Dietary fat is the primary cause of heart disease. A healthy diet should be low in all fats.
The relationship between dietary fat and heart disease is nuanced. Unsaturated fats (olive oil, nuts, fish) are beneficial. Trans fats are harmful. Saturated fat's role is more complex than originally thought. The low-fat dietary advice led to increased consumption of refined carbohydrates, which contributed to obesity.
What changed?
Ancel Keys was a formidable scientist and a formidable personality. His Seven Countries Study, launched in 1958, tracking 12,770 men across Finland, Greece, Italy, Japan, the Netherlands, the United States, and Yugoslavia, became the most influential nutrition study of the 20th century, and the foundation of a dietary consensus that shaped what Americans ate for four decades.
Keys's conclusion was clear: saturated fat raises blood cholesterol, elevated cholesterol causes heart disease, therefore nations eating less saturated fat have lower heart disease rates. The data from Mediterranean countries like Greece and Italy, where olive oil and fish dominated diets and heart disease rates were low, seemed to confirm this. Japan, where dietary fat was low and heart disease rates were very low, seemed to confirm it further.
The McGovern Senate Select Committee on Nutrition and Human Needs translated Keys's research into the first federal dietary goals in 1977: reduce fat consumption to 30 percent of calories, reduce saturated fat to 10 percent. The USDA Food Pyramid that followed in 1992 placed fats at the narrowest tip, "use sparingly", while grains, intended as the foundation of the healthy American diet, occupied the broad base. Six to eleven servings daily.
The food industry responded with breathtaking speed. If fat was the enemy, fat needed to be replaced. The replacement was sugar and refined carbohydrates. Low-fat cookies, low-fat yogurt, fat-free salad dressings, and reduced-fat peanut butter crowded grocery shelves. Snackwell's cookies, fat-free, plenty of sugar, became a cultural phenomenon. Food manufacturers removed fat, added sugar to maintain palatability, and put "low fat" health halos on the packaging.
What no one had adequately studied was whether replacing fat with carbohydrates improved health outcomes. It did not. Obesity rates, which had been relatively stable through the 1970s, began climbing sharply through the 1980s and 1990s. By 2000, one-third of American adults were obese, up from roughly 15 percent in 1978.
The critique of the fat hypothesis began building in the nutrition research community through the 1990s. Walter Willett at Harvard's School of Public Health published mounting evidence that the fat-heart disease link was far weaker than Keys had suggested, that type of fat mattered enormously, and that refined carbohydrates were their own cardiovascular risk factor. The 2015 Dietary Guidelines Advisory Committee removed the dietary fat limit for the first time in nearly four decades. The experiment in population-scale dietary engineering, conducted without the clinical evidence to justify it, had ended, but its effects on the American diet, and on American waistlines, are still being worked through.
