Catching Your Cs
March 2000 -- Nutrition Notes

By: Andrea Platzman, R.D.
Contributing Editor

  Until about 200 years ago, sailors deprived of fruits and vegetables on long voyages commonly died of scurvy. Subsequent British navy experiments, however, proved that the disease could be prevented if a supply of lemon or lime juice was stowed on board. The active ingredient? Vitamin C.

  Vitamin C not only prevents scurvy, it's an essential nutrient with a variety of functions in the body. According to Mark Kantor, Ph.D., food and nutrition specialist, department of nutrition and food science, University of Maryland, College Park, vitamin C aids in wound healing; prevents periodontal disease; enhances absorption of dietary non-heme iron; acts as an in vivo nitrite scavenger to help prevent formation of carcinogenic nitrosamines; maintains collagen and connective tissue in the body; and acts as the most versatile and effective water-soluble dietary antioxidant.

C's activities

Since Linus Pauling's megadose experiments with vitamin C in the 1960s, researchers have conducted more than 20 clinical trials testing the vitamin's supposed cold-shielding powers. Not one of these trials has found that supplements reduce the chance of catching a cold.

  Evidence does suggest, however, that vitamin C helps keep arteries healthy. Researchers now believe that cholesterol starts clogging heart vessels only after low-density lipoproteins (LDLs) become oxidized. Antioxidants, including vitamin C, could prevent LDL oxidation, and thus prevent atherosclerosis. Also, when blood vessels constrict, they cut off the blood supply to the heart - a number of studies show that vitamin C can prevent this.

  Several studies have found that people with high levels of vitamin C in their blood tend to have higher levels of high-density lipoproteins compared to those with low levels of this vitamin. In addition, vitamin C works in tandem with vitamin E, effectively regenerating it and making it a more effective antioxidant by recharging its free-radical ion in the bloodstream.

  One common medical condition that greatly increases the risk of developing heart disease is diabetes. Since cells take up glucose and vitamin C by the same cellular pathway, the two compete to gain entry, and glucose wins. This leaves the diabetic deficient in vitamin C, more vulnerable to oxidative stress, and ultimately more vulnerable to heart disease. Many nutritionists advise diabetics to increase their vitamin C consumption.

  "The research to date has shown some promise in reduction of risk for cancers of the oral cavity, esophagus, mouth, lung, pancreas, cervix and stomach," says Mark Meskin, Ph.D., R.D., associate professor, department of food, nutrition and consumer sciences, California State Polytechnic University, Pomona. Vitamin C's ability to fight oxidation may be particularly useful in the stomach, where the ulcer bacterium Helicobacter pylori produces oxidizing free radicals. Vitamin C may help prevent cancer by neutralizing these free radicals, and/or by blocking the formation of carcinogenic nitrosamines.

  Oxidation can also damage the eye's retina and lens, causing macular degeneration and cataracts respectively. To ward off such damage, the lens normally contains a high concentration of antioxidants, including vitamin C. Observational research on whether dietary vitamin C helps prevent either disease has yielded mixed results, however.

  Additonally, preliminary evidence of vitamin C's beneficial actions against arthritis, asthma and cognitive impairment exists, but results are inconclusive.

The right dose

  The amount of vitamin C needed to prevent scurvy is only 10 mg per day. However, 10 mg does not saturate all of the body's tissues, while the current RDA for adults - 60 mg (100 for smokers) - increases the body's vitamin C pool. "You can maximize the body's vitamin C pool at intakes of 100 to 150 mg per day. There is little point to taking more," says Meskin. However, adds Kantor, the Food and Nutrition Board is scheduled to release new dietary reference intakes (DRIs) for vitamin C within the next few months, which will probably be in the range of 100 to 200 mg per day.

  "Vitamin C is one of the easiest vitamins to get from the diet. From fruits, vegetables and the many foods that are regularly supplemented with this vitamin, one can often exceed the RDA by several times," says Meskin. "Supplements are not necessary unless one rarely consumes vitamin-C rich foods. Then it is entirely safe take supplements; however, there is no reason to take more than 500 mg per day."

  Because it's water-soluble, vitamin C is relatively nontoxic. However, "high doses of vitamin C, 1 to 15 grams, have been associated with diarrhea and abdominal bloating due to osmotic effects of the undigested vitamin in the intestinal tract," says Kantor. "Also, vitamin C maintains iron in its reduced (ferrous) form, so for people with hemochromatosis, it can increase the risk of iron overload by enhancing iron absorption. People susceptible to kidney disease may be at increased risk for oxalate stones, and should avoid excess vitamin C. Finally, large doses of vitamin C in the stool and urine can interfere with diagnostic tests."

  Although most vitamin C research has focused on its positive aspects, in a study entitled "Vitamin C Exhibits Pro-oxidant Properties," published in the April 9, 1998 issue of the British journal Nature, researchers concluded that taking more than 500 mg of vitamin C daily could cause the vitamin to act as a pro-oxidant, possibly damaging adenine bases in DNA, leading to some forms of cancer and arthritis. But several researchers dismissed this study as inconclusive.

  Researchers still do not know for certain how much vitamin C the body actually absorbs, particularly when a person takes supplements, which vary in bioavailability. For this reason, nutritionists recommend getting vitamin C not just from supplements, but also from dietary sources such as citrus fruits, potatoes, cabbage, cantaloupe and broccoli.

Andrea D. Platzman, a registered dietitian, is a consultant to the food industry and regularly writes for nutrition publications. She earned a master's degree in nutrition from New York University, and has a culinary and business background.

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