Vitamin C
Besides the sniffles, cold and flu season also brings renewed interest in vitamin C, or ascorbic acid as it is also known. Beginning in the 1970s, vitamin C has gained prominence in the research community and among consumers as a nutrient with wide-ranging functions and importance to our health. In 1970, Nobel Prize-winner Linus Pauling spurred public interest in the vitamin with his book, Vitamin C and the Common Cold. Up until that time, vitamin C was only notable for its prevention of scurvy, the disease caused by a deficiency of vitamin C, which had been the scourge of sailors over the centuries. Since Pauling's first publication, the controversy has continued and carried into other areas of research about this vitamin. It's not scurvy or the sniffles that has grabbed the national consciousness, but rather the link between vitamin C and diseases involving oxidative damage.
From a historical perspective, man used the vitamin in disease prevention for over 150 years before knowing of its existence. Scientists now think that scurvy accounted for more deaths on long voyages than any other danger sailors faced. In 1753, British physician James Lind seems to have figured it out. He wrote this prescription to prevent scurvy: "Experience indeed sufficiently shows that as greens or fresh vegetables, with ripe fruits, are the best remedies for it, so they prove the most effectual preservatives against it."
Lind recommended that lemon juice be included in the diet of sailors, though it wasn't until thirty years later that Captain Cook followed his advice on a voyage to the Hawaiian Islands and saved countless lives. Throughout history, doctors had described the deficiency disease in ugly detail, but it wasn't until 1928 that the vitamin was isolated by Szent-Gyorgyi. He named the compound hexuronic acid because of the six carbon atoms in the molecule. Although vitamin C functions as a vitamin, from a chemical perspective it can be classified as a carbohydrate because of its structure.
As with many nutrients, our small intestines absorb more vitamin C when we take in less, and less when we take in more, so that absorption depends mostly on dietary intake. If you are like most people and your vitamin C intake is anywhere from 30 to 180 mg, a range which is from half to three times the RDA, you'll absorb 90 percent. If your intake is less than 30 mg, absorption can be as high as 100 percent, while someone taking vitamin C supplements and getting more than 1,500 mg will only absorb half. Dividing the total daily amount of vitamin C into several doses, less than 1 gram each, increases absorption. Like most vitamins, your body isn't picky about where you get vitamin C; you'll absorb the same amount from an orange as you do from a tablet.
Vitamin C Up Close and Personal
Vitamin C has many important roles in the body, most of which have to do with its antioxidant ability. Since vitamin C is water soluble, it is the most important antioxidant in the watery portions of the body, while carotenoids and vitamin E work their protection in the other areas. As an antioxidant, vitamin C acts to become oxidized itself, thereby protecting other compounds inside the water soluble portion of cells and tissues. In addition, this unique ability allows the vitamin to be regenerated after oxidation, so it can go on to fight free radicals yet again. As mentioned, vitamins C and E work in concert to protect water and fat soluble areas, with the former sparing vitamin E by reducing the tocopherol radical back to its active form at the cellular membrane separating the water and lipid soluble compartments.
In addition to that role, vitamin C helps the body absorb essential minerals such as iron by donating an electron, also called reducing. Iron in foods is present as the ferric form having two electrons, but it has to pick up an electron and change to the ferrous form for our intestines to absorb it. Vitamin C's donation enhances the absorption of iron, and the increase in absorption is proportional to the amount of vitamin C present in the same meal. Most women of childbearing age and many children have trouble getting enough iron, so vitamin C's help is even more important.
Another crucial function is vitamin C's part in the formation of collagen, a protein which serves as the matrix, a sort of mortar between bricks, on which bone is formed, as well as other types of connective tissue such as tendons and ligaments. Vitamin C teams up with the enzyme that starts the chemical reactions that stabilize the collagen structure. Supporting this role for the vitamin, scientists point to the connective tissue problems which occur when a person develops scurvy.
