Where plasma levels stop rising
Vitamin C absorption and plasma concentration are tightly regulated. Pharmacokinetic studies in healthy adults found that plasma levels approach saturation at oral intakes around 200 to 400 mg per day, and that the fractional absorption falls as the dose climbs. Past the saturation point, the kidneys clear the surplus, so much of a large dose is simply excreted in urine.
This regulation is why ordinary dietary amounts, spread across the day, fill the body's pool effectively and why very large single doses give diminishing returns.
What saturation implies for megadosing
Because the body resists raising plasma vitamin C much beyond saturation, gram-level supplements mostly add to urinary output rather than tissue stores, and high doses can cause digestive upset and, in susceptible people, contribute to kidney stone risk. The popular idea that very high doses prevent the common cold is not well supported; routine supplementation does not appear to prevent colds in the general population.
This is general educational information, not medical advice. People considering high-dose vitamin C, especially with kidney concerns, should consult a clinician.