Bioavailability

Bioavailability is the proportion of a nutrient or compound that actually reaches your circulation in a usable form. Swallowing a hundred milligrams of something rarely means a hundred milligrams arrives where it is needed.

Several steps take their cut. The compound has to survive stomach acid and digestive enzymes, be absorbed across the gut wall, and pass through the liver, where first pass metabolism can remove a large share before it ever reaches general circulation.

That is why some supplement claims fail at the first hurdle. Collagen is digested into amino acids like any other protein rather than travelling intact to your skin; deer antler velvet is sold on an IGF-1 content that would be digested if swallowed; and swallowed ATP does not become cellular ATP.

Form and context change it substantially. Iron from meat is absorbed far better than iron from plants, and vitamin C alongside improves the plant form. Fat soluble vitamins need fat in the meal. Curcumin is famously poorly absorbed, which is why it is usually sold with piperine to slow its breakdown.

This is also where a lot of premium pricing lives. Fancier salt forms, liposomal delivery and similar claims are sometimes justified and often marginal, and the useful question is whether better absorption of that compound has been shown to produce a better outcome rather than a higher blood level.

Higher is not automatically better either. For nutrients with a plateau, and most have one, improving absorption above sufficiency changes nothing.

In practice, judge a supplement on outcome evidence rather than on absorption claims, and take fat soluble vitamins with a meal containing fat.