More is not better

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Stored or flushed out

EVIDENCEGUIDE 1 · REVIEW 3 · RCT 1

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Extra nutrients you take in do not simply vanish. Where they go depends on which exit the body has left open for them.

Flushed out by the kidneys: vitamin C dissolves in water. In an inpatient study of 7 healthy men, blood plasma was fully saturated at about 1000 mg a day; with single doses of 500 mg or more, the share absorbed fell, and most of what was absorbed left in the urine.

Stored in the liver: vitamin A dissolves in fat. Most of the body's supply is kept as retinyl esters in small lipid droplets inside the liver's stellate cells. Any excess piles up there too: sources note that taking roughly 10 times the recommended amount or more over the long term can lead to chronic toxicity after months to years, harming the liver.

Almost no exit: the body loses only about 1–2 mg of iron a day, mostly as cells are shed from surfaces such as the skin and the gut lining, and that loss is not meaningfully adjusted to how much iron the body holds, so the body's iron total can be regulated almost only by controlling absorption at the gut entrance (bleeding and menstruation take some extra). A child who swallows iron-containing products is an emergency: get medical help or call poison control right away.

So when you meet a nutrient, one question is worth asking first: does the body store it, and how does it get out?

Educational content, not medical advice. See a clinician about your own case.

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