A low-fat diet prevents heart disease.

For decades, official advice equated dietary fat with heart disease, driving a boom in 'low-fat' products (many of which replaced fat with sugar). The strong version of the claim is that cutting total fat prevents cardiovascular disease.

The Women's Health Initiative dietary trial (Howard et al., 2006) randomized nearly 49,000 women to a low-fat diet or usual eating for about eight years. It found no significant reduction in coronary heart disease, stroke, or overall cardiovascular disease. Later evidence shifted the focus from total fat to type: replacing saturated fat with unsaturated fat (not with refined carbohydrates) does help the heart. So the specific 'eat low-fat to prevent heart disease' claim is not supported; what matters is fat quality, not just quantity.