What should—and shouldn’t—we eat? A confusing and sometimes conflicting mass of information seems to arise continually from new research studies, media reports, and popular fads (think gluten-free) related to our diet. The 2014 Consensus Conference on Nutrition, co-sponsored by the Canadian Academy of Health Sciences and the World Heart Federation, was convened to bring together Canadian and international experts on how diet and food policies affect cardiovascular disease globally to highlight both the extent and limitations of our current knowledge. The conference was hosted by World Heart Federation President-Elect and recent Canadian Medical Hall of Fame inductee Salim Yusuf.
Using evidence to guide ourselves towards healthier diets requires not just examining what we eliminate from our diets, but also considering what source of calories we substitute in its place (or else we lose weight – usually a good thing in itself, but a separate issue). Walter Willett, Professor of Epidemiology and Nutrition at Harvard University’s School of Public Health reviewed our current understanding of evidence on dietary fats and oils. He recalled that in decades past, U.S. national dietary recommendations had appropriately emphasized reducing saturated fat consumption, but recommended replacing them with carbohydrates – a strategy that leads to increased risk of cardiovascular disease, as it turns out. The solution instead is to substitute healthier fats for less healthy ones. Along these lines, the greatest diet-based reductions in coronary disease risk have been associated with replacing trans fats with unsaturated fats, as demonstrated perhaps best in the Nurses’ Health Study.
But some confusion has arisen over conflicting conclusions from a recent high-profile systematic review of dietary fats and coronary disease, in which the authors’ bottom line was that the evidence does not conclusively support current guidelines that encourage higher consumption of polyunsaturated fats and lower consumption of saturated fats. Dr. Willett critiqued these findings, emphasizing the strong influence of one outlier, the Sydney Heart Study, in which all fats were replaced with sunflower oil (which has no omega-3 fatty acids) and a margarine high in trans fats. In contrast, he emphasized that studies of diets including a combination of omega-3 and omega-6 polyunsaturated fats have shown more homogeneous evidence of benefit, as do omega-3s alone if studies focused on specific omega-3 fats are pooled with studies looking at combinations of omega-3s.
Is cholesterol bad for us? Ronald Krauss, Senior Scientist and Director of Atherosclerosis Research at Children’s Hospital Oakland Research Institute addressed this from the perspective of how genetic analyses have been able to predict clinical effects of therapeutic interventions on disease biomarkers. Genes that influence LDL cholesterol are associated with cardiovascular disease risk, consistent with benefits documented for treatment that lower LDL. Similarly, genes that influence HDL cholesterol are not associated with cardiovascular disease risk, consistent with the failure of treatments that elevate HDL to show consistent reductions in cardiovascular events. Even for LDL, it may be more complex than we realize to shape our diets to address cardiovascular risk. Lowering saturated fat consumption mainly reduces large LDL particles. Lowering carbohydrate consumption, in contrast, reduces the more atherogenic small and very small LDL particles – independently of saturated fat consumption. Dietary efforts directed against saturated fat therefore may not entirely capture the cardiovascular risk related to cholesterol.
From where should we get our protein intake? Adam Bernstein, Research Director of the Wellness Institute at the Cleveland Clinic, discussed that evidence for associations with red meat consumption and cardiovascular disease has been inconsistent. More compelling, however, have been findings that substituting red meat consumption with nuts, poultry, legumes or fish is associated with reduced mortality. Daan Kromhout, Professor of Public Health Research at Wageningen University, The Netherlands, reviewed evidence that fish consumption is associated with reduced mortality from coronary disease. However, the benefit is seen only with fatty fish, not white fish, and seems to plateau at a level of one serving per week – higher levels of consumption have not shown greater benefits. A potential down side is that fatty fish also contribute relatively high amounts of calories from saturated fat as well as their polyunsaturated fat content. Eggs may be OK—a recent systematic review found that egg consumption at a relatively high average rate of one per day was not associated with coronary disease or stroke, although the large Physicians’ Health Study and Atherosclerosis Risk in Communities (ARIC) studies observed a small associated increase in the incidence of heart failure. Adding further confusion to the story regarding both fish and eggs, Andrew Mente, Assistant Professor of Clinical Epidemiology & Biostatistics at McMaster University, reviewed data from 3 large epidemiologic studies showing variable results regarding protective effects of these foods depending on the presence or absence of baseline cardiovascular disease and on the region of the world being studied. Such studies have typically not captured important potential confounders such as the types of fish, cooking methods, or variation in nutritional composition of the same foods based on geography. David Jenkins, Professor and Canada Research Chair in Nutrition and Metabolism at the University of Toronto, discussed findings that dietary patterns that source proteins from plants are associated with lower risk of coronary disease and diabetes mellitus. Studies of a dietary portfolio constructed on this basis have shown reductions in LDL cholesterol similar in magnitude to that achieved by statins.
The need to focus on dietary portfolios, whole foods and whole diets rather than single nutrients or supplements was another key theme emerging from the discussion. One of the best validated diets to date is the Mediterranean diet, characterized by a predominance of olive oil, fruit, nuts, vegetables, and cereals and moderate amounts of fish, poultry and wine. Miguel Mártinez, Professor and Chair of Preventive Medicine and Public Health at the University of Navarra, Spain, reviewed results of landmark studies that have shown the cardiovascular benefits of the Mediterranean diet, in particular the recent PREDIMED randomized trial, which he led. This large study observed a 30% reduction in the hazard of major adverse cardiovascular events with a Mediterranean diet.
While we in wealthy developed countries may obsess about how diet affects our own health, we must not forget the rest of the world, where rapid changes in economic development are being accompanied by changes in the types of foods available and with this, rising trends of diet-related problems such as cardiovascular disease and obesity. Dariush Mozaffarian, Associate Professor of Epidemiology at Harvard University’s School of Public Health, reviewed recent evidence on global disease burdens showing that 6 of the 20 leading risk factors for disease are dietary, the greatest of which is low fruit consumption. However, because available foods and dietary cultural preferences vary dramatically from one country to another, we cannot assume that recommendations for dietary health derived from Western countries with Western lifestyles should apply the same way globally. Research initiatives are beginning to address this issue. Another Harvard School of Public Health Research Scientist, Vasanti Malik, described work she is doing with the Global Nutrition and Epidemiologic Transition (GNET) study, which is conducting focus groups in different countries to assess the cultural acceptability of different food substitutions, such as replacing refined white rice with whole-grain brown rice, which has been associated with reduced risk of diabetes.
On the final day of the conference, the participating experts met in a working group to craft summary statements arising from the meeting, chaired by Dr. Willett. This workshop was held in camera, but I spoke with Dr. Willett beforehand about future directions for health policy regarding dietary advice for healthy populations. The U.S. Food and Drug Administration is at long last moving towards eliminating trans fats, a development that CMAJ encourages emulation of in Canada. The next battle, according to Dr. Willett, may be over health-related differences in subtypes of carbohydrates (added sugars, high v. low glycemic index, whole grains v. refined products). On this front, if New York City’s experience is any indication, there will be plenty more debate and controversy to come.