Chapter 10.3

Vitamin C

LO 10.3

Identify the main roles, deficiency symptoms, and food sources for vitamin C.

For many centuries, any man who joined the crew of a seagoing ship knew he had at best a 50–50 chance of returning alive—not because he might be slain by pirates or die in a storm, but because he might contract scurvy. As many as two-thirds of a ship’s crew could die of scurvy during a long voyage. Only men on short voyages, especially around the Mediterranean Sea, were free of scurvy. No one knew the reason: that on long ocean voyages, the ship’s cook used up the fresh fruits and vegetables early and then served only cereals and meats until the return to port.

In the mid-1700s, James Lind, a British physician serving in the navy, devised an experiment to find a cure for scurvy. He divided 12 sailors with scurvy into 6 pairs. Each pair received a different supplemental ration: cider, vinegar, sulfuric acid, seawater, oranges and lemons, or a strong laxative. Those receiving the citrus fruits quickly recovered, but sadly, it was almost 50 years before the British navy required all vessels to provide every sailor with lemon or lime juice daily.

The antiscurvy “something” in citrus and other foods was dubbed the antiscorbutic factor. Centuries later, the factor was isolated and found to be a 6-carbon compound similar to glucose; it was named ascorbic acid.

Vitamin C Roles

Vitamin C parts company with the B vitamins in its mode of action. In some settings, vitamin C serves as a cofactor helping specific enzymes perform their jobs, but in others, it acts as an antioxidant participating in more general ways.

As an Antioxidant

Vitamin C loses electrons easily, a characteristic that allows it to perform as an antioxidant. In the body, antioxidants defend against free radicals. Free radicals are discussed fully in Highlight 11, but for now, a simple definition will suffice. A free radical is a molecule with one or more unpaired electrons, which makes it unstable and highly reactive. An antioxidant can neutralize a free radical by donating an electron or two. In doing so, antioxidants protect other substances from free radical damage. Figure 10-16 illustrates how vitamin C can give up electrons and then accept them again to become reactivated. This recycling of vitamin C is key to limiting losses and maintaining a reserve of antioxidants in the body. Other key antioxidant nutrients include vitamin E, beta-carotene, and selenium.

Figure 10-16Active Forms of Vitamin C

The two hydrogens highlighted in yellow give vitamin C its acidity and its ability to act as an antioxidant.

Vitamin C is like a bodyguard for water-soluble substances; it stands ready to sacrifice its own life to save theirs. In the cells and body fluids, vitamin C protects tissues from the oxidative stress inflicted by free radicals and thus may play an important role in preventing diseases. In the small intestine, vitamin C enhances iron absorption by protecting iron from oxidation. (Chapter 13 provides more details about the relationship between vitamin C and iron.)

As a Cofactor in Collagen Formation

Vitamin C helps to form the fibrous structural protein of connective tissues known as collagen. Collagen serves as the matrix on which bones and teeth are formed. When a person is wounded, collagen glues the separated tissues together, forming scars. Cells are held together largely by collagen; this is especially important in the walls of the blood vessels, which must withstand the pressure of blood surging with each beat of the heart.

Chapter 6 describes how the body makes proteins by stringing together chains of amino acids. During the synthesis of collagen, each time a proline or lysine is added to the growing protein chain, an enzyme hydroxylates it (adds an OH group), making the amino acid hydroxyproline or hydroxylysine, respectively. These two special amino acids facilitate the binding together of collagen fibers to make strong, ropelike structures. The conversion of proline to hydroxyproline requires both vitamin C and iron. Iron works as a cofactor in the reaction, and vitamin C protects iron from oxidation, thereby allowing iron to perform its job. Without vitamin C and iron, the hydroxylation step does not occur.

As a Cofactor in Other Reactions

Vitamin C also serves as a cofactor in the synthesis of several other compounds. As in collagen formation, vitamin C helps in the hydroxylation of carnitine, a compound that transports fatty acids, especially long-chain fatty acids, across the inner membrane of mitochondria in cells. It also participates in the conversions of the amino acids tryptophan and tyrosine to the neurotransmitters serotonin and norepinephrine, respectively. Vitamin C also assists in the making of hormones, including thyroxine, which regulates the metabolic rate. When metabolism speeds up in times of extreme physical stress, the body’s use of vitamin C increases.

