Immunity

Vitamin A: An Immune Enhancer

Vitamin A seems to top the A-list of nutrients that are vital for a strong immune system.

While studying the effects of vitamin A in children, Dr. Chandra observed that even a moderate deficiency can weaken the immune defenses of a child’s respiratory tract. Vitamin A deficiency causes damage to the naturally protective mucous membrane barrier of the respiratory tract, and it’s thought that bacteria and viruses take advantage of that damage.

How might that affect a child’s health? After a flu virus attacks, for example, the lining of a normal throat will repair itself. Not so in those who are vitamin A–deficient. “Instead, you might get that once-healthy cell replaced by an abnormal cell,” says Charles B. Stephensen, Ph.D., associate professor in the Department of International Health at the University of Alabama in Birmingham. “That may predispose you to having a more severe episode of an infection or having another infection on top of a viral infection.”

“The link between vitamin A deficiency and the severity of respiratory disease is very well established,” agrees Susan Cunningham-Rundles, Ph.D., associate professor of immunology at Cornell University Medical Center in New York City and editor of Nutrient Modulation of the Immune Response.

Such deficiencies are common in poorer countries, where foods high in vitamin A, such as green, leafy vegetables and fortified milk, are not readily available or are not utilized. And as a result, health officials in those countries routinely prescribe vitamin A supplements to prevent measles and other infections, particularly diarrhea, from becoming life-threatening. This strategy has cut deficiency-related deaths by 30 percent in some countries.

Many experts believe that immune-compromising vitamin A deficiencies are also widespread among children in the United States. Some 28 percent of children in the United States may be vitamin A–deficient, according to Martha Rumore, Pharm.D., associate professor at the Arnold and Marie Schwartz College of Pharmacy and Health Sciences in Brooklyn. In a published review of vitamin A research, Dr. Rumore found that vitamin A deficiency has been correlated with decreased resistance to pneumonia, tuberculosis, whooping cough and infectious diarrhea.

Another study, this one of 20 U.S. children with measles, showed that half were vitamin A–deficient.

How could children in a land so well-supplied with milk shakes and salad bars be vitamin A–deficient? For one thing, measles actually depletes vitamin A. And for another, many people, especially kids, just don’t eat foods that contain vitamin A.

“There’s no doubt about it. All of the continuing food consumption survey studies have shown that vitamin A intakes are low among children, especially in poverty areas,” says Adrianne Bendich, Ph.D., a clinical research scientist in the human nutrition research department at Hoffmann–La Roche in Nutley, New Jersey.

How much vitamin A is enough? The Daily Value for vitamin A is 5,000 international units.

Beta-Carotene: Immune Booster

Beta-carotene is the pigment that helps turn carrots, cantaloupe and other fruits and vegetables orange or yellow. But researchers are discovering that this nutrient does a lot more than add color to your favorite produce.

In fact, studies have shown that beta-carotene does quite a bit of immune-boosting work of its own.

Researchers in one study found that the number of T-helper cells in male volunteers jumped 30 percent after the men took 180 milligrams (almost 299,000 international units) of beta-carotene a day for two weeks. (T-helper cells are important components of the immune system.)

In another study, this time at the University of Arizona in Tucson, groups of men and women were given daily doses of 15 milligrams (about 25,000 international units), 30 milligrams (about 50,000 international units), 45 milligrams (about 75,000 international units) or 60 milligrams (almost 100,000 international units) of beta-carotene for two months, according to Ronald R. Watson, Ph.D., research professor at the University of Arizona College of Medicine. An enhanced immune response was noticeable at 30 milligrams and above, with increased numbers of both natural killer cells and activated lymphocytes, which are also important immune system components.

In studies of AIDS patients and in studies of older patients with precancerous oral lesions, a similar immunological effect was seen with 30 milligrams of beta-carotene a day during a three-month period. The effect declined after the three months, however.

Levels being tested in clinical trials are between 50,000 and 100,000 international units a day. These dosages are considered safe as well as potentially effective in preventing both cancer and heart disease.

No toxic level of beta-carotene has been shown in research so far. A potential side effect is discoloration of the skin, which fades as the dosage is decreased.

Most research has found that smokers benefit the most from beta-carotene supplementation, as lung cancer usually shows the strongest association with low levels of beta-carotene. And lung cancer risk is reduced the most in smokers who consume high levels of beta-carotene-containing foods or supplements. Most doctors and researchers, however, still recommend eating more fruits and vegetables and eliminating smoking.