In a newer role that scientists haven't fully explained, vitamin C appears to be involved in one of the body's critical systems for stopping the action of drugs and toxins, as well as in compounds the body makes, such as hormones. The importance of inactivating toxins is easy to understand, but the same function applied to hormones and prescribed drugs may not be. Hormones exert powerful effects, as do prescription drugs, but if they continue their action unchecked, it could prove harmful or even deadly. Besides fighting free radicals, the vitamin may also work in this way to protect against cancer, by detoxifying carcinogens. Scientists think that vitamin C probably helps out in the chemical reactions that convert various substances to a more water soluble state, making them easy to excrete into the urine. As proof, studies have shown that vitamin C deficiency decreases a person's ability to inactivate and excrete various drugs. And the reverse can occur as well-some drugs, such as oral contraceptives and aspirin, interfere with vitamin C metabolism and lower blood levels of the nutrient. Another interesting interaction relates to heart disease; vitamin C changes the structure of cholesterol in the liver as the first step in excreting it.
Vitamin C is also involved in the synthesis of several key compounds. It speeds up the production of adrenal gland hormones, epinephrine (formerly called adrenalin), and norepinephrine, that act to transmit nerve impulses and are involved in the "fight or flight" response. Vitamin C also helps make the compound carnitine, which helps transport long-chain fatty acids across the cell membrane in order to be oxidized for energy. One more for the list is one of the thyroid hormones which controls metabolic rate.
Vitamin C's connection to the thyroid hormone is the reason why physiological stress, such as fever or infection, increases the metabolic rate and hence the need for more vitamin C. It's this relationship that sparked Dr. Pauling's interest in vitamin C and the common cold.
Pauling recommended taking a dose of vitamin C that exceeds the RDA by 1,600 percent at the first sign of a cold. Studies to prove the connection yielded mixed results, with some showing a modest effect of the vitamin and others showing nothing. The lack of consistent results led most researchers to conclude that Pauling's theories and recommendations were never substantiated. The consensus had been that while vitamin C supplementation doesn't prevent colds, it may lessen the severity and the duration in some people. Recently, researchers did what they call a meta-analysis in which they lump together results from all available studies on the topic and then apply statistics to a cumulative result. The meta-analysis on vitamin C suggests that supplementation reduces the rate of upper respiratory infections by 50 percent in some people, probably those who engage in more strenuous activity.
How Much Do We Need?
You can answer that question by again using that old disclaimer for most nutrients: it depends on who you are. The most recent RDA update increased vitamin C from 45 to 60 mg from the previous level. It also added a new group of people who need to get more vitamin C, people who smoke cigarettes, who should aim for 100 mg every day. The best sources are fruits and vegetables, but not all fruits and veggies are created equal when it comes to this important vitamin. In general, citrus fruits and a few other tropical fruits are the best sources, with one orange and one glass of grapefruit juice surpassing the RDA in one fell swoop. In the vegetable kingdom, your best bets are dark leafy greens and members of the cabbage family, with a mere half cup of broccoli providing more than 90 percent of the vitamin C you need for the entire day.
Even though fruits and vegetables are excellent sources, they also lose significant amounts of vitamin C because of how we tend to store and handle them in food preparation. Because they dissolve in water, most of the water soluble vitamins are touchy, but even within the entire group, some are touchier than others. Vitamin C is sensitive to heat, so cooking vegetables causes a drop in vitamin content, and the longer the food is cooked, the more vitamins you lose. The water soluble nature of vitamin C means that you also lose a significant amount in the water used for preparation and cooking. Try following the tips outlined in Section 2, especially the Southern tradition of recycling the water used to cook green vegetables into soups and stews; this helps reclaim vitamin C lost in the cooking water.
