Was it fat or sugar that was hurting hearts in the 1960s?

In the 1960s nobody could say for sure: each side had evidence and each side had critics. One camp was led by Ancel Keys of the University of Minnesota, who pointed at saturated fat; the other by John Yudkin, who pointed at sugar. Keys was puzzled by a pattern: American business executives, presumably among the best-fed people, had high rates of heart disease, while in post-war Europe heart-disease rates had fallen sharply when food supplies shrank. In 1955 he presented a correlation between heart-disease deaths and the percentage of fat in the diet across six countries, and two other epidemiologists heavily critiqued it.

The Seven Countries Study, which Keys directed from 1956, enrolled 12,763 men aged 40 to 59 in 16 groups across seven countries, and it was the world's first multicountry study of how lifestyle and diet relate to heart disease and stroke. Keys travelled widely with his wife Margaret, who tested blood cholesterol, and they sent the samples back to Minnesota. The study suggested that heart attack and stroke risk correlated with total blood cholesterol, and Keys concluded that saturated fats in milk and meat were harmful while unsaturated vegetable-oil fats were beneficial. Of the 12,000 men studied in 1960, he said, those least likely to develop cardiovascular disease lived in Crete.

In London, Yudkin, a British physiologist and nutritionist, had argued since at least 1957 that sugar was dangerous to health, and in 1972 he published Pure, White and Deadly. Both men attracted critics. Yudkin's studies were criticised at the time for failing to account for confounding factors, such as the fact that sugar intake was associated with smoking. Keys's study was criticised by low-carbohydrate advocates for excluding countries that did not fit, and in 2017 the True Health Initiative replied that those criticisms were historical inaccuracies.

Both ideas live on. Health organisations' dietary recommendations today corroborate Keys's narrower hypothesis that replacing saturated fat with polyunsaturated fat reduces cardiovascular disease, while interest in Yudkin's work revived from the late 2000s and his book was republished in 2012.