The last decade has seen nearly 20 papers reviewing the totality of the data on saturated fats and cardiovascular outcomes, which, altogether, have demonstrated a lack of rigorous evidence to support continued recommendations either to limit the consumption of saturated fatty acids or to replace them with polyunsaturated fatty acids. These papers were unfortunately not considered by the process leading to the most recent U.S. Dietary Guidelines for Americans, the country’s national nutrition policy, which recently reconfirmed its recommendation to limit saturated fats to 10% or less of total energy intake, based on insufficient and inconsistent evidence. Continuation of a cap on saturated fat intake also fails to consider the important effects of the food matrix and the overall dietary pattern in which saturated fatty acids are consumed.

Purpose of review: This article recounts the history of the diet-heart hypothesis from the late 1950s up to the current day, with revelations that have never before been published in the scientific literature. Insights include the role of authorities in launching the diet-hypothesis, including a potential conflict of interest for the American Heart Association; a number of crucial details regarding studies considered influential to the hypothesis; irregularities in the scientific reviews on saturated fats, for both the 2015 and 2020 Dietary Guidelines for Americans; and possible conflicts of interest on the relevant subcommittee reviewing saturated fats for the 2020 Dietary Guidelines Advisory Committee. Information obtained via the Freedom of Information Act (FOIA) on emails from the 2015 process is published here for the first time. These findings are highly relevant to the 2025–2030 Dietary Guidelines process, now underway, which has plans for a new review on saturated fats. Recent findings: Recent findings include shortcomings in the scientific review processes on saturated fats, for both the current 2020–2025 Dietary Guidelines for Americans and the previous edition (2015–2020). Revelations include the fact the 2015 Advisory Committee acknowledged, in an e-mail, the lack of scientific justification for any specific numeric cap on these fats. Other, previously unpublished findings include significant potential financial conflicts on the relevant 2020 guidelines subcommittee, including the participation of plant-based advocates, an expert who promotes a plant-based diet for religious reasons, experts who had received extensive funding from industries, such as tree nuts and soy, whose products benefit from continued policy recommendations favoring polyunsaturated fats, and one expert who had spent more than 50 years of her career dedicated to ‘proving’ the diet-heart hypothesis. Summary: The idea that saturated fats cause heart disease, called the diet-heart hypothesis, was introduced in the 1950s, based on weak, associational evidence. Subsequent clinical trials attempting to substantiate this hypothesis could never establish a causal link. However, these clinical-trial data were largely ignored for decades, until journalists brought them to light about a decade ago. Subsequent reexaminations of this evidence by nutrition experts have now been published in >20 review papers, which have largely concluded that saturated fats have no effect on cardiovascular disease, cardiovascular mortality or total mortality. The current challenge is for this new consensus on saturated fats to be recognized by policy makers, who, in the United States, have shown marked resistance to the introduction of the new evidence. In the case of the 2020 Dietary Guidelines, experts have been found even to deny their own evidence. The global re-evaluation of saturated fats that has occurred over the past decade implies that caps on these fats are not warranted and should no longer be part of national dietary guidelines. Conflicts of interest and longstanding biases stand in the way of updating dietary policy to reflect the current evidence.

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