Vitamin C: Benefits, Absorption, and How Much You Need

TL;DR

• Vitamin C is authorised in the EU to support normal collagen formation, normal immune function, protection of cells from oxidative stress, and reduction of tiredness and fatigue.
• Humans can't make their own vitamin C, so intake has to come from food or a supplement.
• Taking vitamin C with plant based iron raises how much of that iron you absorb.
• Plasma saturates around 200mg a day for most people. Above roughly 1,000mg a day the extra is mostly excreted.

What vitamin C does

Vitamin C (ascorbic acid) is a water soluble vitamin and antioxidant with a wide range of jobs. It's the cofactor for the enzymes that stabilise collagen, so without enough of it your body can't build collagen properly. It's also needed to make carnitine, which moves fat into mitochondria for energy, and to convert dopamine into norepinephrine.

Humans are one of the few mammals that can't synthesise vitamin C. A change in the GULO gene removed that ability in the primate line around 60 million years ago, described by Nishikimi et al. (Journal of Biological Chemistry, 1994). That's why a steady dietary intake matters across your whole life.

Under EU Regulation 432/2012, vitamin C is authorised to contribute to normal collagen formation for the normal function of skin, bones, cartilage, gums, teeth, and blood vessels, normal function of the immune system, protection of cells from oxidative stress, normal energy yielding metabolism, and the reduction of tiredness and fatigue. It also increases iron absorption.

Vitamin C and iron absorption

This is one of the most useful everyday applications. Hallberg et al. (1986) showed that vitamin C taken alongside plant based iron sources increases non haem iron absorption several fold. For someone with low ferritin who eats little or no meat, having a vitamin C source at the same meal as iron rich food is a simple lever. It's also an authorised claim under EU regulation.

What research has looked at

What follows is context on the research base, not a set of claims about outcomes for you.

Hemila and Chalker (Cochrane review, 2013) pooled 29 randomised trials covering more than 11,000 people. Regular supplementation didn't stop colds happening in the general population. The reviewers noted a shorter average cold duration, and a larger effect in people under heavy physical stress such as marathon runners.

Pullar et al. (Nutrients, 2017) reviewed vitamin C's role in skin and collagen, noting that even moderate insufficiency reduces the hydroxylation steps that give new collagen its structure.

If you have specific health concerns, talk to your doctor before changing anything.

Blood markers that indicate a need

Plasma ascorbic acid below 23 micromol/L indicates deficiency. Between 23 and 50 micromol/L suggests suboptimal status. Levine et al. (1996) found plasma saturates around 70 micromol/L at intakes of 200mg or more per day, and that intake above that threshold is largely excreted.

Low ferritin alongside a plant heavy diet is an indirect reason to make sure vitamin C is present at meals, so iron absorption is as high as possible.

Genetics and vitamin C

SLC23A1 and SLC23A2 encode the transporters that move vitamin C from the gut into the blood and from blood into cells. Cahill and El-Sohemy (2011) found that variants in these transporter genes were linked to lower plasma vitamin C for the same dietary intake. Carriers may need a higher intake to reach the same blood level.

How it fits into a personalised plan

At Get Body Data, we see vitamin C status track closely with diet and stress. It's a common inclusion because of its role in collagen, iron metabolism, and immune function. We calibrate the dose to your plasma ascorbic acid level and your absorption, which is partly set by your SLC23A variants. For most adults 200 to 500mg a day holds plasma saturation. Doses above 1,000mg a day raise urinary oxalate, which matters for anyone with a history of kidney stones.

Frequently asked questions

Is more vitamin C always better?

No. Once your plasma is saturated, extra vitamin C is mostly passed in urine. Very high daily doses also raise oxalate excretion, which is a consideration if you're prone to kidney stones.

Should I take vitamin C with food?

If your goal includes better iron absorption, yes, take it with the meal that contains your iron. Otherwise timing is flexible.

Does vitamin C prevent colds?

The research doesn't support that for most people. Regular intake has been studied mainly in the context of cold duration and in people under extreme physical stress. Read it as research background rather than a promise.

What foods are high in vitamin C?

Peppers, blackcurrants, kiwi, citrus, strawberries, broccoli, and Brussels sprouts are all strong sources. Vitamin C degrades with heat and storage, so raw or lightly cooked keeps more of it.

Disclaimer

This article is for information only. It isn't medical advice. Statements about the role of vitamin C in the body refer to authorised health claims under EU Regulation 432/2012. Research citations describe the scientific literature and are not claims about outcomes for individual readers. Always talk to your doctor before starting or changing a supplement plan.

References

Cahill LE, El-Sohemy A. Haptoglobin genotype modifies the association between dietary vitamin C and serum ascorbic acid deficiency. Molecular Nutrition and Food Research. 2011;55(10):1477-1483.

Hallberg L, Brune M, Rossander L. Effect of ascorbic acid on iron absorption from different types of meals. Human Nutrition: Applied Nutrition. 1986;40(2):97-113.

Hemila H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews. 2013;(1):CD000980.

Levine M, Conry-Cantilena C, Wang Y, et al. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. Proceedings of the National Academy of Sciences. 1996;93(8):3704-3709.

Nishikimi M, Fukuyama R, Minoshima S, Shimizu N, Yagi K. Cloning and chromosomal mapping of the human nonfunctional gene for L-gulono-gamma-lactone oxidase. Journal of Biological Chemistry. 1994;269(18):13685-13688.

Pullar JM, Carr AC, Vissers MCM. The roles of vitamin C in skin health. Nutrients. 2017;9(8):866.