that some of the results reported in the meta-analysis are “controversial” and notes the authors’ decision to exclude from their study some research that found replacing saturated fat with polyunsaturated fat could offer some heart health benefits.
“Small differences in the criteria used to include studies in different parts of the analysis can lead to large differences in reported results,” he explains.
He also suggests that RCTs might not capture the long-term benefits of changes in real world settings, and therefore observational studies should be considered before making any changes in the health recommendation.
“The existence of nutritional guidelines can be beneficial through altering the content of food available, changing how food is packaged and through setting normative standards for what is considered healthy. Small reductions in risk factors at a population level might then be expected to have large effects on the rates of the disease,” he said, adding: “Such mechanisms are difficult to replicate in a trial setting.”
Public policy does not require RCTs
Evidence from RCTs is not generally required for public policy, so advocating for the withdrawal of the dietary fat recommendation on this basis is unusual, Bahl also notes.
Sanders agrees that “public health guidelines do not operate in the same arena as drug trials,” which require evidence based data using death or disease incidence as endpoints, and therefore a demonstrated statistical significant reduction in heart disease mortality is unnecessary.
Rather “eminence based” data, or “the best guesses available from experts in the area at the time,” can be sufficient, he said.
Plus, he noted, the impact of the recommendation appears to “have turned out OK, as the [heart disease] rates fell in countries that adopted the policy of replacing saturated with polyunsaturated fatty acids.”
With that in mind, he suggested the guidance to follow a “prudent diet” continues to make sense – a sentiment that Bahl also espoused.