Vitamin C

Vitamin C, also commonly called ascorbic acid, is a water-soluble micronutrient essential in the diet for humans, although most other mammals can readily synthesize it. Vitamin C’s ability to easily donate electrons makes it a highly effective antioxidant. It can effectively scavenge reactive oxygen species, reactive nitrogen species, and many other free radicals. It protects lipids by disabling free radicals and by aiding in the regeneration of vitamin E. As an antioxidant, vitamin C may protect white blood cells, LDL-cholesterol from oxidation, and the lungs from damage due to cigarette smoke and the air pollutant ozone.

In addition to its role as an antioxidant, vitamin C is a required part of several enzymes like signaling molecules in the brain, some hormones, and amino acids. Vitamin C is also essential for the synthesis and maintenance of collagen.

Collagen is the most abundant protein in the body and is used for different functions such as the structure of ligaments, tendons, blood vessels, and scars that bind wounds together. Vitamin C acts as the glue that holds the collagen fibres together and without sufficient levels of vitamin C in the body, collagen strands are weak and abnormal (Figure 11.2 “The role of vitamin C in collagen synthesis”).

Vitamin C is essential for the synthesis of DNA, bile, neurotransmitters such as serotonin, carnitine (which helps to transport long-chain fatty acids into the mitochondria), and hormones such as thyroxin, epinephrine, norepinephrine, and steroids. Vitamin C also plays a role in enhancing iron absorption, regenerating vitamin E after it has been oxidized, and it may play a role in reducing nitrosamines, which are cancer-causing agents found in cured and processed meats.

Vitamin C levels in the body are affected by the amount in the diet, which influences how much is absorbed and how much the kidney allows to be excreted, such that the higher the intake, the more vitamin C is excreted. Vitamin C is not stored in any significant amount in the body, but once it has reduced a free radical, it is very effectively regenerated and can therefore exist in the body as a functioning antioxidant for many weeks.

Dietary Reference Intakes for Vitamin C

The RDAs and ULs for different age groups for vitamin C are listed in Table 11.4 “Dietary reference intakes for vitamin C”. They are considered adequate to prevent scurvy. Vitamin C’s effectiveness as a free radical scavenger motivated the Institute of Medicine (IOM) to increase the RDA for smokers by 35 milligrams, as tobacco smoke is an environmental and behavioral contributor to free radicals in the body.

Dietary Sources of Vitamin C

Citrus fruits are great sources of vitamin C and so are many vegetables. British sailors in the past were often referred to as “limeys” as they carried sacks of limes onto ships to prevent scurvy. Vitamin C is not found in significant amounts in animal-based foods.

Because vitamin C is water-soluble, it leaches away from foods considerably during cooking, freezing, thawing, and canning. Up to 50 percent of vitamin C can be boiled away. Therefore, to maximize vitamin C intake from foods, you should eat fruits and vegetables raw or lightly steamed. For the vitamin C content of various foods, see Table 11.5 “Vitamin C content of various foods”.

Consuming Too Little Vitamin C

A vitamin C deficiency is rare, but the classic condition associated with vitamin C deficiency is scurvy. The signs and symptoms of scurvy include skin disorders, bleeding gums, painful joints, weakness, depression, and increased susceptibility to infections. Scurvy can be prevented by having an adequate intake of fruits and vegetables rich in vitamin C.

Those consuming too little vitamin C are also at risk of anemia.

Consuming Too Much Vitamin C

High supplemental doses of vitamin C have been reported to cause numerous problems, but the only consistently shown side effects are gastrointestinal upset and diarrhea. To prevent these discomforts, the UL for adults is 2,000 milligrams per day (greater than twenty times the RDA).

At very high supplemental doses in combination with iron, vitamin C has sometimes been found to increase oxidative stress, reaffirming that getting your antioxidants from foods is better than getting them from supplements, as that helps regulate your intake levels. There is some evidence that taking vitamin C supplements at high doses increases the likelihood of developing kidney stones, however, this effect is most often observed in people who already have multiple risk factors for kidney stones.