Scientists don’t know anything about diet and health … says the BMJ

John Ioannidis, the scourge of poor biomedical science, has shown the great unreliability of most studies linking nutrition to disease and mortality,7 and perhaps we fail to recognise the complexity of relations between diet and disease when we pick out single components, whether it’s total fat, saturated fat, trans fats, sugar, or salt.

Attributing disease or mortality to diet is scientifically difficult. Associations are first made through observational studies, but recording exactly what people eat is hard. We eat very varied diets, and maybe over time our diets change. Then converting our diet into components of fat, carbohydrate, protein, and the like is unreliable. So to make a link between diet recorded over a short period of time and diseases and deaths encountered perhaps decades later is inevitably difficult.

Then intervention trials are unreliable. Unlike with a drug trial, where there will be one variable (taking or not taking the drug), trials of diet include more than one variable: for example, a diet of less fat probably means more carbohydrate so as to supply enough energy. Adherence is an important problem in drug trials but a much bigger problem in trials of diets, as people may find it very difficult to follow an unfamiliar diet. Also, the trials are usually short term and rarely include hard outcomes such as cardiovascular events or deaths.

Despite continuing doubts, it became, and still is, the global orthodoxy that saturated fat was an important cause of cardiovascular disease and that people should eat low fat diets. The biggest test of the saturated fat hypothesis came with the Women’s Health Initiative, which enrolled 49 000 premenopausal women in a randomised trial of the low fat diet and cost $725m (£460m; €580m).17 The women were followed for 10 years, and those in the low fat arm successfully reduced their total fat consumption from 37% to 29% of energy intake and their saturated fat from 12.4% to 9.5%. But there was no reduction in heart disease or stroke, and nor did the women lose more weight than the controls. [link]

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