Vitamin C, also called ascorbic acid, is an essential water-soluble vitamin that the human body cannot manufacture and has to take in from food. It acts as a cofactor for the enzymes that build stable collagen and as an antioxidant in the water phase of blood and cells. With no large storage depot, levels follow recent intake rather than lifetime habits. Prolonged lack of it causes scurvy, the first nutrient deficiency ever treated by diet.
What it measures
Vitamin C is a nutrient rather than a reading, but it has a lab test. Chemically it is an electron donor, and that one property explains most of what it does: it keeps the iron atoms inside several enzymes in working order. Two of them hydroxylate collagen strands, which is why connective tissue, blood vessel walls, gums and wound healing fail first when it runs out. Others build carnitine and norepinephrine. It also regenerates oxidized vitamin E and keeps dietary iron in the form the gut can absorb. A fuller reference on the vitamin itself is Vitamin C (ascorbic acid).
The test, when ordered, is plasma or serum ascorbic acid from a blood draw. It is not a routine panel item: the sample degrades quickly in light and warmth, and the result reflects what you ate in the last day or two more than your long-term status. Leukocyte ascorbate tracks tissue stores better but is mostly a research measure. In practice a deficiency is recognized from diet history and physical signs, not from a number.
Typical values
| Figure | What is commonly published | Notes |
|---|---|---|
| US recommended daily intake, adults | 90 mg for men, 75 mg for women | National recommendations differ; several countries set lower targets |
| Smokers | An extra 35 mg a day on top | Turnover is higher, so the same diet leaves less |
| Amount that prevents scurvy | On the order of 10 mg a day | Far below the intake that fills tissues |
| US tolerable upper intake level, adults | 2,000 mg a day | Above this, loose stools and stomach upset become common |
| Plasma ascorbic acid | Read against the interval printed on your own report | Deficiency is generally defined below roughly 0.2 mg/dL, but cut-offs vary by lab and assay |
Why it matters for longevity
The floor matters more than the ceiling. Real deficiency still occurs, mostly with very restricted diets, heavy alcohol use, malabsorption and in isolated older adults, and it reverses once intake is restored. Above that floor the picture changes. In observational cohorts, higher plasma vitamin C is associated with lower cardiovascular disease and lower all-cause mortality, but it is also one of the best markers of how many plants a person eats, so the association is hard to separate from the diet behind it.
Randomized trials of supplements in well-nourished adults have generally not shown fewer cardiovascular events, less cancer or longer life. Regular supplementation does not prevent colds in the general population; it shortens them slightly. The antioxidant role is oversold: the body runs many overlapping defenses against Oxidative stress and flooding one of them with a pill has not translated into outcomes.
What changes it
Food moves it easily. Bell peppers, broccoli, brussels sprouts, kiwi, berries, citrus, tomatoes and potatoes all carry meaningful amounts, and a couple of servings a day covers most people. The vitamin is heat-sensitive and dissolves into cooking water, so long boiling costs you much of it while raw, steamed or quickly cooked portions keep more.
Small regular amounts beat occasional large ones: the fraction absorbed falls as the dose rises and the kidneys clear the rest, so plasma plateaus however much you swallow. Smoking, alcohol use disorder, dialysis and malabsorption all lower status. Vitamin C taken with a plant-based meal improves Iron absorption, which helps if your iron is low and does nothing if it is not. High doses raise urinary oxalate, which matters if you form kidney stones, and can distort some home glucose meters.
Related reading
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.