Dietary is the adjective for everything relating to food eaten — dietary habits, dietary intake, dietary guidelines, dietary fibre. It marks the nutritional perspective on eating: where “food” names the stuff, “dietary” names its relationship to the eater’s health and requirements. The word does heavy lifting in nutrition science and regulation — dietary reference values set the targets, dietary surveys measure what populations actually eat, dietary fibre names the indigestible plant components, and dietary supplements are the regulated category of pills and powders. In labelling law it qualifies claims with legal meaning. Simple as it looks, the adjective carries the whole shift from food as commodity to food as a determinant of health; to call something dietary is to view it through the lens of what it does to the body.