Are avocados good for us? Is dairy in or out? Should we all be drinking red wine? The media supplies us with an endless list of “dos and don’ts” but the challenge is to realize that these recommendations change quickly and often contradict each other.
Prioritizing a healthy lifestyle can become a struggle when we don’t know where to get our information. And this is information that is particularly significant – historically, cardiovascular disease has been the costliest disease in Canada while obesity is estimated to have an economic burden of $4.6 to $7.1 billion annually.
Coronary artery disease refers to blockages in the blood vessels of the heart that can lead to heart attacks. It is the number one cause of death worldwide. Risk factors include high cholesterol, high blood pressure, diabetes, obesity and smoking, all of which are considered modifiable risk factors, with the first four modifiable by diet. Data from the Organization for Economic Cooperation and Development tell us that almost 60 per cent of the population of member countries are overweight, with 40 per cent of these individuals classified as obese. Morbid obesity, in particular, is on the rise, with morbid obesity accounting for more than 70 per cent of total obesity growth in the United States. The Heavy Burden of Obesity, an OECD study, says the increasing number of people who are overweight will curb Gross Domestic Product by an estimated 3.3 per cent on average and that for every dollar spent on preventing obesity, the economic return is up to six dollars. These factors are what make having good data on diet and nutrition so important.
In the scientific world, the “randomized controlled trial” is considered the highest form of primary evidence. It splits study participants into at least two groups – the “control” group, which does not receive the intervention, and the “experimental” group, which does. For example, in a randomized controlled trial examining avocados, the control group would eat its normal diet and the experimental group would consume a diet high in avocados. Who is in which group would be determined by random chance, ensuring that any differences measured between the groups should, in theory, be attributable to the avocados alone. This is considered to be superior to cohort and case-control studies, case series and case reports.
But even with the mighty “randomized controlled trial,” we need to be careful. As doctor, epidemiologist and writer Amitha Kalaichandran puts it, “One study is just one study … if findings from one study were enough to change medical practices and public policies, doctors would be practicing yo-yo medicine where recommendations would change from day to day.” She warns that we must be wary of headlines that state “a study found” (often what we see in headlines about nutrition), as one study alone often is not enough evidence to change behaviours – study results should be replicable. Red meat illustrates the “yo-yo,” garnering a bad reputation over the years, linked to health risks like heart disease and various cancers. A study published in Annals of Internal Medicine in 2019, however, led to news headlines on variations of the general idea that red meat is not so bad. Since then, a number of researchers have questioned the methodology and accuracy of these results, once again pushing us to believe red meats may in fact be bad for us.
Randomized controlled trials are generally resource intensive and difficult to conduct – randomized controlled trials relating to diet, however, are exceptionally difficult. Unlike studies where the randomization is a drug versus a placebo, diets, much like human behaviours, are far more complicated, thus maintaining consistency is more difficult. Given the heterogeneity, it becomes difficult to ascertain whether any differences we may detect are truly attributable to the diet in question. Additionally, these trials often follow subjects for short time periods, making it difficult to extrapolate to long-term outcomes since the sustainability of these diets is called into question. These points are nicely illustrated in a study published in Jama Network Open indicating that 86 per cent of diet trials had a significant discrepancy from the initial registration with ClinicalTrials.gov while this rang true for only 22 per cent of drug trials.
Also important to recognize are the stakeholders involved in making recommendations. For example, the accuracy of recommendations put forward in the latest Canada Food Guide, published in January 2019, was questioned by organizations like the Dairy Farmers of Canada and Alberta Beef Producers. Unsurprisingly, the new guide deemphasizes dairy and pushes plant-based protein for a healthy diet.
In direct contradiction to the greater challenges we face with diet trials compared to drug trials, diet trials receive much less funding. David Ludwig, co-director of the New Balance Foundation Obesity Prevention Centre at Boston Children’s Hospital and a professor of pediatrics at Harvard Medical School, and Steven Heymsfeld, professor and director of the Metabolism and Body Composition Laboratory at Pennington Biomedical Research Centre, Louisiana State University, put it eloquently in their New York Times article: Issues with quality control in diet trials lead to “underinvestment in nutrition research and in how we tackle the mysteries of a healthy diet.” They argue that “nutrition research to prevent disease must have the same quality and rigor as pharmaceutical research to treat disease.”
Despite these challenges, a select few diets have been proven to be beneficial. For example, a study published in the New England Journal of Medicine, effectively showed that a Mediterranean diet, high in extra-virgin olive oil or nuts, is associated with a lower risk of major cardiovascular events, specifically heart attack, stroke or death from a cardiovascular cause, than a reduced-fat diet. This study, overall, is felt to be a high-quality study based on metrics such as the number of participants enrolled (7447 participants) and the long follow-up period (median 4.8 years).
Another example is the Portfolio diet, a plant-based diet that encourages cholesterol-lowering foods including nuts, plant proteins, soluble fibre and plant sterols. While the randomized controlled trial is considered the highest form of primary evidence, the meta-analysis takes it a step further by pooling the results of multiple studies to find trends and draw conclusions based on all available data. The Portfolio diet emphasizes a pattern of eating as opposed to a strict regimen to make it practical and to encourage uptake. A meta-analysis on this diet and cardiovascular disease found that following its recommendations leads to improvements in lowering “bad” cholesterol, blood pressure and 10-year coronary artery disease risk.
Important to remember is that while lifestyle modification is important, some of the roles of medications cannot be completely substituted by changes to lifestyle. For example, statins, the group of medications generally considered first-line therapy for high cholesterol, also have anti-inflammatory effects and protective effects post-heart attack and post-stroke. Beta blockers, medications that reduce blood pressure, have a number of other uses, including improving heart function in those with heart failure, controlling heart rate in those with abnormal heart rhythms, reducing symptoms of chest pain and heart protection after a heart attack. ACE inhibitors, another class of blood pressure medication, also improve heart function in heart failure, protect the heart after a heart attack, and even have protective effects for some people with kidney disease. Thus, while lifestyle modification is always indicated, whether or not supplementation with medications is necessary requires a thorough discussion with your physician.
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