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November 18 2022
An interesting analysis of nutrition and aging was recently published in BMC Biology, in which the authors used a multi-dimensional modeling technique...
A paper published recently in the journal Nutrients fires shots across the bow of the continued debate about the supposed dangers of dietary saturated fat. The title asks a potentially controversial question: Dietary Saturated Fats and Health: Are the U.S. Guidelines Evidence-Based? The question deserves an answer, because despite numerous reviews and meta-analyses that have shown no association between saturated fat intake and increased risk for cardiovascular disease, the most recent iteration of the Dietary Guidelines for Americans (DGA) still recommends restricting saturated fat intake to less than 10% of daily calories for all individuals starting at age two. (The 2020-2025 DGA also advise limiting sodium intake to less than 2300mg per day—a recommendation which has also been seriously questioned.)
The paper’s authors don’t mince words. Citing federal law, they note, “The guidelines are updated every five years, and the policy must, according to U.S. law, serve the ‘general public’ and reflect ‘the preponderance of the scientific and medical knowledge which is current at the time the report is prepared.’” However, the paper then reviews epidemiological data as well as results from clinical trials and concludes that the 10% cap on daily saturated fat intake “is not supported by rigorous scientific studies.”
It’s puzzling that this debate continues to rage, despite the publication over a decade ago of two reviews by Harvard and UC Davis researchers in the American Journal of Clinical Nutrition (AJCN), both of which concluded that evidence was lacking to support an association between saturated fat intake and increased risk of cardiovascular disease (CVD) or coronary heart disease (CHD). It would certainly make the “French paradox” much less paradoxical if dietary saturated fat does not influence risk for CVD or CHD.
But getting to the bottom of the saturated fat issue may influence much more than dinner party conversations about why a population that revels in butter, lardons, heavy cream, and goose fat has lower rates of cardiovascular problems than a population that was specifically cautioned against consuming large amounts of these foods. The DGA hold powerful influence over the nutritional requirements of foods supplied to the US military, the school lunch program, Supplemental Nutrition Assistance Program (SNAP), Special Nutritional Program for Women, Infants and Children (WIC), feeding programs for the elderly, and more. Moreover, they may also influence food manufacturers to focus on certain types of products (e.g., low-fat or low-sodium versions of various packaged foods), not to mention the nutritional guidance provided by medical and dietetics organizations. So, it’s critical that the guidelines be based on solid evidence, yet it increasingly appears that may not be the case.
One of the AJCN reviews from 2010 raised concerns about potential adverse effects of limiting dietary saturated fat, depending on what those calories were replaced with. For example, replacing them with polyunsaturated fats may have beneficial effects, but replacing them with carbohydrates—refined carbs, in particular—had greater potential to worsen cardiovascular health: “Replacement of saturated fat by polyunsaturated or monounsaturated fat lowers both LDL and HDL cholesterol. However, replacement with a higher carbohydrate intake, particularly refined carbohydrate, can exacerbate the atherogenic dyslipidemia associated with insulin resistance and obesity that includes increased triglycerides, small LDL particles, and reduced HDL cholesterol.” (Emphasis added.)
These concerns go back even farther than that, though. Dr. Gerald Reaven, a Stanford University researcher who coined the term “Syndrome X" (now called metabolic syndrome), sounded this alarm as far back as 1986. He was not specifically addressing saturated fat, but rather, the notion of a higher-carb diet that was lower in total fat across the board, expressing concern that in many people, high-carbohydrate diets would induce hyperinsulinemia and elevate triglycerides, increasing risk of coronary artery disease. More recent research echoes these concerns. There appears to be more evidence linking excessive sugar intake to increased coronary risk compared to anything to do with saturated fat.
The main rationale for limiting dietary saturated fat intake is that, in many individuals, saturated fat may raise serum LDL-cholesterol concentration, and elevated LDL-C is considered a risk factor—if not an outright cause—for CVD. However, serious doubts have been raised regarding a causal link between LDL-C and CVD, with many researchers starting to rethink this relationship. (Some have gone as far as to state outright that “LDL-C does not cause cardiovascular disease.”) LDL-C may have a very small positive association with CHD, but as we explored in a recent article, a paper in no less than the special cardiology issue of the Journal of the American Medical Association concluded that among women younger than age 55, the hazard ratio for CHD associated with elevated LDL-C was just 1.38, compared to a staggering 10.7 for diabetes and over 6.0 for metabolic syndrome and lipoprotein insulin resistance (LPIR). So if there is a link between elevated LDL-C and CHD, it is far eclipsed by the increased risk conferred by diabetes and insulin resistance.
It’s worth remembering that Dr. Robert Atkins (creator of the popular low-carb diet that bears his name), was a cardiologist. He was the target of much personal and professional criticism and slander for daring to suggest that people could eat butter, egg yolks, cheese, and juicy steaks and actually improve their health—that is, as long as they avoided so-called “heart-healthy” whole grains and other starchy and sugary foods. The popular thinking of the time was that low-carb diets worked for weight loss, but surely they would “clog your arteries” and cause a heart attack, so you would simply “make a thinner corpse.” But it’s hard to believe that a physician would have deliberately harmed his patients.
Indeed, low-carbohydrate diets may have a beneficial influence on cardiovascular risk even when they are not expressly limited in saturated fat. Apart from LDL-C, if there are concerns regarding elevated saturated fats in the blood and increased risk for cardiovascular problems, then here again, reducing dietary carbohydrate may be more effective than reducing dietary saturated fat: in adults with metabolic syndrome, increased carbohydrate intake was shown to increase plasma saturated fatty acid concentration, while modulating saturated fat intake had little to no effect.
As one researcher stated, “…the lack of any clear evidence that saturated fats are promoting any of the conditions that can be attributed to PUFA [polyunsaturated fats] makes one wonder how saturated fats got such a bad reputation in the health literature. The influence of dietary fats on serum cholesterol has been overstated, and a physiological mechanism for saturated fats causing heart disease is still missing.”
One thing is for certain: nutritional guidelines that were once considered “settled science” are anything but settled.
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