Vitamin C and your health: C for crucial, C for controversial

Content provided by the Faculty of the Harvard Medical School

Vitamin C and your health: C for crucial, C for controversial

Call it ascorbic acid, and it sounds like a fierce industrial solvent that should be stored under lock and key. Call it vitamin C, and it sounds like something that belongs in every man's medicine chest. Call it an essential part of the human diet, and you'll be right. Call it a superfluous supplement, and you'll also be right. Call it what you will, but understand why vitamin C is essential for your health and why a good diet makes supplementary doses unnecessary.

Soluble C

Humans are unique members of the animal kingdom. Among other things, we are one of the few species that require vitamin C. That requirement was first demonstrated in 1747, but the vitamin itself was not discovered until 1932. Since then, scientists have learned an enormous amount about vitamin C, but they have not answered all the questions about it.

Vitamins are organic chemicals that are essential for the body's metabolism. Because humans can't produce any of the 13 recognized vitamins except vitamin D, vitamins must be obtained from food to prevent disease. Only tiny amounts are required, however, and a balanced diet can provide adequate doses of all the vitamins. The question is not whether men need vitamins (we do), but whether extra doses provide extra benefits.

Like the eight B vitamins, vitamin C is water soluble. It's easily absorbed from food but can be removed from food by boiling, especially if large amounts of water are used; prolonged storage of fresh fruits and vegetables can also reduce their vitamin C content, but microwaving and cooking with small volumes of water won't cut the C in food. Once in the body, vitamin C makes its way into the fluid component of all the tissues and cells. But unlike fat-soluble vitamins, it is not stored in the body to any appreciable degree. Instead, excessive amounts quickly pass out in the urine.

See what it can do

Vitamin C has many functions. It starts working right in the intestinal tract, where it promotes the absorption of iron. It helps produce healthy collagen, the protein that is the backbone of connective tissue. As a result, it is essential for healthy skin and blood vessels, good wound healing, and sound bones. And in the brain and nervous system, vitamin C is essential for the production of neurotransmitters, chemicals that convey messages between nerve cells. Vitamin C has a role in the immune system, especially by enhancing the function of the white blood cells that gobble up invading bacteria. In the endocrine system, it appears to improve glucose (sugar) metabolism. And it also affects prostaglandins and nitric oxide, chemicals that have major roles in blood clotting, vascular function, and lung function.

Any one of these roles would qualify vitamin C as essential. But the function that has attracted the most interest is vitamin C's role as an antioxidant. In fact, it is one of the most powerful and versatile antioxidants in nature. Acting on its own, it can protect DNA from oxidative damage, perhaps reducing the risk of cancer, protecting against memory loss and various neurological diseases, and possibly even slowing the aging process itself. Vitamin C helps protect LDL ("bad") cholesterol from oxidative damage — and since oxidation puts the "bad" into "bad cholesterol," it might reduce the risk of heart disease, stroke, and peripheral artery disease. And vitamin C also helps regenerate vitamin E, boosting supplies of another powerful antioxidant.

It's an impressive array of functions; still, vitamin C's only proven therapeutic role is to prevent scurvy — but indirect evidence suggests it may do a lot more.

C on the high seas

Scurvy is rare in the Western world, but it wasn't always that way. The disease was rampant in men who sailed the seas in the 15th–18th centuries, the great era of exploration. Accomplished seafarers, the British were particularly hard hit, developing the bleeding and rotten gums, skin rash, swollen joints, muscle weakness, and profound fatigue that characterize the disease. In 1740, for example, Admiral George Anson set out to circle the globe with six ships. He returned four years later with only one, having lost almost 1,400 men, many to scurvy.

Dr. James Lind, a Scottish ship's surgeon, set out to rectify this problem. While sailing aboard HMS Salisbury in May 1747, he conducted the first controlled clinical trial in medical history. The subjects were 12 sailors who all had "putrid gums, the spots and lassitude, with weakness of their knees" — in other words, scurvy. Lind divided the sailors into six groups of two and gave each pair a different daily supplement: A quart of cider, two spoonfuls of vinegar before each meal, half a pint of sea water, elixir of vitriol, a paste of garlic, mustard seeds, and herbs, or one lemon and two oranges. Within just six days, the sailors who ate the citrus fruits were fit enough to return to duty. But despite his success, Dr. Lind experienced the same frustration as many of today's scientists: It took 48 years for the bureaucrats of the British Admiralty to add citrus fruits to ships' rations.

Does high C help?

It doesn't take much vitamin C to prevent scurvy. That's why many countries recommend as little as 30 mg a day, a third of the amount recommended for American men. And even though vitamin C isn't stored in the body, people who have been getting even modest dietary amounts have enough in their systems to prevent any signs of deficiency for at least two weeks of complete vitamin C deprivation.

A little vitamin C is more than good, it's essential. But is more better?

