Kevin Lomangino is the managing editor of HealthNewsReview.org. He tweets as @KLomangino.
After decades of scientific squabbles over the role of saturated fat in the development of heart disease, the American Heart Association (AHA) claimed last week to have put the debate behind us in a news release that began this way:
“Replacing saturated fats with healthier ones found in some vegetable oils can reduce cholesterol levels and heart disease risk as much as statins.”
Soybean oil as good as a statin? That’s a stretch.
Statin drugs have been the subject of multiple gold-standard clinical trials involving many thousands of patients. The research unequivocally demonstrates that these drugs lower the rate of heart attacks and strokes in individuals at high risk of cardiovascular disease. (Note: there is a great deal of controversy, however, about the value of statins in those who are not at high risk of cardiovascular disease.)
There’s simply way less evidence that substituting unsaturated fat for saturated fat is beneficial. Many of the clinical trials that have tested this hypothesis were performed 50 or more years ago and are riddled with design problems.
Yet those issues are nowhere to be found in the AHA news release, based on a “presidential advisory,” which instead leans heavily on data showing that higher saturated fat intake raises LDL cholesterol levels — a “surrogate” for outcomes that we really care about like heart attacks, strokes, and deaths.
Nor were those concerns raised in many of the news stories that I read about the advisory. They cast the advisory as an opportunity to “set the record straight” and featured only proponents of the AHA guidance.
Yes, saturated fat does raise LDL cholesterol, and LDL levels are unquestionably associated with higher rates of cardiovascular disease.
But correlation is not causation, and you can’t draw a straight line from hamburgers to heart attacks.
There is no doubt that a healthy overall eating pattern is good protection against heart disease and preferable to drug therapy. But things get tricky when you try to isolate one specific component of a complicated diet and recommend its avoidance or replacement with another component.
Researchers have been wrong about diet recommendations before, and these errors have the potential to cause harm. Guidelines advocating lower fat intake, for example, may have encouraged people to eat more refined carbohydrates and sugar.
The reality is that there’s very little news here. The AHA scientific advisory focuses on four supposedly “core trials” that all date to the 1960s and which all show benefits from replacing saturated fat with unsaturated fat. Other trials which came to different conclusions were excluded. Why? The AHA advisory says the four studies were selected on “the basis of quality of study design, execution, and adherence.”
But according to Andrew Mente, PhD, a nutritional epidemiologist at McMaster University, the AHA approach amounted to “cherry picking” what studies to include or not include without using predetermined criteria.
“With this approach, you can include and exclude whichever studies you want, depending on whether they fit your preconceptions or beliefs,” he said. “For instance, they include the Finnish Mental Hospital Study which was a cluster trial with two clusters. Therefore, a non-randomized trial. Meanwhile, they exclude some of the largest randomized trials conducted including the Minnesota Coronary Experiment, the Sydney Heart Study, and the Women’s Health Initiative (the single largest clinical trial ever and the most expensive). Each of these trials refuted the hypothesis that reducing saturated fat reduces clinical outcomes. Furthermore, other recent reviews that used predefined inclusion and exclusion criteria of trials found no reduction in cardiovascular mortality or total mortality with saturated fat replaced with polyunsaturated fat.”
Similar concerns were raised by journalist and low-carb diet advocate Gary Taubes in this rebuttal to the AHA statement, which points out that the four AHA-sanctioned trials have substantial — possibly fatal — design flaws, including failure to include a true control group. (Disclosure: Taubes has previously received funding from the Laura and John Arnold Foundation, which also supports this project.)
Other researchers have looked at the evidence and concluded, like the AHA, that dietary fat modification may protect against cardiovascular disease. But they also cautioned, like Taubes, that many of the trials they looked at “had important design limitations.”
In fact, contrary to its own certain-sounding news release, the AHA readily acknowledges that there are no definitive trials of dietary fat modification. In the advisory itself the AHA researchers describe what it would take to conduct such a study:
The core trials reviewed in this section were started in the late 1950s and early 1960s. Readers may wonder why at least 1 definitive clinical trial has not been completed since then. Reasons include the high cost of a trial having upward of 20000 to 30000 participants needed to achieve satisfactory statistical power, the feasibility of delivering the dietary intervention to such a large study population, technical difficulties in establishing food distribution centers necessary to maintain high adherence for at least 5 years, and declining CVD incidence rates caused by improved lifestyle and better medical treatment. These linked issues, which must be managed to obtain a definitive result, remain the central considerations for dietary trials on CVD and indeed are the overarching reason why few of these trials have ever been done.
Translation: Studies of dietary fat are hard and expensive to do and that’s why there are no conclusive ones. There may never be conclusive studies in this area.
But instead of burying these concerns, as the AHA news release does, beneath claims that vegetable oil is as good a statin, why not acknowledge these problems and give more emphasis to the ongoing uncertainty?
And if you’re a reporter working on this story, why not turn to some of the many researchers who’ve raised legitimate questions about the evidence on saturated fat replacement to balance the narrative?
I particularly like this nugget from Christopher Ramsden, MD and colleagues from a 2016 BMJ study that challenged the AHA position.
“Given the limitations of current evidence, the best approach might be one of humility, highlighting limitations of current knowledge and setting a high bar for advising intakes beyond what can be provided by natural diets,” they said.