How do Americans stack up against the RDA for vitamin C? Recent dietary intake surveys reported that 20 to 30 percent of American adults consume less than the RDA, and less than 15 percent of children and adults meet the recommended intake. Using certain drugs, many of which people use regularly such as aspirin, can alter the vitamin's metabolism and increase its need. Some scientists are also concerned that, although most people will never develop scurvy, a marginal deficiency of vitamin C may be common. A marginal deficiency of a nutrient won't necessarily cause noticeable symptoms, but it will impair some of the important functions in the body that the nutrient carries out.
A person could develop a marginal deficiency in two ways: not getting enough of the vitamin in the diet, or in situations in which the need for the vitamin is higher. Smoking actually breaks down vitamin C because the smoke contains oxidants, which use up this antioxidant. A variety of conditions that cause stress in the body also increase need: aging, being in a hospital or other institution for prolonged periods, having a chronic disease, alcohol abuse, chronic infections, major operations, extensive burns, exposure to temperature extremes, and intake of toxic heavy metals such as cadmium, lead, and mercury. Of course, pregnant and breast-feeding women have a higher need for vitamin C, with an RDA of 70 and 95, respectively.
It's clear that we fall far short on our intake of this important nutrient, and even those people hitting the RDA may not be on target if some researchers are right about how much we really need. They think that we need more vitamin C than the level the RDA has set. They point to research suggesting a role for the vitamin in fighting diseases such as heart disease and cancer. In a recent and simple study evaluating how the body handles vitamin C, scientists found that a single dose of 200 mg was completely used in human subjects. The authors concluded that based on this and other studies, the RDA for vitamin C should be increased to 200 mg daily.
Supplement Safety
The safety of vitamin C supplementation has been the focus of as much attention as the vitamin C/cold connection. Several studies have attempted to answer the safety question, but, as with other aspects of this vitamin, the verdict is not yet in. One of the potential problems over which the vitamin C naysayers voice concern is that a high intake of vitamin C may cause kidney stones. The theory behind this suggests that vitamin C helps to make a compound, oxalate, that is the main building block of most kidney stones.
More recent and well-controlled studies have concluded that vitamin C does not cause kidney stones. In fact, a recent epidemiologic study showed that vitamin C intake above 1,500 mg was associated with a lower risk for oxalate kidney stones than levels closer to the RDA! The reason for earlier studies showing this connection may have been the result of analytical problems which scientists have since overcome with major advances in technology.
Another purported problem with vitamin C supplements is rebound scurvy. In rebound scurvy, also called conditioned scurvy, high intake of vitamin C might predispose someone to developing the deficiency if they stop taking the supplement. The original theory was based on a case of scurvy in an infant born to a mother who took vitamin C supplements. However, a recent review of the studies appeared in a respected publication, the American Journal of Clinical Nutrition, and concluded that the claim for rebound scurvy is "speculative but not substantiated."
So why were people concerned about vitamin C supplement safety to begin with? The answer lies in the numerous studies showing that this nutrient fights heart disease and cancer, but probably at levels well above the RDA. You could achieve high vitamin C levels without supplements, but you'd have to work hard, including hefty portions of citrus fruits and leafy greens on a daily basis.
Although the connection is mostly indirect, based on data from dietary intake studies, the evidence suggests that vitamin C is protective against cancer. Scientists have theorized that vitamin C may work on several fronts to fight cancer. Besides its powerful antioxidant ability, vitamin C may be protective by boosting immune system function including stopping tumor growth and spread through its role in collagen synthesis and by blocking the formation of potent cancer-causing agents called nitrosamines. Fear of these carcinogens caused many Americans to give up hot dogs, which, like other cured meats, contain nitrates and nitrites that convert to nitrosamines in the stomach.
As for heart disease, it's vitamin C's antioxidant punch that is probably responsible for any protective effect, although some studies suggest it may boost HDL levels. But as with cancer, the positive studies to date are mostly population studies and not the more powerful intervention trials. It may well be that vitamin C is only protective against these killers when it works with its teammates. Only more studies will provide the answer, but below are some research highlights.