Can you get too much vitamin C?

In 1971, biochemist Linus Pauling, winner of the Nobel Prize in Chemistry, advocated for high doses of vitamin C for preventing colds and other illnesses. Even though this notion was roundly rejected by the medical community, some patients still follow it. On National Vitamin C Day, let’s take a closer look at why getting too much of this good thing can be a bad thing.

Vitamin C is water-soluble and not stored in the body, yet you can still be exposed to toxic quantities if you take > 2 g in a single dose. Because vitamin C is sold over the counter as tablets and gummies, in 500 mg or 1,000 mg dosages, simply taking too many could increase the risk of abdominal pain, diarrhea, and nausea. In other words, vitamin C gummies may look like candy, but they certainly are not.

What is vitamin C?

Vitamin C comes in two bioactive forms: ascorbic acid and dehydroascorbic acid. Of note, only the L-isomer of ascorbic acid is bioactive.

Vitamin C offers antioxidant activity and helps with nonheme iron absorption. It also plays a role in peptide hormone biosynthesis, carnitine biosynthesis, and conversion of dopamine to norepinephrine. Carnitine, in turn, plays an important role in energy production at the level of the mitochondria. Furthermore, vitamin C is necessary for connective tissue metabolism, as well as cross-linking. Finally, vitamin C is a component of drug-metabolizing enzyme systems, including the mixed-function oxidase system.

Dietary sources

Vitamin C is found in citrus fruits, tomatoes, potatoes, and green vegetables such as broccoli. The current recommended dietary allowance (RDA) for vitamin C is 90 mg/day for men and 75 mg/day for women, which equates to five servings of fruits and vegetables. Moreover, about 40% of Americans take vitamin C in the form of a dietary supplement. Smoking, pregnancy, hemodialysis, stress, and lactation can boost an individual’s need for vitamin C.

Absorption

Vitamin C is completely absorbed at single doses less than 100 mg. At single doses greater than 1 g, ≤ 50% is absorbed. Moreover, at higher levels of intake, there is increased urinary and fecal excretion of vitamin C.

Deficiency

People who experience vitamin C deficiency can develop scurvy. In the United States, this condition is mostly observed among the elderly and indigent. People with alcohol dependence, who often consume fewer than 10 mg/day of vitamin C, also are at risk for scurvy. Furthermore, adherence to a macrobiotic diet can also increase the risk for scurvy.

Importantly, although uncommon, young people can develop scurvy if they are eating very unbalanced diets.

Vitamin C deficiency can be detected via low plasma or leukocyte levels.

Symptoms of scurvy include fatigue and impaired formation of mature connective tissue. Bleeding dyscrasias can also result at the level of the skin (eg, petechiae and ecchymoses), gums, pericardium, adrenal glands, and peritoneum.

Treatment for scurvy at a dosage of 200 mg/day can help to relieve symptoms after several days. Treatment at even higher dosages (as much us several grams per day) may slightly boost symptom resolution, as well as resolution of upper respiratory tract infection.

Other health benefits of vitamin C

Vitamin C supplementation may help with a variety of conditions, including Chédiak-Higashi syndrome—a rare autosomal recessive disease characterized by recurrent bacterial infections and oculocutaneous albinism—and osteogenesis imperfecta, or “brittle bone disease.” Furthermore, diets rich in vitamin C have been suggested to decrease the incidence of certain cancer types, including esophageal and gastric cancers.

The anticancer effects of vitamin C may be secondary to the prevention of the conversion of nitrites and secondary amines to carcinogenic nitrosamines. Moreover, researchers are exploring the evidence for a pro-oxidative role of parenteral ascorbic acid in the treatment of advanced cancers.

Toxicity

Taking high-dose vitamin C supplements (> 2  g in a single dose) for an extended period might increase the risk of kidney stones, although findings on this topic are mixed. If a person has preexisting kidney disease, renal insufficiency, or kidney stones, it may be best to avoid taking vitamin C supplements.

Another risk of taking high doses of vitamin C on a long-term basis could be iron overload and toxicity. High doses of vitamin C can trigger hemolysis in patients with glucose-6-phosphate dehydrogenase deficiency.

With respect to lab tests, vitamin C dosages of more than 1 g/day can yield false-negative guaiac reactions and alter tests for urinary glucose.

Finally, high vitamin C doses could interfere with the activity of some prescription drugs, including bortezomib in patients with myeloma.

The healthiest approach to getting enough vitamin C would be to rely on dietary sources. Eating five servings of fruits and vegetables per day will provide you with more vitamin C than the RDA. If you do want to take a supplement, remember that only 50% or less of the vitamin C will be absorbed by your body from supplements that contain greater than 1 g. Problems can ensue with regular consumption of more than 2 g in a single dose.