Vitamin C: what it does, how much you need, deficiency and foods rich in it

What vitamin C does

Vitamin C, or ascorbic acid, is a water-soluble vitamin that the human body cannot make, so it depends on what you eat. According to EFSA, it acts as a cofactor for several enzymes, and that is where its functions come from: it is essential for making collagen, the protein that acts as the “scaffolding” of skin, gums, bones, cartilage and blood vessels; it is needed to produce carnitine, which carries fats into the mitochondria to be used for energy, and catecholamines such as noradrenaline; it helps convert cholesterol into bile acids, and it neutralises reactive oxygen species as part of the body’s antioxidant network. It also makes iron from plant foods easier to absorb. Without vitamin C, the collagen scaffolding weakens, which is why a lack of it shows up in the gums and in wounds before anywhere else.

How much vitamin C you need a day

In 2013 EFSA calculated how much vitamin C is needed to replace what the body uses each day and keep its stores nearly full (a blood concentration of about 50 µmol/L). These are its population reference intakes:

Other institutions use different figures because they start from a different criterion: the UK NHS says adults aged 19 to 64 need 40 mg a day. It is not a contradiction but a different question: EFSA aims to keep stores nearly full, not just to avoid deficiency. In practice it hardly matters which you use as a reference, because the average intake of European adults from food alone is already between 69 and 130 mg a day in men and between 65 and 138 mg in women, according to the data compiled by EFSA itself.

Foods with the most vitamin C (mg per serving)

Values from USDA FoodData Central with typical servings. Fruit and vegetables are raw unless stated; cooked vegetables are boiled and drained:

Three things that change the maths. First: two servings from the top of the table already take you past EFSA’s 110 mg, without supplements. Second: orange is a good source, but raw red pepper has more than twice as much per 100 g (128 versus 53 mg), and a mandarin only gives 20 mg. Third: apples and bananas, the most common fruit in many homes, provide very little, so they should not be your only fruit of the day. To check the calories of other foods, see the food calorie table.

How much is lost in cooking

Vitamin C is sensitive to heat and dissolves into cooking water. USDA’s own data show this when you compare the same food raw and boiled, per 100 g:

• Broccoli: 89 mg raw and 65 mg boiled (27% less).
• Brussels sprouts: 85 mg raw and 62 mg boiled (27% less).
• Cauliflower: 48 mg raw and 44 mg boiled (8% less).
• Spinach: 28 mg raw and 10 mg boiled (65% less).

The loss depends on the food and how it is cooked, but the practical conclusion is the same: eat some vegetables raw (pepper, tomato, cabbage in a salad), steam the rest or cook it briefly with little water, and do not leave cut fruit out for hours. Even boiled, broccoli and Brussels sprouts are still among the best sources per serving.

Deficiency symptoms and who is most at risk

Severe deficiency is scurvy, rare today in places with access to fruit and vegetables, but not gone. Its symptoms come from faulty collagen:

• Swollen, tender gums that bleed easily.
• Bruises that appear at the slightest knock, because of fragile blood vessels.
• Wounds and cuts that take a long time to heal.
• Intense tiredness, weakness and irritability.
• Joint and muscle pain.

What is more common is low status that falls short of scurvy. In the US NHANES 2003-2004 survey (7,277 people tested), 7.1% were deficient in vitamin C (below 11.4 µmol/L in the blood). The risk was higher in people on lower incomes and, above all, in smokers: adult smokers had about one third less vitamin C in their blood than non-smokers. People on very restrictive or monotonous diets, with hardly any fruit or vegetables, are also at higher risk. Recognising yourself in one of these groups does not mean you are deficient, but it is a reason to pay more attention to eating vitamin C sources every day.

Vitamin C and iron: it helps, but less than people say

Iron from plant foods (non-haem iron, found in pulses, spinach, tofu or cereals) is absorbed less well than iron from meat, and vitamin C makes it easier. In the classic study by Cook and Monsen (1977), with 63 men and a test meal, adding 25 mg of vitamin C multiplied iron absorption by 1.65, and adding 1,000 mg by 9.57; when the meal contained meat, the increase was much smaller. Hallberg (1989) also showed that vitamin C partly offsets the brake that phytates in cereals put on absorption. The important caveat came later: when Cook and Reddy (2001) measured absorption over 5 days on a complete diet, going from 51 to 247 mg of vitamin C a day did not significantly change average absorption. In other words, the effect is real, but in a real diet it is much smaller than in a single meal. Pairing pulses with pepper, tomato or fruit for dessert is a good habit, especially on a vegetarian diet; if you have anaemia, what corrects it is the treatment your doctor prescribes. More in the guide on iron deficiency.

Vitamin C and colds: what the evidence says

It is the most entrenched myth, so it is worth looking at the numbers. The Cochrane review by Hemilä and Chalker (2013) pooled placebo-controlled trials that used at least 200 mg a day:

• Prevention: in 29 comparisons with 11,306 participants, taking vitamin C daily did not reduce the risk of catching a cold in the general population (relative risk 0.97).
• Duration: in people already taking it daily, colds lasted 8% less in adults and 14% less in children.
• Treatment: starting it at the first symptoms showed no consistent effect on duration or severity.
• The exception: in 598 marathon runners, skiers and soldiers on subarctic exercises, the risk of catching a cold was halved (relative risk 0.48).

Translated into real life: a 7-day cold in an adult who takes vitamin C daily would last, on average, a little over half a day less. It is a real but small effect, and it does not justify taking supplements all year round to avoid colds.

Supplements and megadoses: more is not better

Most people do not need to supplement: food covers vitamin C with room to spare. And high doses have a clear ceiling. In Levine’s study (1996), with volunteers kept in hospital for months, absorption was complete with 200 mg in a single dose; at 1,000 mg a day the blood was already saturated, from 500 mg in a single dose absorption fell and what was absorbed was excreted in urine, and the authors concluded that above 400 mg a day there was no evident benefit. If you ever consider a supplement, for example on a diet very low in vegetables, discuss it with your doctor or pharmacist, and remember that a supplement does not fix a poor diet.

Renzy does not measure vitamin C: from a photo of your plate it gives you an estimate of calories and macros that you can correct. But that same photo lets you see, at the end of the week, whether pepper, fruit and vegetables really appear in your meals or stay a good intention. With the table above, two well-chosen servings a day are enough.