Vitamin C: needs, supplements and the limits of health claims

The European Food Safety Authority (EFSA) sets a reference of 110 mg per day for adult men and 95 mg for women, a bit more during pregnancy and notably more during breastfeeding. The US NIH adds 35 mg per day for smokers. These are population-level benchmarks, not prescriptions.

Anses (the French food safety agency) cites blackcurrant, citrus fruits, parsley, and peppers among the best sources; kiwi, strawberries, broccoli, and cabbage also contribute. Vitamin C is water-soluble and heat-sensitive: short cooking, steamed or in the microwave, preserves more of it than long cooking in a lot of water. Eating raw fruit and vegetables every day is often enough to cover needs.

For its role in iron absorption, see the article dietary iron: animal and plant sources.

A severe deficiency causes scurvy: extreme fatigue, bleeding gums, loose teeth, bruising, joint pain, and poor wound healing. These signs are not very specific on their own: fatigue or repeated infections aren't enough to conclude there's a deficiency.

The risk increases with a diet very low in fruit and vegetables, malabsorption, certain chronic illnesses, or smoking. If a deficiency is suspected, a professional weighs diet, symptoms, treatments, and, if needed, a blood test.

The 2013 Cochrane review is the reference synthesis. It reaches three conclusions:

• Prevention: across 29 comparisons covering 11,306 participants, regular intake did not reduce the risk of catching a cold in the general population.
• Duration: regular intake shortened colds by an average of 8% in adults and 14% in children, or about half a day for a week-long cold in adults.
• Treatment: started after symptoms began, vitamin C had no consistent effect on duration or severity.

A different signal appears in people undergoing extreme, brief physical exertion: across five trials covering 598 marathon runners, skiers, and soldiers, the risk of a cold was reduced by about half. This result doesn't transfer to adults in general.

The body tightly regulates its vitamin C content. The higher the dose, the smaller the fraction absorbed, and the excess is eliminated in urine. Taking more pills therefore doesn't reproduce the effect of a varied diet, and a normal biological function doesn't prove that a much higher intake prevents disease in someone already well-nourished.

"Liposomal," "extended-release," powder, or acerola-based forms aren't, on their own, proof of clinical superiority. Compare the amount per serving rather than the slogan.

High doses can cause nausea, abdominal cramps, and diarrhea. The NIH sets the upper limit at 2,000 mg per day for adults. This ceiling isn't a target, and multivitamins, "immunity" products, and fortified drinks add up.

Report your supplements before a lab test or a course of care, especially if the treatment concerns iron, the kidneys, or a chronic illness.

1. 01Clarify the need: a diet low in fruit and vegetables, a documented deficiency, or a current cold.
2. 02Look at your usual intake over a few days.
3. 03Read the label and spot the ascorbic acid present in several products.
4. 04Check your situation: kidneys, iron, pregnancy, treatments.
5. 05Reassess: if a supplement causes discomfort or doesn't meet the need, don't increase the dose.