A low-fat diet is the healthiest way to eat and prevent heart disease.

The low-fat dogma had remarkable staying power. From its institutionalization in the 1977 Dietary Goals through the fat-free food boom of the 1990s, the idea that a healthy diet was primarily a low-fat diet shaped everything: grocery product formulations, school lunch programs, hospital meal trays, medical advice, and the quiet guilt that accompanied eating cheese or avocado at a time when both were coded as nutritional failures.

The theory had intuitive appeal. Fat contains nine calories per gram compared to four for carbohydrates and protein. Fat is fat; eat fat, accumulate fat. The biochemistry was obviously more complicated, but the simplicity of the message was part of its power. It could fit on a food package. It could animate a federal dietary guideline. It could sell Snackwell's cookies.

What the low-fat consensus failed to address was what people would eat instead. When fat was removed from food, something had to replace the calories, texture, and palatability it provided. The answer was almost always refined carbohydrates and sugar. In eliminating fat from the American diet, the dietary guidelines inadvertently engineered a massive increase in glycemic load.

The clinical evidence for low-fat diets never matched the policy confidence placed in them. The Women's Health Initiative Dietary Modification Trial, the largest randomized controlled trial of a low-fat diet ever conducted, involving nearly 49,000 women over eight years, found no significant reduction in cardiovascular disease, breast cancer, or colorectal cancer in the low-fat intervention group compared to controls. The trial results, published in JAMA in 2006, were received with genuine surprise.

Meanwhile, Mediterranean diet studies were producing consistent evidence of cardiovascular benefit from a dietary pattern explicitly high in fat, olive oil, nuts, fatty fish. The PREDIMED trial, published in the New England Journal of Medicine in 2013, found that a Mediterranean diet supplemented with extra-virgin olive oil or nuts reduced major cardiovascular events by approximately 30 percent compared to a low-fat control diet.

The 2015 Dietary Guidelines for Americans quietly removed the total fat restriction that had governed federal nutrition policy since 1977. The guidelines noted that fat type, not fat quantity, was the relevant variable. Unsaturated fats support cardiovascular health; trans fats are harmful; saturated fat's role is more nuanced. The food supply had spent decades optimizing for the wrong variable.