There is no Daily Value for beta-carotene, but nutrition experts generally recommend getting 8,300 to 10,000 international units a day. Most people get 1,600 to 3,300 international units a day from foods.

B₆: You May Need More

Researchers at Tufts University School of Nutrition in Medford, Massachusetts, discovered that when healthy elderly people had vitamin B₆ almost completely taken out of their diets, immune response went down. Even more telling: The amount of B₆ needed to restore strength to the immune system was higher than the Daily Value of 2 milligrams. When the study participants were provided with 50 milligrams of B₆ daily, immunity was boosted to a level that was even better than before the study began.

“These data tell us two things,” says Dr. Bendich. “One is that the Daily Value of vitamin B₆ is not high enough for optimum function in the elderly. The other is that taking a B₆ supplement enhances immunity.”

Several other studies, in fact, have shown that older folks in general don’t seem to eat enough vitamin B₆. One study of older residents of New Mexico showed that they eat barely one-fourth of the B₆ that they need each day.

Whether you’re young or old, you can boost your vitamin B₆ intake by eating chick-peas, prune juice, turkey, potatoes and bananas. A banana provides 33 percent of your Daily Value of B₆, while an eight-ounce glass of prune juice provides 28 percent.

Vitamin C Gets Votes

There is general agreement in the medical community that vitamin C is vital to the production of white blood cells, the foot soldiers of your immune system.

“The best experimentation that I’ve seen suggests that vitamin C is in some way or another stimulating the white blood cells to function better,” explains Elliot Dick, Ph.D., professor of preventive medicine and chief of the respiratory virus research laboratory at the University of Wisconsin–Madison and one of the country’s leading cold researchers. “White blood cells attack the infected cell, gather around it, destroy it and then clean up.”

By the same token, at least one study has shown that levels of vitamin C don’t have to be very low, even in otherwise healthy men between ages 25 and 43, to cause a decline in immune function. During a three-month study, researchers at the U.S. Department of Agriculture Western Human Nutrition Research Center in San Francisco found that getting 20 milligrams or less of vitamin C a day caused a delayed reaction to a skin test designed to provoke an immune response, such as swelling or a rash. What’s more, “the researchers could not bring the levels of vitamin C back up to where they were before the deficient diet until the volunteers were given 250 milligrams of vitamin C a day for three weeks,” says study investigator Robert A. Jacob, Ph.D., research chemist in micronutrients at the Western Human Nutrition Research Center.

While the Daily Value for vitamin C (60 milligrams) is lower than the amounts used in many of these studies, many people don’t get even that much.

“You’ll find a significant number of people who do not get even 75 percent of the Daily Value of vitamin C, a vitamin that is very abundant, to say the least,” says Vishwa Singh, Ph.D., director of the Human Nutrition Research Department at Hoffmann–La Roche. No one should be deficient in vitamin C, he maintains, not when so many fruits and vegetables have such high amounts. An eight-ounce glass of orange juice has 200 percent of the Daily Value, for example. And a half-cup of chopped raw red bell peppers provides 158 percent.

But is the Daily Value enough to keep your immune system functioning in tip-top form? That’s the question. And researchers simply don’t know the answer yet. Many nutrition experts, however, recommend that you get at least 500 milligrams a day.

Delivering Potential Benefits with Vitamin D

Vitamin D is also developing a reputation as a key player in a healthy immune system. When researchers at the University of Wisconsin–Madison tested vitamin D–deficient laboratory animals, they found that the thymus gland was not doing its job of generating a sufficient number of immune system cells. And it took eight weeks of a diet with normal vitamin D levels to restore proper immunity.

The Daily Value for vitamin D is 400 international units. The nutrient is found in eggs and fortified milk. You even create your own supply as a natural reaction when sunlight touches your skin.

So who wouldn’t get enough? Experts say people who avoid milk to bypass stomach problems or who stay out of the sun for fear of getting wrinkles could be putting themselves at risk for a shortage. “It could be more of a problem in the elderly than in the young,” says Dr. Bendich. “What we’re finding is that elderly people aren’t able to make vitamin D in their skin as well as younger people. They don’t go out in the sun as much. They use more skin protectors—sunscreens that block the formation of vitamin D. And they don’t drink a lot of milk.” The elderly need to get their Daily Value of vitamin D from a multivitamin/mineral supplement, according to Dr. Bendich.