That depends on your standards. Population surveys from around the world have studied the relationship between vitamin C and disease. Some estimate vitamin C consumption; others measure the actual amount of the vitamin in the blood. Although there are some dissenting results, many of the studies link high levels of vitamin C to a reduced risk of heart attacks, strokes, memory loss and dementia, and impaired lung function in smokers. The results of cancer studies are more variable. For example, in 2006 the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial reported that a high consumption of vitamin C did not protect men from prostate cancer. All in all, however, a high consumption of vitamin C has generally been associated with reduced mortality rates.

Studies like these have made vitamin C a best-selling supplement. But they don't prove vitamin pills are helpful. Far from it. For one thing, the dissenting research includes some large studies such as Harvard's Health Professionals' Follow-Up Study as well as a 2004 trial that linked a high consumption of vitamin C from supplements to an increased risk of heart disease in diabetics. More important, the hopeful results are all derived from observational studies, not clinical trials. Observational studies can never establish a cause-and-effect relationship — and they can sometimes be wrong. For example, observation suggested that hormone replacement therapy could help postmenopausal women and that vitamin E and beta carotene could help both men and women — but randomized clinical trials showed just the opposite.

It seems likely, even probable, that a high level of vitamin C is a marker for a good diet and a healthy lifestyle, but it is not necessarily protective in its own right. Fruits and vegetables are the main dietary sources of vitamin C, but they provide other beneficial nutrients as well. People who eat lots of fruits and vegetables also tend to eat fewer high-fat, high-salt, animal-based and processed foods, which have a negative impact on health. And since smoking reduces vitamin C levels, the diseases linked to low levels may result from tobacco, not deficiencies of the vitamin.

To date, no clinical trials of vitamin C and heart disease or cancer have been completed, though several are in progress. And the trials of vitamin C and respiratory infections have been negative.

Until scientists finish the randomized trials started by Dr. Lind more than 250 years ago, we can conclude that a high consumption of vitamin C from foods is very beneficial (see table), and that supplementary doses are unlikely to help people who eat well.

Should C cause concern?

Small amounts of vitamin C are essential for good health, but are large amounts harmful? High doses increase the urinary levels of oxalate and uric acid, two chemicals that can cause kidney stones. It's a theoretical risk, but actual cases of vitamin C–related stones are rare and have been limited to people with kidney disease. Large doses of vitamin C can cause nausea, indigestion, and diarrhea, but these side effects are uncommon and temporary. People with a certain enzyme deficiency (glucose-6-phosphate dehydrogenase, or G6PD) can develop anemia from high doses, but this, too, is uncommon. A much more common, but much less serious, problem is that large amounts of vitamin C can interfere with lab tests for sugar in the urine or blood in the feces.

Although there are few well-documented clinical side effects of vitamin C, some experimental results counsel caution. Although normal amounts of vitamin C have a powerful antioxidant action, large amounts may promote oxidative damage. In a well-publicized experiment, British scientists gave a daily dose of 500 mg of vitamin C to 30 volunteers; after six weeks, their blood cells showed increased traces of oxidative damage to DNA. Similarly, an American team reported that high levels of vitamin C enhance the production of DNA-damaging substances called genotoxins. And several small studies have linked high doses of supplements to an accelerated thickening of the carotid artery, a marker of atherosclerosis; other studies, however, show the reverse.

All this research is preliminary, and none of it proves that vitamin C is harmful, even in large amounts. But it does provide some support for that common-sense standby, moderation.

Counting Cs

After reviewing the latest information about vitamin C, in 2000 the National Academy of Sciences revised the recommendations that had been in effect since 1989. They increased the recommended daily allowance (RDA) from 60 mg a day to 90 mg for men and 75 mg for women. They also suggested an extra 35 mg for smokers of either gender. Finally, the panel set 2,000 mg a day as the maximum safe daily dose of vitamin C.

It's the new standard, but it doesn't mean you should settle for 90 mg a day, much less push to 2,000 mg. Experts from the National Institutes of Health suggest that 200 mg a day is the best amount — and they also suggest you get it from foods rather than supplements. In fact, the widely publicized five servings of fruits and vegetables a day should do the trick. A simple daily multivitamin that provides 100% of the RDA for vitamin C is surely safe. Doses above 500 mg a day may be unwise, especially for people with excessive iron levels, kidney disease, or kidney stones. And since vitamin C can mask blood in the stool, you should cut down on it for three days before if you're having fecal occult blood tests to screen for colon cancer.

Vitamin C is crucial for good health. A diet that includes lots of fruits and vegetables is the best way to get the C you need. Although the jury is still out, modest supplementary doses appear safe but are unlikely to help, and large doses may do more harm than good. This advice cuts through the confusion and controversy. As usual, your mother got it right: Eat your fruits and vegetables.

Last updated:	September 05, 2008