Vitamin C, or ascorbic acid, is essential for human health. Most animals can synthesize vitamin C in their bodies. However, people must obtain it from dietary sources to prevent scurvy, a potentially fatal disease characterized by degeneration of your connective tissues. Although healthy adults only need around 100 mg of vitamin C daily to prevent deficiency, many people take large doses – several grams daily – for vitamin C’s purported health benefits. In some individuals, overconsumption of vitamin C can cause health problems.
Vitamin C is required for the synthesis of collagen, the most abundant protein in your body. It also plays a role in the formation of hormones, amino acids and carnitine, a molecule needed for fatty acid metabolism. Vitamin C is a potent free-radical scavenger, and it supports immune function and improves intestinal iron absorption. Although high-dose vitamin C usually only causes nausea and diarrhea, people with certain medical conditions are at higher risk for toxicity.
Following absorption from your intestine, vitamin C is ultimately metabolized to oxalate, which your kidneys excrete. High urinary oxalate concentration theoretically increases your risk for kidney stones, but a 2004 Harvard Medical School study, involving more than 45,000 men with a history of kidney stones, showed only a slightly increased risk of stone formation with vitamin C doses over 1,000 mg daily. In contrast, the 2011 issue of “International Journal of Nephrology” cited a case of vitamin C-induced kidney failure caused by oxalate accumulation in the kidneys of a 72-year-old man who was taking up to 960 mg of vitamin C daily. The authors also reviewed several other similar episodes.
Vitamin C alters the chemical nature of iron in your diet and makes it more absorbable. This is particularly true of “nonheme” iron, which is typically found in plants and supplements. Since intestinal iron absorption is influenced by a variety of factors, including your body’s need for iron at any given time, vitamin C’s influence does not represent a problem for healthy adults. However, for people with certain genetic conditions, such as hemochromatosis or thalassemia, the consumption of several grams of vitamin C daily could contribute to iron overload.
In 1994, scientists at Arizona State University demonstrated that “megadoses” of vitamin C – 2 g daily for 2 weeks – resulted in delayed insulin response to a glucose challenge in healthy adults, presumably because vitamin C interferes with the absorption of glucose into the pancreatic cells that secrete insulin. This leads to a prolonged period of elevated blood glucose following a meal. The study’s authors suggested this is probably not a problem for healthy people, but it could prove troublesome for diabetics.
Recommendations and Precautions
The recommended dietary allowance for vitamin C – the dose required to avoid deficiency – varies from 40 mg to 125 mg, depending on age, gender, pregnancy and smoking status. Large doses of vitamin C – more than 2 g daily – usually only cause gastrointestinal symptoms for healthy people, and some tolerance to these side effects develops with prolonged use of higher doses. However, if you have kidney disease, diabetes, hemochromatosis or thalassemia, or if you have a history of kidney stones, you are at increased risk for health problems when taking extra vitamin C. Ask your doctor before taking supplemental vitamin C.