Vitamin C Levels: What a Blood Test Says About This Antioxidant

Written by Dr. Ryan Kneessi, ND

Quick answer: A vitamin C blood test measures ascorbic acid in plasma or serum and mostly reflects intake over the past few weeks, because the body stores little of it. Low levels are more common in people who smoke or eat few fruits and vegetables. In a large English cohort, people with the highest levels had about half the mortality risk of those with the lowest.

Humans are one of the few mammals that cannot make their own vitamin C, so every bit of it has to come from food or supplements. The body also stores very little of it. That makes a vitamin C blood test one of the more direct windows into recent eating habits, and it explains why levels can change within weeks when someone eats more or fewer fruits and vegetables. I already have an article on who might consider more vitamin C, and this one focuses on the lab marker itself.

This article is educational. It explains what the marker reflects and how researchers connect it to the deeper mechanisms of aging, so you can have a more informed conversation with your own provider.

What does a vitamin C blood test show?

A vitamin C test measures ascorbic acid in plasma or serum. Because vitamin C is water-soluble and the body keeps only a modest pool, the result mostly reflects intake over the past few weeks. Blood handling matters too. Vitamin C breaks down quickly once blood is drawn, so samples need to be processed and protected promptly, and a surprisingly low result can sometimes reflect sample handling rather than the person.

In the national survey described below, the average level in the United States was about 51 micromoles per liter, and deficiency was defined as a level below 11.4 micromoles per liter. Levels between those two points are often described as low or marginal. Very low levels over time are linked to scurvy, whose early signs include fatigue, easy bruising, bleeding gums and slow wound healing.

There is a ceiling to how high plasma vitamin C can go from oral intake. The gut absorbs less as single doses get larger, and the kidneys release the extra into the urine. A high result usually reflects recent supplements or a diet very rich in fruits and vegetables. Very large doses can raise urinary oxalate, which matters for people prone to kidney stones.

How is vitamin C connected to aging and root causes?

Vitamin C touches several of the core processes researchers associate with aging. It is a key water-soluble antioxidant, and it works hand in hand with vitamin E by recycling it after vitamin E has neutralized an oxidized fat in a cell membrane. When vitamin C runs low, that whole antioxidant network carries more of the load, and oxidative damage to DNA, proteins and fats can build up faster.

Vitamin C is also a required helper for the enzymes that build collagen, the protein framework of skin, blood vessel walls, gums, bones and joints. That is why gum and skin changes are some of the first signs of a serious shortfall. Collagen changes with age, and vitamin C status is one of the factors that shape how well connective tissue keeps up its normal repair.

Immune cells concentrate vitamin C at levels many times higher than plasma, which points to its role in immune function and in the balance between helpful and chronic inflammation. Vitamin C is also involved in making carnitine, which carries fats into mitochondria for energy, and in enzymes that help regulate how genes are switched on and off, an area of research that is still developing.

From a root-cause view, the most common drivers of a low level are simple. Low intake of fruits and vegetables is the biggest. Smoking and secondhand smoke use up vitamin C faster. Ongoing illness, infection or inflammation, heavy alcohol use, some restrictive diets, and conditions that limit absorption can also pull levels down, while genetics and body size add individual variation.

What does the research show about vitamin C levels?

The clearest picture of how intake relates to blood levels comes from a study at the National Institutes of Health. In a 1996 depletion and repletion study in the Proceedings of the National Academy of Sciences, Levine and colleagues kept 7 healthy young men in the hospital for four to six months on a diet with less than 5 mg of vitamin C a day, then gave them seven increasing daily doses from 30 to 2,500 mg until their levels stabilized. Plasma levels rose steeply between 30 and 100 mg a day, 200 mg was the first dose past the steep part of the curve, and plasma was completely saturated at 1,000 mg a day. White blood cells were saturated at 100 mg a day and held at least 14 times more vitamin C than plasma. Absorption was complete for a single 200 mg dose but fell at single doses of 500 mg and above, and oxalate excretion rose at 1,000 mg a day. With only seven men, the results may not apply to women, older adults or people who smoke.

How common are low levels? In a 2009 analysis in the American Journal of Clinical Nutrition, Schleicher and colleagues measured serum vitamin C in 7,277 people aged 6 and older in the nationally representative NHANES 2003 to 2004 survey. The average was 51.4 micromoles per liter, and about 7.1 percent of people were deficient, down from about 13 percent in the survey a decade earlier. Adult smokers had levels about one-third lower than nonsmokers, women ran higher than men, and deficiency was more common at lower income levels. Recent supplement use or adequate dietary intake lowered the risk. This was a cross-sectional snapshot, so it describes who tends to run low without showing what happens to them later.

The most widely cited long-term study is EPIC-Norfolk in England. In a 2001 study in The Lancet, Khaw and colleagues measured plasma vitamin C in 19,496 men and women aged 45 to 79 and followed them for about four years. People in the top fifth of vitamin C had about half the risk of dying during follow-up compared with those in the bottom fifth, and the relationship held across the whole range of levels. Each 20 micromole per liter higher level, roughly what one extra serving of fruit or vegetables a day would provide, was associated with about 20 percent lower all-cause mortality, after adjusting for age, blood pressure, cholesterol, smoking, diabetes and supplement use. Because it was observational and followed people for only about four years, plasma vitamin C may partly be a marker of an overall healthier diet and lifestyle rather than the direct cause.

Related reading

If you would like to explore related topics, my articles on who might consider more vitamin C, vitamin E levels and 8-OHdG and oxidative DNA damage go deeper. You can also see the specialty tests I commonly recommend on my functional labs page.

Common questions about Vitamin C

What is considered a low vitamin C level?

In the United States national survey, deficiency was defined as a serum level below 11.4 micromoles per liter, and the average level was about 51 micromoles per liter.

Why do smokers tend to have lower vitamin C?

Smoking increases oxidative stress, which uses up vitamin C faster. In the national survey, adult smokers had levels about one-third lower than nonsmokers.

Can vitamin C levels keep rising with bigger doses?

No. In a careful NIH study, plasma levels were fully saturated at about 1,000 mg a day, absorption fell at single doses of 500 mg and above, and the extra was released in the urine.

Sources

This article is for educational purposes only and is not medical advice. It does not replace an individual evaluation. Talk with your healthcare provider before making changes to your diet, exercise, supplements, or medications, especially if you are pregnant, nursing, or managing a health condition.