In what would become known as the Seven Countries Study, Keys, the epidemiologist Henry Blackburn, and their colleagues recruited groups of middle-aged men for a long-term project not only in Finland, but also in the United States, Japan, Italy, the Netherlands, Greece, and Yugoslavia. Each subject in the study was asked questions about his diet and given a battery of physical tests. Duplicates of everything they ate were collected, frozen, and sent to a University of Minnesota lab for analysis. Then, at five-year intervals, the study checked in on the subjects again. A pattern soon emerged: The farther north the men lived, the more animal products they consumed and the more heart attacks they suffered. In Greece and Italy, where people ate mostly a plant-based diet, men were largely free of heart disease—an observation that eventually informed our understanding of the value of the traditional Mediterranean diet. (Keys has been criticized for omitting government data on diet and heart disease from certain countries that he compared early on. But Keys had good reason to leave out the data: Death certificates were undependable, and World War II had disrupted the food supply in those countries.) In places like North Karelia (the study’s northern extreme), conversely, men were 30 times more likely to die of heart attacks than in places like Crete. In fact, North Karelian men on average were dying 10 years earlier than their counterparts in the south. It got so bad that, by 1972, North Karelian men achieved the dubious distinction of having the highest rate of heart attacks in the world.

To Puska and the researchers, the roots of the disease were clear. Before World War II, North Karelian men were largely lumberjacks whose diets revolved around hunting game, picking berries, and fishing. Besides the occasional bear mauling, their main health concerns were tuberculosis, infectious diseases, and death at childbirth. After the war, veterans, as part of their compensation, were given small plots of land. Lacking agriculture skills, they cleared the land to raise pigs and cows. Predictably, pork and diary consumption skyrocketed. Butter soon made its way into almost every meal: butter-fried potatoes, buttered bread. Even traditional fish stew was half butter. They had fried pork or meat stew for dinner, chased with buttered bread and milk. Vegetables were considered food for the animals. Adding to the problem, GIs had returned home with a new habit: By 1972, more than half of all men smoked.

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I interviewed Puska in his Helsinki office on a cold June afternoon not long ago. He had recently retired as the director of noncommunicable-disease prevention for the World Health Organization; wall photos of him with heads of state and diplomas recorded a stellar career. “In my wild youth I was very active in student politics,” he told me, gesturing expansively. Now 68 years old, he looked decidedly non-wild, wearing a bureaucrat’s khaki pants and crooked knit tie, but still retained Steve McQueen good looks with limpid blue eyes and sandy brown hair. “That was the time for thinking you could change the world.”