Study under review: The effect of replacing saturated fat with mostly n-6 polyunsaturated fat on coronary heart disease: a meta-analysis of randomized controlled trials
Fatty acids are broadly classified as saturated, monounsaturated, or polyunsaturated based on how many double-bonds exist within the fatty acid tail (zero, one, and more than one, respectively). Many studies beginning in the 1950s identified saturated fats and polyunsaturated fats as major dietary influences of blood cholesterol levels. These observations, in combination with observational research regarding the association between diet and heart disease, led Dr. Ancel Keys to propose the diet-heart hypothesis (the history of which is outlined in Figure 1), which posits that dietary saturated fat raises blood cholesterol levels and increases the risk of heart disease.
Despite some researchers arguing that there were significant flaws in the data Keys used to support his claims, the diet-heart hypothesis persisted and resulted in the belief that a heart-healthy diet should limit saturated fat. However, accumulating evidence over the last decade has suggested that saturated fat may not be as detrimental to cardiovascular disease risk as once thought. Additionally, there has been research on fatty acids having different physiological effects in the body, regardless of their structural classification as saturated or unsaturated. Nonetheless, many health authorities, such as the USDA and American Heart Association, continue to advocate for reductions in saturated fat intake.
These health authorities not only recommend limiting saturated fat, but also replacing it with unsaturated oils, the most common of which are those rich in omega-6 polyunsaturated fatty acids. For example, the USDA’s 2015 Dietary Guidelines for Americans list of healthy oil examples includes canola, corn, peanut, safflower, soybean, and sunflower oils. The American Heart Association lists soybean oil, corn oil, and sunflower oil. The preference of polyunsaturated fats over saturated fats stems from the diet-heart hypothesis and evidence that doing so lowers blood cholesterol levels. According to the diet-heart hypothesis, this reduction in cholesterol should also reduce rates of heart disease.
Several controlled trials have directly investigated the diet-heart hypothesis by altering dietary fat intake between groups and observing the development of adverse cardiovascular events or dying from heart disease and all causes. The results of these trials have been inconsistent. Several meta-analyses of these trials have also reported conflicting findings, likely owed at least in part to which trials were included in the analyses and how they were categorized.
Another reason for the discrepancies between meta-analyses and controlled trials may be the intervention used in the controlled trials. Ideally, a controlled trial would isolate the variables of interest without affecting other aspects of the participants’ lives. However, many of the controlled trials to date have substantial differences between the intervention groups that were not related to saturated fat or omega-6 polyunsaturated fat intake. Examples include giving advice to only one of the intervention groups to limit trans-fat intake from margarines, shortenings, and partially hydrogenated oils, recommending increased consumption of omega-3 polyunsaturated fats from fish and seafood, and advice to consume more plant-based foods and limit sugar intake.
Without adequate control for confounding, many of the controlled trials testing the diet-heart hypothesis and, consequently, the meta-analyses aggregating their results may not accurately represent the effect of replacing saturated fat with omega-6 polyunsaturated fat on heart disease risk. Accordingly, the study under review was a meta-analysis that sought to determine the effect of replacing saturated fat with omega-6 polyunsaturated fat on heart disease risk after accounting for study quality.
The diet-heart hypothesis states that saturated fat increases blood cholesterol levels and therefore increases the risk of heart disease. Several controlled trials have tested this hypothesis and reported conflicting results. However, many of the controlled trials’ interventions involved more than simply altering the saturated fat to polyunsaturated fat ratio, which could confound the results. The study under review is a meta-analysis that sought to examine the effect of replacing saturated fat with omega-6 polyunsaturated fat on heart disease risk after accounting for study quality.
Other Articles in Issue #32 (June 2017)
Can chondroitin save knee cartilage?
Chondroitin’s mixed results for slowing the progression of osteoarthritis may be due to the low-quality or lower-dose chondroitin used in some studies. Looking at the structural effects of higher-dose, pharmaceutical-grade chondroitin could shed more light on its efficacy
Protein, fast and slow
Fast-digesting whey helps with post-workout muscle protein synthesis, and some studies suggest slower-digesting casein reduces muscle protein breakdown. This opens the possibility that their combination could be better than either alone
Timing protein before bed for gains
Sleep is one big fast, which could put muscles’ protein balance into the red. Could taking slow-acting casein before bed put the balance back into the black overnight?
Interview: Matthew Dalby, PhD
Dr. Dalby’s research explores the links between diet, obesity, and the microbiome. In this interview, we discuss fecal transplants, the role of animal models in research, and more.
Should 1000 IU be the new RDA for vitamin D?
Since it was set in 2010, the 600 IU vitamin D RDA has been widely circulated. But a close look at individual patient data may give a more accurate estimate of vitamin D needs.
Better performance with nitrate supplementation?
Nitrates are one of the few supplements that consistently show promise for athletic performance. But should you take them acutely or chronically for best effect?
Interview: Kenneth Brown, MD
Dr. Brown is an accomplished gastroenterologist who has conducted clinical trials on supplements and drugs for GI conditions. We pick his brain here