Dietary fat

Dietary fat supplies about nine calories per gram, more than twice protein or carbohydrate. It comes as saturated, monounsaturated and polyunsaturated fats, and two polyunsaturated fatty acids — linoleic and alpha-linolenic acid — are essential, meaning the body cannot make them.

It matters because fat is not optional. It carries the fat-soluble vitamins A, D, E and K, forms the structure of every cell membrane, and provides the substrate for steroid hormone production, including testosterone. Cutting it very low for long periods is a genuine problem rather than an aggressive diet tactic.

In practice, the composition matters as much as the amount: replacing saturated fat with unsaturated sources measurably improves lipid profiles, while the very-low-fat advice of the 1980s has not held up. A sensible floor sits around a fifth to a quarter of total calories for most people.