Vitamin A is Fat-soluble.  At first glance, these two sources of nutrients seem far, far apart. One source is the plant world, the other is all animal. Once the beta-carotene from plant sources gets inside the body, however, it undergoes a transformation. Through a number of chemical processes, it can be converted into a nutrient that’s identical to the vitamin A that comes from animal sources. The plant-source vitamin A has all the benefits of animal-source vitamin A. Not only that, but once the transformation is complete and the body has all the vitamin A it can use, any excess beta-carotene goes on to do some other good works.

The best source of vitamin A is a beta-carotene supplement or a multivitamin that contains a nontoxic amount of vitamin A along with an ample amount of beta-carotene. Both are readily available in drugstores and health food stores. And here’s why you might want to pick some up—along with carrots, squash, broccoli, and dark green, leafy vegetables.

Vitamin A plays vital roles throughout the body. In our eyes, it helps maintain a crystal-clear outer window, the cornea. Without enough vitamin A, the cornea clouds over. "At the back of the eye, in the retina, vitamin A is part of the pigment that reacts chemically when struck by light and helps create the nerve impulse that goes to the brain and creates a visual message," says James Allen Olson, Ph.D., professor of biochemistry and director of the vitamin A research group at Iowa State University in Ames. For this reason, vitamin A can help with the condition known as night blindness.

Without adequate vitamin A, your eyes recover very slowly after flashes of bright light at night, or you’re unable to see in dim light. In Indonesia, where vitamin A deficiency is common, this condition is called chicken eyes because chickens can’t see at night and go to sleep when the sun goes down.

Vitamin A also has another basic function. "It helps cells to mature and develop certain definite characteristics and properties, a process called cell differentiation," Dr. Olson says. It acts as a kind of traffic cop for cells in a developing embryo. As the cells start to divide and multiply, vitamin A helps guide them in the direction they need to go, putting this one on its way to becoming a muscle cell, steering another one toward becoming a liver cell, and so on.

Vitamin A helps maintain the surfaces of the skin, the mucous membranes lining the nose and throat, and the tissues lining the intestines, bladder, and other internal cavities. All of these benefits can help boost immunity, since mucous membranes help prevent invasion by bacteria and viruses.

The vitamin also plays a direct role in immunity by helping the immune cells change into the special forms necessary to fight off infection. "One special kind of cell, called a T-helper cell, which helps to direct other immune cells, is very sensitive to vitamin A status," Dr. Olson says. "There’s no question that the immune system doesn’t function very well with inadequate vitamin A, and you don’t necessarily need to be clearly deficient for this to happen."

Vitamin A even helps bones. It’s involved in the dismantling and reforming of bone, an important part of making new bone.

Given the many roles of vitamin A in the body, it’s fortunate that we can get it from so many sources—plant as well as animal. While the transformation from beta-carotene into vitamin A is chemically complex, it doesn’t take long to happen.

Beta-carotene is what’s known as a precursor of vitamin A, which means that it’s an essential part of the production process. As your body needs vitamin A, it can split beta-carotene in half, producing two molecules of vitamin A. Alternatively, the beta-carotene might split other ways, leaving one molecule of vitamin A.

Apart from the role it plays in vitamin A production, beta-carotene gives you other benefits that have been studied by researchers. Studies often involve other forms of plant chemicals, including beta-carotene, that are known as carotenoids.

Given these results, scientists have tried to find some evidence of how beta-carotene can produce a protective effect in our bodies. What they’ve found is that beta-carotene acts as an antioxidant. Like vitamins E and C, it can neutralize free radicals, the free-roaming, unstable molecules that can damage cells. Vitamin A is a staunch cell protector. Once incorporated into the fatty membranes around a cell, it also guards structures inside it. Among its protectorates is the very heart of the cell, the nucleus, which houses essential genetic material.

With beta-carotene—along with other antioxidant—fortifying the cell membrane from attack by free radicals, the cell is still vulnerable, but it’s far better protected. Since cancer seems to get rolling when cell damage is at its peak, any form of protection is likely to reduce risk. At least, that’s the theory—one that would explain why beta-carotene seems to play a cancer-protective role.

