Carotenoid — Glossary

Carotenoids are a large class of fat-soluble pigments synthesised by plants, algae, and some fungi and bacteria, responsible for yellow, orange, and red colouration in many fruits, vegetables, and flowers.

Structurally, carotenoids are tetraterpenoids built from eight isoprene units, giving a characteristic 40-carbon backbone with an extended system of conjugated double bonds. This conjugated chain is what absorbs visible light and produces the pigments' distinctive colours. The class divides into two broad groups: carotenes, which are pure hydrocarbons (such as alpha- and beta-carotene and lycopene), and xanthophylls, which carry oxygen-containing functional groups (such as lutein, zeaxanthin, and astaxanthin).

In plants, carotenoids serve two primary roles in photosynthesis: they act as accessory light-harvesting pigments that transfer absorbed energy to chlorophyll, and they quench excess light energy and reactive oxygen species to protect the photosynthetic apparatus from oxidative damage. This photoprotective function is intrinsic to plant physiology and does not, of itself, confer equivalent effects in other biological contexts.

Some carotenoids are provitamin A precursors. Beta-carotene, for example, is cleaved in the intestinal mucosa by the enzyme beta-carotene 15,15'-monooxygenase (BCMO1) to yield retinal, which the body converts to retinol (vitamin A). Conversion efficiency varies considerably between individuals and is influenced by dietary fat intake, food matrix, and genetic polymorphisms in the BCMO1 gene. Not all carotenoids are provitamin A compounds; lycopene, lutein, and zeaxanthin, for instance, have no provitamin A activity.

Carotenoids are found in concentrated form in foods such as carrots, sweet potato, tomatoes, capsicum, dark leafy greens, and egg yolk (where they are deposited from the hen's diet). Because they are fat-soluble, absorption from food is generally improved when consumed alongside dietary lipids. In the context of phytomedicine research, carotenoids are studied for their photochemical properties and their role as precursor compounds; their precise physiological roles in human biology remain an active area of investigation.

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