The diet-heart hypothesis
Starting in the late 1950s, University of Minnesota physiologist Ancel Keys developed the idea that fats and, subsequently, saturated fats increased blood serum levels of cholesterol and LDL-cholesterol. Many studies show significant associations between elevated blood serum LDL-cholesterol levels and increased risk of cardiovascular disease. Thus, reducing dietary intake of saturated fats should reduce the risk of cardiovascular disease. This idea became known as the ‘diet-heart hypothesis.’ It rapidly gained favor and has guided US dietary advice for the past 60 years.
Critiques of the diet-heart hypothesis
While the diet-heart hypothesis has enjoyed a long life, contrary evidence began appearing in scientific journals and books in the late 2000s. The 2007 book, Good Calories, Bad Calories: Fats, Carbs, and the Controversial Science of Diet and Health, by journalist Gary Taubes showed that the diet-heart hypothesis was not nearly as ironclad as its proponents claimed. In 2014, journalist Nina Teicholz published The Big Fat Surprise: Why Butter, Meat, and Cheese Belong in a Healthy Diet, which further undermined the diet-heart hypothesis.
A 2013 review by Glen Lawrence supported the critique of the of the diet-heart hypothesis by Gary Taubes. Lawrence found that the demonization of saturated fat was not supported by the scientific literature. In fact, recent evidence suggested that saturated fatty acids, especially those in dairy products and coconut oil, could improve health. Evidence that omega-6 polyunsaturated fatty acids promote inflammation and several diseases, while omega-3 fatty acids seem to produce opposite effects, continues to accumulate. Adverse health effects that have been reported for saturated fat may arise from consumption of processed meat and/or oxidation of polyunsaturated fatty acids in meat due to cooking the meat at high temperatures. Replacing saturated fats with carbohydrates, particularly sugars, promotes obesity and associated health complications. Lawrence says that it’s time to re-evaluate dietary advice that calls for minimizing saturated fatty acids, given the lack of scientific evidence of adverse health effects.
Is the scientific report scientific?
Every 5 years, the US government convenes a dietary guidelines committee of experts to review the evidence pertaining to certain aspects of diet. An expert report serves as the main source of scientific information for the committee’s deliberations. In addition, the panel operates under guidelines that specify, among other things, that recommendations in the guidelines must reflect the preponderance of the available evidence. Nina Teicholz argued that the 2010 scientific report from the committee wasn’t scientific. According to Teicholz, the committee did not include in its deliberations numerous original randomized controlled trials and longitudinal studies and meta-analyses of these studies that found no significant links between intake of dietary saturated fats and risk of cardiovascular disease and mortality. In fact, Teicholz found that the evidence used by the committee did not bolster its own conclusions. Teicholz also argued that many committee members had major conflicts of interest, such as being consultants for food companies that would benefit from finding a significant link between dietary saturated fat and cardiovascular disease and death and being advocates of vegetarian or vegan diets.
The American Heart Association’s support of the diet-heart hypothesis
The American Heart Association has long advocated reducing intake of saturated fats in order to reduce risk of cardiovascular disease. A meta-analysis that confirmed AHA’s 60-year old recommendation to limit saturated fats and to replace them with polyunsaturated fats was based on 4 Core Trials. Researcher Jeffrey Heileson at Baylor University examined the evidence pertaining to risk of cardiovascular disease and consumption of saturated fats. He identified 9 observational studies and 10 randomized controlled trials. Meta-analyses of the observational studies showed no significant association between saturated fat intake and risk of heart disease. Meta-analysis of the randomized controlled trials found inconsistent results that tended to favor lack of a significant association. Heileson found that the 4 Core Trials, which AHA relied upon, had design and methodology problems. Plus, the 4 Core Trials did not meet all of the predefined inclusion criteria. Thus, evidence from Heileson’s meta-analyses and the deficiencies of the 4 Core Trials cast doubt on AHA’s recommendation to limit dietary saturated fats and to replace them with polyunsaturated fats.