In Stress

Among the stresses known to increase vitamin C needs are infections; burns; extremely high or low temperatures; intakes of toxic heavy metals such as lead, mercury, and cadmium; the chronic use of certain medications, including aspirin, barbiturates, and oral contraceptives; and cigarette smoking. During stress, the adrenal glands—which contain more vitamin C than any other organ in the body—release vitamin C and hormones into the blood.

When immune system cells are called into action, they use a great deal of oxygen and produce free radicals. In this case, free radicals are helpful. They act as ammunition in an “oxidative burst” that demolishes the offending viruses and bacteria and destroys the damaged cells. Vitamin C steps in as an antioxidant to control this oxidative activity.

In the Prevention and Treatment of the Common Cold

Vitamin C has been a popular option for the prevention and treatment of the common cold for decades, but research supporting such claims has been conflicting and controversial. Some studies find no relationship between vitamin C and the incidence of colds, whereas others report modest benefits—fewer colds, fewer days, and milder symptoms. A review of the research on the use of vitamin C in fighting against infections reveals a slight reduction in the duration of the common cold in favor of those taking large doses (6 to 8 grams per day) of vitamin C supplements.

Discoveries about how vitamin C works in the body provide possible links between the vitamin and the common cold. Anyone who has ever had a cold knows the discomfort of a runny or stuffed-up nose. Nasal congestion develops in response to elevated blood histamine, and people commonly take antihistamines for relief. Like an antihistamine, vitamin C comes to the rescue and deactivates histamine.

In Disease Prevention

Whether vitamin C may help in preventing or treating cancer, heart disease, cataracts, and other diseases is still being studied; findings are presented in the Highlight 11 discussion of antioxidants. Conducting research in the United States can be difficult, however, because diets typically contribute enough vitamin C to provide optimal health benefits.

Vitamin C Recommendations

How much vitamin C does a person need? As is true of all the vitamins, recommendations are set generously above the minimum requirement to prevent deficiency disease and well below the toxicity level (see Figure 10-17).

Figure 10-17Vitamin C Intake (Mg/Day)

Recommendations for vitamin C are set generously above the minimum requirement and well below the toxicity level.

The requirement—the amount needed to prevent the overt symptoms of scurvy—is only 10 milligrams daily. Consuming 10 milligrams a day does not saturate all the body tissues, however; higher intakes can boost the body’s total vitamin C further. At about 100 milligrams per day, 95 percent of the population reaches tissue saturation. (For perspective, 1 cup of orange juice provides more than 100 milligrams of vitamin C.) Recommendations are slightly lower, based on the amounts needed to provide antioxidant protection. At about 200 milligrams, absorption reaches a maximum, and there is little, if any, increase in blood concentrations at higher doses. Excess vitamin C is readily excreted.

As mentioned earlier, cigarette smoking increases the need for vitamin C. Cigarette smoke contains oxidants, which greedily deplete this potent antioxidant. Exposure to cigarette smoke, especially when accompanied by low dietary intakes of vitamin C, depletes the body’s vitamin C. People who chew tobacco also have low vitamin C. Because people who smoke cigarettes regularly suffer significant oxidative stress, their requirement for vitamin C is increased an additional 35 milligrams. Nonsmokers regularly exposed to cigarette smoke should also be sure to meet their RDA for vitamin C. Smokers are among those most likely to suffer vitamin C deficiency.

Vitamin C Deficiency

Early signs of nutrient deficiencies can be difficult to recognize. Two of the most notable signs of a vitamin C deficiency reflect its role in maintaining the integrity of blood vessels. The gums bleed easily around the teeth, and capillaries under the skin break spontaneously, producing pinpoint hemorrhages (see Figure 10-18).

Figure 10-18Vitamin C–Deficiency Symptoms—Scorbutic Gums and Pinpoint Hemorrhages

Biophoto Associates/Science Source; Dr. P. Marazzi/Science Source

When vitamin C concentrations fall to about a fifth of optimal levels (this may take more than a month on a diet lacking vitamin C), scurvy symptoms begin to appear. Inadequate collagen synthesis causes further hemorrhaging. Muscles, including the heart muscle, degenerate. The skin becomes rough, brown, scaly, and dry. Wounds fail to heal because scar tissue will not form. Bone rebuilding falters; the ends of the long bones become softened, malformed, and painful; fractures develop. The teeth become loose as the cartilage around them weakens. Anemia and infections are common. There are also characteristic psychological signs, including hysteria and depression. Sudden death is likely, caused by massive internal bleeding.

