Carotenoids

Every photosynthetic tissue makes them, though green chlorophyll usually masks the colour until autumn. The concentrated dietary sources are mostly fruits, roots and flowers:

Saffron is the informative outlier: its pigments are cleaved carotenoid fragments carrying sugars, which makes them water-soluble where the rest of the class is not.

Carotenoids split cleanly by whether they carry oxygen, and the distinction predicts where they end up in the body:

Carotenes

pure hydrocarbons: beta-carotene, lycopene

Xanthophylls

oxygen-bearing: lutein, zeaxanthin, astaxanthin, capsanthin

Only a subset — those retaining an unsubstituted beta-ionone ring — can be cleaved to vitamin A. Beta-carotene can; lycopene and lutein cannot.

The provitamin-A carotenes are the dietary route to vitamin A for most of the world's population, and their deficiency remains a leading cause of preventable childhood blindness.

Lutein and zeaxanthin accumulate selectively in the macula, where they filter blue light and are the basis of the AREDS2 formulation for age-related macular degeneration — one of the better-evidenced nutrient interventions in ophthalmology. Because all of these are fat-soluble, absorption depends heavily on eating them with fat, and on cooking, which breaks the cell walls that trap them.