Carrots. Squash. Broccoli. Kale. Orange and yellow vegetables. Dark leafy greens. They all contain beta-carotene. Liver. Milk. Eggs. They all contain vitamin A.
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.
As for the supplement forms of these nutrients, they’re often separate, but not equal. The problem is that the pure form of vitamin A, called preformed vitamin A, can create a number of nasty toxic effects if you take too much of it. Although beta-carotene isn’t without its problems, low doses may be taken in supplement form.
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.
All the Roles of A
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.
SUPPLEMENTSNAPSHOT
| Beta-Carotene and Vitamin A May help: Cancer of the lung, stomach, esophagus, mouth, cervix, and colon; angina; genital herpes; colds and flu; osteoarthritis; and low immunity in people with HIV. Daily Value: Beta-carotene—no DV; vitamin A—5,000 international units (IU), or 1,500 retinol equivalents (RE). Special instructions: For maximum absorption, take supplements with meals that contain some fat. Do not take with meals or supplements that contain large amounts of pectin, a type of soluble fiber found in citrus fruits. Who’s at risk for deficiency: Cigarette smokers, alcoholics, and people who eat fewer than three servings a day of fruits or vegetables. Good food sources: Beta-carotene—dark green, leafy vegetables, orange and yellow vegetables, and yellow fruits. (One large carrot—one of the best food sources—has about 10,600 IU.) Vitamin A—fortified milk and milk products, such as cheese, cream, and butter, and fortified margarine. Cautions and possible side effects: Avoid taking more than 25,000 IU of beta-carotene as supplements. There is evidence that it causes lung cancer in smokers taking 50,000 IU in supplement form. Do not take preformed vitamin A supplements unless you are under a doctor’s supervision. Taking more than 50,000 IU (15,000 RE) of preformed, animal-source vitamin A a day over a long period of time can lead to headaches, blurred vision, hair loss, joint pain, dry skin, drowsiness, diarrhea, and enlargement of the liver and spleen. Symptoms slowly disappear once the dosage is reduced. Do not take more than 5,000 IU if you are pregnant or trying to conceive. |
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.
The Many Beta Benefits
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.
In either case, the presence of beta-carotene leads to the production of vitamin A when it’s needed. "When the body is lacking vitamin A, more beta-carotene is converted to vitamin A by an enzyme in the intestines as well as elsewhere, in other tissues in the body," says Susan Taylor Mayne, Ph.D., director of cancer prevention and control research at the cancer center at Yale University School of Medicine.
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.
For years, researchers have seen a growing body of evidence to suggest that the more foods we eat that contain beta-carotene, the less likely we are to develop certain types of cancer. It looks as if beta-carotene may help prevent cancers of the lung, stomach, and esophagus. There’s also been some evidence that beta-carotene might lower the risk of developing cancer of the cervix and colon as well. Mouth and throat cancers may also be on the hit list.
"A reduction in the risk of lung cancer really stood out, followed by mouth and throat cancer," says Dr. Mayne, summing up some studies. "Again, beta-carotene and other carotenoids showed very strong preventive activity. In fact, an international group of researchers reviewed this data, and the evidence of a protective effect remains very compelling."
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 antioxidants—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.
Making Good Neighbors
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.
The Carotene Conundrum
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.
Another study, called the Carotene and Retinol Efficacy Trial, was conducted at a number of U.S. medical centers that specialize in the prevention of lung cancer. Again, the study focused on very high-risk groups—in this case, heavy smokers and asbestos workers who were also smokers. But scientists had to end the study almost two years earlier than planned because the supplement seemed to increase health risks. The supplemented group, who were taking 30 milligrams (50,000 IU) a day of beta-carotene in combination with 25,000 IU of vitamin A (five times the Daily Value), were showing a 28 percent greater risk of developing lung cancer. They also had a 25 percent higher risk of death from heart disease. "It’s hard to draw conclusions from this study, since it was stopped early and used a combination of nutrients," Dr. Mayne says. "But the data did indicate that there was the possibility of harm from beta-carotene, and little likelihood of benefit."
In a third study, the Physicians’ Health Study, participants took 50 milligrams (83,000 IU) of beta-carotene every other day. Although the study went on for 12 years, the researchers found that beta-carotene had no effect—either beneficial or harmful—on the risk of cancer or cardiovascular disease. This study included relatively fewer smokers, however.
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’s the Right Dose?
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.
Judicious Approaches
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.
