It’s not every day that a heart diet controversy is truly settled. But earlier this month a report in the British Medical Journal solved the cold case of a nearly 50-year old heart study with a hidden, and crucial, finding. Better still, a landmark study published this week sharpens the point.
In 1966 the Sidney Diet Heart Study, named after the city in which it originated, randomly assigned 448 middle-aged men with a prior heart attack to either eat as they chose, or to eat a diet low in trans fats and cholesterol but rich in omega-6 polyunsaturated fats (primarily safflower oil). For seven years the researchers tracked heart attacks and deaths in both groups. The results shocked everyone: despite a sharp drop in cholesterol 6% more men died in the diet group, suggesting that 1 out of 18 died because of the diet. But the 1978 report offered no information about the causes of the deaths, and experts have often discounted the study as a potential fluke, arguing that men in the diet group may have died from unrelated, non-cardiac issues.
There is heated controversy over what constitutes a heart friendly diet. The American Heart Association recommends increasing intake of omega-6 fatty acids, while some researchers have suggested that doing so may increase heart attacks. Moreover, the widely accepted science impugning trans (‘saturated’) fats, and supporting polyunsaturated fats, is based largely on observational studies in which researchers observe diet and other habits among large groups. These studies often raise tantalizing questions, but they offer dubious answers. One can never tell if such findings represent the chicken or the egg: Does eating more fish lead to heart health, or do heart healthy people eat more fish? This kind of confounding explains why study results about the dangers or benefits of certain foods come and go with the wind, cycling like fashion trends.
Less trumpeted, however, is a small core of reliable answers from randomized trials of diet. The best known example may be the Lyon Diet Heart Study which showed dramatic effects of a Mediterranean diet (think Lyon, France). The Mediterranean diet, studied in hundreds of heart patients over nearly a decade, emphasizes fruits and vegetables, grains, white meat over red meat, and olive oil. Compared to an AHA diet focused on lowering cholesterol, the Mediterranean diet prevented deaths for 1 in 30 heart patients, and heart attacks for 1 in 18. That makes it three times more powerful than a statin drug for heart patients. Moreover, in an unprecedented effort, researchers from Spain this week published a paper in the New England Journal of Medicine showing that the same diet works equally well in men and women attempting to prevent a first heart attack or stroke.
Unfortunately, years-long, rigorous diet trials like the Lyon and Spain study are rare. An excellent example, however, would be the Sidney study.
Christopher Ramsden, a National Institutes of Health investigator and a researcher at the University of North Carolina, has been compiling data from diet trials for years. To determine once and for all the effect of omega-6 fatty acid diets Ramsden reached out to Boonseng Leelarthaepin, a young research assistant at the study’s inception and now the only surviving researcher from the Sidney group. Improbably, Leelarthaepin managed to locate the 9-track tapes containing the study data, and after converting the obsolete format into readable information, Ramsden’s group extracted the causes of death for each subject in the study.
The results are both remarkable and instructive: blood tests during the study showed that cholesterol and triglyceride levels dropped substantially in the diet group, precisely the intended effect. But the final outcome was a 6% increase in fatal heart problems—accounting completely for the difference in survival. The Sidney diet, using an exclusive increase in omega-6 fatty acids, increased coronary and cardiac deaths.
What do the findings mean for a heart healthy diet? First, diets rich in both omega-3 and omega-6 fatty acids—together—have, paradoxically, reduced heart attacks in randomized trials, and the Mediterranean diet is the most powerful, dependable example. With the study published this week we now have formidable proof that this works not just for those with heart problems but also for those at lowest risk, patients who have never had a heart attack or stroke.
The success of combining the two polyunsaturated fats, however, raises the question of whether omega-3 fatty acids may be the beneficial ingredient and omega-6 the dangerous one. Sadly, supplementing a diet with omega-3 fish oil pills has failed to provide beneficial effects on heart health in dozens of studies, (though the trials were short, averaging about two years). Thus until larger, longer trials can separate out the critical effects of each element, it appears that polyunsaturated fats should be taken in the form of food—not pills—and consumed together, not alone. Most importantly, exclusively increasing omega-6 fatty acids while lowering fats and cholesterol, is likely to be dangerous.
It took nearly a half century to get them, but these are answers about eating for heart health that are likely to last even longer.