Vitamin D is in most Vitamin E: A Well-Known Aid

The story on vitamin E and immunity is long and positive. For years, researchers have been finding dramatic immune-enhancing effects using vitamin E supplements, including increased levels of interferon and interleukin. Both of these biochemicals are produced by the immune system to fight infection.

In one study, Tufts University researchers divided older volunteers into two groups: 18 who took daily 800-milligram vitamin E supplements and 14 who took placebos (inactive look-alike pills). At the end of 30 days, the researchers found that the vitamin E takers had a 69 percent increase in levels of interleukin-2 and a decrease in levels of a substance called prostaglandin, which can reduce the number of white blood cell soldiers patrolling your body.

“The work at Tufts has very clearly shown that giving the elderly doses of vitamin E can improve immune response,” says Dr. Singh.

Vitamin E also helps prevent oxidative damage in the body. This is a kind of damage that has been linked to lowered immune response. It seems that when immune system killer cells such as macrophages do their jobs of attacking and absorbing viruses, bacteria and other foreign invaders, dreaded free radicals are created as a by-product. Free radicals are unstable molecules that steal electrons from healthy molecules to balance themselves, weakening or damaging cells in the process. Vitamin E tames these free radicals by offering them its own electrons, helping to shield healthy cells from abuse.

How much vitamin E is enough to create this immune-boosting effect? Experts generally recommend getting 400 international units a day.

Iron: Tops for Immunity

Not only is iron an important mineral for healthy immune system functioning, but iron deficiency is fairly common, says Dr. Sherman. The deficiency can be caused by not eating enough iron-containing foods such as red meat and green, leafy vegetables. Menstruating women often have low iron stores because of the monthly loss of iron-rich blood. Stomach problems such as ulcers also cause the loss of blood, as do parasitic infections and, of course, serious injuries, she says.

Stored in the liver, spleen and bone marrow, iron is used first and foremost to produce hemoglobin in the blood. You’re not considered iron-deficient until your blood hemoglobin level begins to fall, according to Dr. Sherman.

The Daily Value for iron is 18 milligrams, which is considered enough to keep the immune system up to par. Many researchers urge caution in taking more. Getting too much can cause abdominal pain, diarrhea and consti pation.

Think Zinc

Like iron, zinc is crucial for making sure that the first wave of immune fighters, lymphocytes, has enough troops.

“When the body is exposed to a pathogen, one of the things that happens is that immune cells begin to proliferate. That is the beginning of all of the steps in killing the offender,” says Dr. Sherman. “And both zinc and iron are involved in that process.” Less zinc means that lymphocytes will respond more slowly to the foreign invader and that fewer will even make it to the battlefield.

Fortunately, serious zinc deficiencies are rare. Far more common, however, are moderate zinc deficiencies. Strict vegetarians are often at the greatest risk for zinc deficiency because they shun meats and seafood, the best sources of zinc.

Getting the Daily Value of zinc (15 milligrams) should be enough to keep the immune system functioning properly. And getting that amount shouldn’t be a problem. Just three ounces of any lean red meat delivers about 32 percent of that amount, while six steamed oysters provide five times the amount of zinc that you need.

Taking Some Multivitamin Insurance

There’s also a good deal of research that supports the idea of taking a multivitamin/mineral supplement every day. In a yearlong study of 100 elderly Canadians, for example, Dr. Chandra gave half of the group daily Since then, another multivitamin/mineral study has produced similar results. Researchers used skin tests to measure immune responses to proteins from bacteria and fungi that cause tuberculosis, diphtheria, tetanus and other ailments. After one year, the supplement takers in the study had significantly more virile immune systems than did the people who took placebos for the same amount of time, according to research leader John Bogden, Ph.D., professor in the Department of Preventive Medicine and Community Health at the University of Medicine and Dentistry of New Jersey/New Jersey Medical School in Newark.

Why would a simple multivitamin/mineral tablet make such a difference in the performance of your immune system? Dr. Bogden thinks he has the answer, at least for the elderly. “We think it may be either that older people have increased requirements or that the Recommended Dietary Allowances, or levels near the Recommended Dietary Allowances, are not adequate to support optimum immunity,” he says.