“Correlation does not prove that there is a causal relationship”
(Tim Chico, cardiologist)
Correlation but not causation
Armitage says the cholesterol theory is science-based. He admits, however, that it is difficult to draw direct conclusions about heart disease from nutritional research. In randomized, controlled trials that represent the gold standard, saturated fat in the diet is replaced with polyunsaturated fats, such as those found in vegetable oil. It lowers LDL cholesterol in the blood. Interestingly, this has not resulted in any reduction in mortality in most studies. Many other nutritional studies on heart disease are observational. They are based on dietary questionnaires that participants fill in by memory and as such have limitations. “These approaches give a general indication of which foods are associated with heart disease,” says Tim Chico, a cardiologist at the University of Sheffield in the UK. “However, this correlation does not prove that there is a causal relationship,” he adds.
Many conflicting findings could indicate that saturated fat was overestimated and that other dietary components that contribute to heart disease may be overlooked. Four years before Keys appeared on the cover of Time Magazine, British physiologist John Yudkin stated that the real danger to human health was sugar. At the time, his discoveries were largely ignored. However, in 2016 it found that influential 1960s research downplaying the role of sugar in coronary heart disease was funded by the sugar industry.
Shortly after this discovery, the results of the Prospective Urban Rural Epidemiology (PURE) study suggested that diets high in carbohydrates, not high in fat, could drastically reduce life expectancy. The authors found no association between high fat consumption and the incidence of heart attacks or cardiovascular disease. On the contrary, the study found that a diet high in saturated fat reduces the risk of stroke by about 20 percent. “The new data shows that refined sugars, not fat, are probably the main problem in our diets,” said study leader Mahshid Dehghan, a nutritionist at the Population Health Research Institute in Hamilton, Canada. However, the PURE study is also questionnaire-based and therefore subject to the same limitations as many other observational studies.
DuBroff would not call himself an absolute cholesterol skeptic. Blaming all the blame for bad cholesterol paints an incomplete picture at best. “Focusing solely on LDL is oversimplifying a very complex disease process,” says the cardiologist. He points to a study co-authored by Ravnskov in which people with the highest LDL cholesterol levels lived longer than those with the lowest. A 2019 study also suggests that levels of a specific subset of LDL may be a better predictor of a heart attack than total LDL present. More research is needed to clear up the confusion about the effects of cholesterol, says DuBroff. Other biochemical mechanisms and dietary components can also cause heart disease, such as insulin resistance and inflammation.
Although critics questioned his life’s work, the approach seemed to work for Keys. He died in 2004 at the age of 100 after eating the Mediterranean diet for most of his life. Lots of olive oil, starchy foods and vegetables, low-saturated animal fats – Chico also recommends this to his patients. He also thinks it’s okay if they don’t eat pasta and want to maintain a low-carb diet. “Why does it always have to be either / or?” He asks. “I’d like to see a more constructive discussion of how we’re tackling the many impacts on heart disease, rather than holding a popularity contest.”