Once diagnosed, scurvy is readily resolved by raising vitamin C intake. Moderate doses of 100 milligrams per day are sufficient, curing the scurvy within about 5 days. Such an intake is easily achieved by including vitamin C–rich foods in the diet.

Vitamin C Toxicity

The availability of vitamin C supplements and the publication of books recommending vitamin C to prevent colds and cancer have led many people to take large doses of vitamin C. Not surprisingly, side effects of vitamin C supplementation such as gastrointestinal distress and diarrhea have been reported. The UL for vitamin C was established based on these symptoms.

Several instances of interference with medical regimens are also known. Large amounts of vitamin C excreted in the urine obscure the results of tests used to detect glucose or ketones in the diagnosis of diabetes. In some instances, excess vitamin C gives a false positive result; in others, a false negative. People taking anticlotting medications may unwittingly counteract the effect if they also take massive doses of vitamin C. Those with kidney disease, a tendency toward gout, or a genetic abnormality that alters vitamin C’s breakdown to its excretion products are prone to forming kidney stones if they take large doses of vitamin C.

Vitamin C supplements may adversely affect people with iron overload. As Chapter 13 explains, vitamin C enhances iron absorption and releases iron from body stores, and too much free iron causes the kind of cellular damage typically inflicted by free radicals. These adverse consequences illustrate how vitamin C can act as a prooxidant when quantities exceed the body’s needs.

Vitamin C Food Sources

Fruits and vegetables can easily provide generous amounts of vitamin C. A cup of orange juice at breakfast, a salad for lunch, and a stalk of broccoli and a potato for dinner alone provide more than 300 milligrams. (For perspective, review Figure 10-17.) Clearly, a person making such food choices does not need vitamin C supplements.

Figure 10-19 shows the amounts of vitamin C in various common foods. The overwhelming abundance of purple and green bars reveals not only that the citrus fruits are justly famous for being rich in vitamin C, but that other fruits and vegetables are in the same league (see Photo 10-9). A half cup of broccoli, bell pepper, or strawberries provides more than 50 milligrams of the vitamin (and an array of other nutrients). Because vitamin C is vulnerable to heat, raw fruits and vegetables usually have a higher nutrient density than their cooked counterparts. Similarly, because vitamin C is readily destroyed by oxygen, foods and juices should be stored properly and consumed within a week of opening.

Figure 10-19Vitamin C in Selected Foods

Meeting vitamin C needs without fruits (purple) and vegetables (green) is almost impossible. Most meats, legumes, breads, and milk products are poor sources. See How To 10-1 for more information on using this figure.

The potato is an important source of vitamin C, not because one potato by itself meets the daily need, but because potatoes are such a common staple that they make significant contributions. In fact, scurvy was unknown in Ireland until the potato blight of the mid-1840s, when some 2 million people died of malnutrition and infection.

The lack of yellow, blue, brown, and red bars in Figure 10-19 confirms that grains, milk (except breast milk) and milk products, and most protein foods are poor sources of vitamin C. Organ meats (liver, kidneys, and others) and raw meats contain some vitamin C, but most people don’t eat large quantities of these foods. Raw meats and fish contribute enough vitamin C to be significant sources in parts of Alaska, Canada, and Japan, but elsewhere fruits and vegetables are necessary to supply sufficient vitamin C.

Because of vitamin C’s antioxidant activity, food manufacturers sometimes use an additive similar to vitamin C in some beverages and most cured meats, such as luncheon meats, to prevent oxidation and spoilage. This compound safely preserves these foods, but it does not exhibit vitamin C activity in the body. Simply put, “Ham and bacon cannot replace fruits and vegetables.”

Review Vitamin C

Vitamin C acts primarily as an antioxidant and a cofactor. Recommendations are set well above the amount needed to prevent the deficiency disease scurvy. A variety of fruits and ­vegetables—most notably citrus fruits—provide generous amounts of vitamin C. The accompanying table provides a review of vitamin C facts.

Photo 10-9

Citrus fruits and juices are rightly known for their vitamin C contents, but many other fruits and vegetables are also good sources.

Angel Tucker

Vita means life. After this discourse on the vitamins, who could dispute that they deserve their name? Their regulation of metabolic processes makes them indispensable to the healthy growth, development, and maintenance of the body. The following table condenses the information provided in this chapter for a quick review. Descriptions of the remarkable roles of the vitamins continue in the next chapter.