Beta-carotene may also help to regulate the proteins that enable cell-to-cell communication. "Every cell in the body has the ability to communicate with its neighbors, as though it were calling out across the backyard fence," Dr. Mayne says. This ability to communicate is thought to play a role in inhibiting cancer.

When normal cells are grown in the laboratory, they won’t pile up on top of one another, she says. "Once they have filled up the top of a layer in a cell culture system, they send messages to each other that say, in effect, ‘We are done growing.’ This makes all the cells stop."

To cancer researchers, that’s a fascinating effect, because cancer cells don’t stop growing, and that’s the problem. "In cancer cells, that communication goes awry, and they just continue to grow and grow," says Dr. Mayne.

In their frenetic growing process, the cancer cells eventually pile up, forming tumors. As the pile-up continues, the cells eventually invade the space of normal, healthy cells and generally take over the neighborhood. If beta-carotene is really a key link in cell-to-cell communication, maybe it can help to keep cells from overgrowing. At least, that’s what researchers are hoping.

Starting with some studies in the 1980s and 1990s, however, doctors began to see that the scenario is even more complex. While some doses of beta-carotene in certain forms may have the cancer-preventive effect that we want, there are many variables. In fact, in certain high-risk groups, certain kinds of beta-carotene in high doses may tilt the balance the other way and actually increase the risk of one kind of cancer.

A study conducted in Finland showed an unexpected outcome among heavy smokers, ages 50 to 69, who took 20 milligrams (33,200 international units, or IU) a day of beta-carotene for five to eight years. Compared with other smokers who had similar lifestyles but took no beta-carotene, the people who took the supplements seemed to have an increased rate of cancer. In fact, they had an 18 percent higher incidence of lung cancer than the heavy smokers who did not take supplements. This study also found an 11 percent increase in deaths from heart disease or stroke among the smokers who were taking beta-carotene.

These results have researchers puzzled because they’re so contrary to what scientists would expect, given the positive results of earlier studies with people involving foods rich in beta-carotene and animal studies involving supplements. "There’s no doubt that now there are more questions than answers about beta-carotene," says Dr. Mayne.

What does the uncertainty about beta-carotene mean to you if you are wondering whether to take a supplement?

Well, if you don’t smoke, you probably shouldn’t worry. Those who were adversely affected were heavy smokers, and they were taking large amounts of supplements. "The only evidence of possible harm we have is in people who smoke more than one pack a day of cigarettes," Dr. Mayne says.

Researchers still don’t know why harm occurs in this select group, but the very large doses are certainly a factor. "Antioxidants can promote free radical formation rather than inhibit it under certain circumstances, which could perhaps occur in someone with lots of oxidative damage going on in their body," Dr. Mayne says. "In smokers, there’s a lot of oxidative damage."

A heavy smoker’s mouth, throat, lungs, and blood are constantly being exposed to chemical reactions that involve oxygen, Dr. Mayne points out. When oxygen is involved, trouble can follow. "These compounds can damage the cell membranes, which hinders the cell’s ability to function properly. And they can go right through the cell membrane and chemically react with the cell’s genetic material, setting the stage for cancer."

"It’s possible that one dose of beta-carotene might offer protection from oxidative damage, while a higher dose could promote it, especially in already-damaged cells," Dr. Mayne says.

People at risk for vitamin A deficiency can benefit from beta-carotene supplements. In the United States, most people get enough vitamin A from food; worldwide, vitamin A deficiency is a common cause of blindness. Those most likely to have low supplies are children from poor families, who may not get enough vitamin A–fortified milk or beta-carotene–rich fruits and vegetables. In some areas of Africa and Asia, both children and adults die as a result of lack of vitamin A, usually from infections.

Others who may suffer from vitamin A deficiency are alcoholics, people who can’t absorb fat properly, and people with liver disease (vitamin A is stored in the liver). If you don’t get enough zinc, an essential trace mineral, you can also become deficient, because your body needs zinc to make a protein that carries vitamin A in your bloodstream from the liver to other organs. If zinc’s not there, vitamin A is not used effectively.

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