Important nuances of LDL cholesterol
A team of researchers took to task the long-standing recommendation to limit intake of saturated fats, which supposedly reduce risk of cardiovascular disease. The authors cite recent meta-analyses of randomized trials that do not show reduced risk of CVD following lower saturated fat intake. Increased intake of saturated fats does elevate serum LDL-cholesterol levels. But this increase of overall LDL-cholesterol does not increase levels of the small, dense LDL particles that pose greater risk of atherosclerosis. Rather, the larger LDL particles, which are less strongly related of CVD risk, increase in abundance with higher saturated fat intake.
The health effects of foods cannot be determined simply by focusing on saturated fats (or any particular nutrient). Instead, researchers must consider the overall dietary macronutrient composition. For example, whole-fat dairy, unprocessed meat, and dark chocolate have abundant saturated fats but are not linked to increased CVD or diabetes risk. Therefore, there’s no sound basis for limiting intake of these foods from a CVD risk standpoint. Plus, the current guidance to limit saturated fat intake across the entire US population has no sound basis. As a practical matter, the guidance to limit saturated fats has had the unintended consequence of Americans replacing these fats with sugar and refined grains, which may lead to worse health outcomes than eating saturated fats. Much more nuanced guidance is needed to promote healthy eating.
Since 1960, the US Dietary Guidelines for Americans have consistently advised limiting saturated fat intake to reduce LDL-cholesterol, thereby reducing risk of atherosclerotic cardiovascular disease. A recent review by an international team of researchers found evidence to support this advice unpersuasive. The preponderance of randomized controlled trials and observational studies does not support the idea that increased intake of saturated fats reduces all-cause mortality or cardiovascular disease mortality. There is little doubt that reducing consumption of saturated fat reduces the concentration of blood serum total cholesterol and LDL cholesterol.
As noted above, LDL cholesterol comes in several forms, particularly the small, dense form and the large, buoyant form. The former is atherosclerotic, while the latter is not. Saturated fat restriction tends to reduce the non-atherosclerotic large, buoyant LDL form. Plus, increased saturated fat intake increases HDL cholesterol, which is associated with better cardiovascular health. The food matrix serves as an unappreciated factor in human nutrition. Foods with substantial saturated fat can also contain a host of beneficial nutrients including other fatty acids, vitamins, and minerals, the health value of which is not reflected in US Dietary Guidelines. People who reduce their intake of saturated fats tend to increase their intake of carbohydrates, much of which includes sugars and starches. An unintended result of advice to limit saturated has been a reduction in overall US diet quality. Overall, the best available evidence does not support advice to limit saturated fat to less than 10 percent of calories consumed or to replace saturated fats with polyunsaturated fats.
The diet-heart hypothesis has enjoyed a remarkable longevity, especially given the weak evidence to support it. In a recent article, Nina Teicholz noted that over 20 review articles have largely concluded that higher saturated fat intake does not predict increased risk of cardiovascular disease or cardiovascular-related death or all-cause deaths. How, then, do the Dietary Guidelines for Americans still recommend eating less saturated fat? Teicholz attributes this disconnect to the unwillingness of the Dietary Guidelines Advisory Committees to consider published evidence that conflicts with the diet-heart hypothesis. Conflicts of interest and deeply entrenched beliefs in the validity of the diet-heart hypothesis may be underlying reasons.
What to do
If you want to reduce your risk of cardiovascular disease, I suspect that that reducing your intake of saturated fats to less than 10 percent of daily calories would not be your most effective strategy. If you’re like most Americans and your diet includes mostly ultra-processed foods with loads of sugar and refined grains, move toward a diet that features whole fruits, vegetables, beans, whole grains, and dairy. Plus, if you’re not getting at least 20 minutes of moderate or strenuous exercise per day, find an enjoyable way to do so. And if your life has too many chronic stresses (which likely lead to eating junk food), find ways to defuse those chronic stresses, such as taking daily, after-meal walks.
