Anthocyanins

Only six anthocyanidin aglycones account for the great majority of what occurs in nature; the thousands of named anthocyanins are these six wearing different sugars:

Cyanidin

the most widespread; red-magenta

Delphinidin

blue-purple, and the pigment behind true-blue flowers

Pelargonidin

orange-red, as in strawberry and radish

Peonidin, petunidin and malvidin

the methylated members; malvidin dominates red grape

The colour shifts with acidity — red in acid, purple near neutral, blue-green in alkali — which is why red cabbage changes colour in a pan and why the same pigment paints both a strawberry and a cornflower.

Anthocyanins are the textbook case of a compound whose measured bioavailability makes no sense against its measured effects. Recovery of intact anthocyanins in plasma and urine is famously poor — often well under one percent of the dose.

The resolution is that the parent pigment is mostly not what acts. Gut bacteria degrade anthocyanins to smaller phenolic acids, protocatechuic acid chief among them, and those metabolites circulate at far higher concentrations and for far longer. Any account of anthocyanin pharmacology that stops at the intact pigment is missing most of the story.

The best-supported effects are vascular. Anthocyanin-rich extracts improve endothelial function and capillary integrity, which is the basis of bilberry and blackcurrant preparations in venous insufficiency and capillary fragility.

The wartime story that bilberry sharpened RAF night vision was disinformation, and controlled trials have not supported an acute night-vision effect — worth knowing, because the claim is still repeated. What bilberry does have is reasonable evidence in diabetic retinopathy and microvascular protection. Elsewhere, tart cherry anthocyanins have small trials in exercise recovery and sleep, and cranberry's are incidental to its proanthocyanidin activity.