New study on saturated fats leads to collision of politics, science

Saturated fats are having a moment, one that has ensnared researchers in a political debate they never intended to enter. 

A new systematic review of 17 randomized clinical trials found that reducing intake of saturated fats reduced the chance of serious cardiovascular events — but only for some people. The paper, published Monday in Annals of Internal Medicine, arrives as members of the Make America Healthy Again movement are questioning the national dietary guidelines’ recommendation to limit saturated fats to no more than 10% of daily calories. 

Health secretary Robert F. Kennedy Jr. is promoting full-fat dairy, red meat, and beef tallow ahead of the 2025-2030 version of the recommendations, used to guide policy and food benefits for Americans, from WIC to SNAP to military meals. Due in December, the guidelines have been delayed until next year, administration officials said, citing this fall’s government shutdown.

The review has findings that both mainstream nutrition experts and fat-forward Kennedy allies might welcome. First, among people considered at high risk for cardiovascular death, heart attacks, and strokes, cutting down on saturated fats lowered their cholesterol and their risk over the next five years. If they substituted polyunsaturated fats, they fared better as their cholesterol fell, including LDL, or the “bad” cholesterol known to clog arteries that precede heart attacks and strokes.

Second, people at low to moderate risk of these cardiovascular problems did not see the same benefit from a diet low in saturated fats. 

That’s where the conflict begins.

“The findings of this review align with the current emerging recognition that dietary SFAs per se are unlikely deleterious for cardiometabolic health for the general population, but may be deleterious for people at high risk for CV events,” Ramon Estruch and Rosa M. Lamuela-Raventós of the University of Barcelona  wrote in a companion editorial, referring to saturated fatty acids.

That interpretation flies in the face of what the reviewers concluded. Their results confirmed what previous analyses have shown about reductions in saturated fat intake mirroring lowered LDL cholesterol, followed by fewer strokes and heart attacks, the reviewers reiterated in a message to reporters. The editorial incensed the review’s co-authors so much that on Friday, they raised their objections to the journal. 

“The editorial by Estruch and Lamuela-Raventos is unfortunately poorly contextualized and contains both errors and omissions that are likely to cause unnecessary confusion and controversies surrounding the results of our review and their relevance to current Dietary Guidelines,” the co-authors wrote in a message to the journal, a draft of which was shared with journalists. “We are writing to request that this Editorial be reconsidered, or that we have an opportunity to respond,” their final message said.

The journal encouraged the authors of the systematic review to share their concerns by posting a comment to the editorial once both the review and the editorial have been published. 

“Research on diet and dietary guidelines always generate vigorous debate,” Christine Laine, editor-in-chief of the journal, told STAT in a statement Monday.

Among criticisms of the editorial submitted by the review authors to the journal: 

• “The authors characterize the saturated fat ‘lipid hypothesis’ as being based on only ‘weak evidence of association, not causation,’ citing a narrative review from a popular diet book author.” The reviewers did not name the person, but the editorial’s first footnote lists Nina Teicholz, journalist and author of “The Big Fat Surprise.”
• Referring to a trial in very sick individuals during their deinstitutionalization, “Estruch and Lamuela-Raventos’ presentation of this trial as though it is blockbuster and questions current dietary recommendations either points to an inability to critically examine the literature or an intentionally contrarian approach to their editorial.”
• The discussion of fatty acids is “replete with basic errors related to both physiology and food science.” 

For the systematic review, the researchers arrived at their conclusions with “low to moderate certainty” after weighing evidence from 17 randomized clinical trials involving 66,337 participants, whittling the research down from more than 900 candidates of varying quality. To be more rigorous, their analysis looked at three outcomes — death, heart attack, and stroke — down from the Cochrane review of 10 outcomes, which included angina, for example. 

“The results of our review are relevant mostly to medical guidelines and for clinicians to prioritize interventions to reduce cardiovascular risk in patients,” the review co-authors said in the message to reporters. “Indeed, our stratification approach assessed baseline risk over a five-year period, consistent with a medical nutrition therapy approach.”

The review itself did not surprise Alice Lichtenstein, a nutrition scientist at Tufts University who was not involved in it, but in her eyes there’s a missing piece.

“The data are sound but there is a fundamental flaw within the context of interpretation: It highlights the difference between prevention and treatment,” she  told STAT. “They found that there was a relationship between the type of dietary fat and cardiovascular outcomes in the higher-risk group, right? They didn’t find it in those that were at low risk. Well, that’s where prevention comes in.”

Studies would have to follow low-risk people for something like 30 years to really see the effects of a diet low in saturated fats, she said, noting that two-thirds of Americans are at risk of developing cardiovascular disease because they are overweight or have obesity. Biomarkers such as cholesterol and high blood pressure have proven effective in quickly identifying people heading to serious heart disease or stroke. Substituting polyunsaturated fats for saturated fats is a well-established strategy to lower risk, she said.

The review authors did say in the paper’s discussion that their results have limited relevance to the general population because of the trials they included, the risk factors of the participants, their diets during the trials, and the time frame of only five years. 

Tom Sanders, a professor emeritus of nutrition and dietetics at King’s College London who was not involved in the review, noted that most of the people who participated in trials that were reviewed had preexisting cardiovascular disease, putting them at greater risk of heart attacks, strokes, or death from cardiovascular disease.

 “This is important because individuals can be at high risk if they have not had a cardiovascular event and absolute risk increases greatly with age,” Sanders said in a statement to the Science Media Centre. He also said that while a low-fat diet is beneficial for most people, statins are far more effective at lowering non-HDL cholesterol in people at risk. 

For Lichtenstein, the editorial saying saturated fats were not bad for the general population provoked a stronger reaction. 

“I totally disagree with that. I think it really, really misses the point,” she said. 

Lichtenstein also questioned the editorial’s focus on various kinds of fatty acids having different effects. “There’s no way to contrive a diet where you’re going to just restrict one individual saturated fatty acid.”

In response to STAT’s request for comment, one of the editorial writers said their aim was to emphasize that not all saturated fats are the same.

“Some may not be harmful to health — perhaps even the contrary,” Estruch wrote in an email Monday. “We believe that, in future updates, the expert groups responsible for developing clinical guidelines should consider that saturated fats do not exert a uniform effect on cardiovascular health. If our perspective is taken into account, we will be pleased. If not, we will continue to advocate for additional research to clarify these important issues.”

Kevin Klatt, a co-author of the new review and an incoming assistant professor of nutritional sciences at the University of Toronto, said the editorial writers got the science wrong by relying on weak evidence.

“It is riddled with factual errors and poorly contextualized to the point that either the authors do not understand critical evaluation of the evidence and basic food chemistry concepts or are intentionally aiming to be contrarian,” he told STAT.

Beyond their concerns about the editorial, the study authors also want to make it clear that their work should not be construed as a blueprint for new dietary guidelines. They call their results just a small piece of the evidence to support the dietary guidelines for Americans, replicating other research that shows the more saturated fat and LDL cholesterol go down, the more strokes and heart attacks fall, too. 

“These results, along with the much larger body of evidence linking saturated fat intakes with LDL-C, may partially inform Dietary Guidelines that aim to manage risk factors for cardiovascular disease in the broad population,” their statement to reporters said.

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