Vitamin A, vitamin D, vitamin E, and vitamin K are relatives that have two characteristics in common: All dissolve in fat, and all are stored in your fatty tissues. But like members of any family, they also have distinct personalities. One keeps your skin moist. Another protects your bones. A third keeps reproductive organs purring happily. And the fourth enables you to make special proteins.
Which does what? Read on.
Vitamin A is the moisturizing nutrient that keeps your skin and mucous membranes (the slick tissue that lines the eyes, nose, mouth, throat, vagina, and rectum) smooth and supple. Vitamin A is also the vision vitamin, a constituent of 11-cis retinol, a protein in the rods (cells in the back of your eye that enable you to see even when the lights are low) that prevents or slows the development of age-related macular degeneration, or progressive damage to the retina of the eye, which can cause the loss of central vision (the ability to see clearly enough to read or do fine work). Finally, vitamin A promotes the growth of healthy bones and teeth, keeps your reproductive system humming, and encourages your immune system to churn out the cells you need to fight off infection.
Two chemicals provide vitamin A: retinoids and carotenoids. Retinoids are compounds whose names all start with ret: retinol, retinaldehyde, retinoic acid, and so on. These fat-soluble substances are found in several foods of animal origin: liver (again!) and whole milk, eggs, and butter. Retinoids give you preformed vitamin A, the kind of nutrient your body can use right away.
The second form of vitamin A is the vitamin A precursor, a chemical such as beta-carotene, a deep yellow carotenoid (pigment) found in dark green and bright yellow fruits and vegetables. Your body transforms a vitamin A precursor into a retinol-like substance. So far, scientists have identified at least 5oo different carotenoids. Only 1 in io — about 5o altogether — are considered, like beta-carotene, to be sources of vitamin A.
Help! I'm turning orange
Because you store retinol in your liver, megadoses of preformed vitamin A can build up to toxic levels. Not so with carotenoids, which serve up another form of that vitamin. They aren't stored in the liver, so these red and yellow pigments in fruits and vegetables are safe even in very large amounts.
But that doesn't mean that excess carotenoids don't have side effects. Carotenoids, like retinoids, are stored in body fat. If you wolf down large quantities of carotenoid-rich foods like carrots and tomatoes every day, day after day, for several weeks, your skin — particularly the palms of your hands and the soles of your feet — will turn a nifty shade of dusty orange, brighter if your skin is naturally light, darker if it's naturally dark. It sounds fantastic, but it has actually happened to people eating two cups of carrots and two whole tomatoes a day for several months. When they cut down on the carrots and tomatoes, the color faded.
Now, let's see . . . it's September 1, and you've been invited to a Halloween party. Maybe this year you'll go as a pumpkin. If you start packing in the carrots and tomatoes right now. . . .
Hand in hand: How vitamins help each other
All vitamins have specific jobs in your body. Some have partners. Here are some examples of nutrient cooperation:
I Vitamin E keeps vitamin A from being destroyed in your intestines.
I Vitamin D enables your body to absorb calcium and phosphorus.
I Vitamin C helps folate build proteins.
I Vitamin B1 works in digestive enzyme systems with niacin, pantothenic acid, and magnesium.
Taking vitamins with other vitamins may also improve body levels of nutrients. For example, in 1993, scientists at the National Cancer Institute and the U. S. Department of Agriculture (USDA) Agricultural Research Service gave one group of volunteers a vitamin E capsule plus a multivitamin pill; a second group, vitamin E alone; and a third group, no vitamins at all. The people getting vitamin E plus the multivitamin had the highest amount of vitamin E in their blood — more than twice as high as those who took plain vitamin E capsules.
Sometimes, one vitamin may even alleviate a deficiency caused by the lack of another vitamin. People who do not get enough folate are at risk of a form of anemia in which their red blood cells fail to mature. As soon as they get folate, either by injection or by mouth, they begin making new healthy cells. That's to be expected. What's surprising is the fact that anemia caused by pellagra, the niacin deficiency disease, may also respond to folate treatment.
Isn't nature neat?
Traditionally, the recommended dietary allowances of vitamin A are measured in International Units (IU). However, because retinol is the most efficient source of vitamin A, the modern way to measure the RDA for vitamin A is as retinol equivalents, abbreviated as RE. One microgram (mcg) RE = 3.3 IU. However, many vitamin products still list the RDA for vitamin A in IUs.
If I say "bones" or "teeth," what nutrient springs most quickly to mind? If you answer calcium, you're giving only a partial picture. True, calcium is essential for hardening teeth and bones. But no matter how much calcium you consume, without vitamin D, your body can't absorb and use the mineral. So vitamin D is vital for building — and holding — strong bones and teeth.
Researchers at the Bone Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston say vitamin D may also reduce the risk of tooth loss by preventing the inflammatory response that leads to periodontal disease, a condition that destroys the thin tissue (ligaments) that connects the teeth to the surrounding jawbone. Finally, a report in the February 2oo6 issue of The American Journal of Public Health suggests that taking 1,ooo international units (IU) of vitamin D a day may cut in half a person's risk of developing some forms of cancer, including cancer of the colon, breast, or ovaries.
Vitamin D comes in three forms: calciferol, cholecalciferol, and ergocalciferol. Calciferol occurs naturally in fish oils and egg yolk. In the United States, it's added to margarines and milk. Cholecalciferol is created when sunlight hits your skin and ultraviolet rays react with steroid chemicals in body fat just underneath. Ergocalciferol is synthesized in plants exposed to sunlight. Cholecalciferol and ergocalciferol justify vitamin D's nickname: the Sunshine Vitamin.
The RDA for vitamin D is measured either in International Units (IUs) or micro-grams (mcg) of cholecalciferol: io mcg cholecalciferol = 4oo IU vitamin D.
Every animal, including you, needs vitamin E to maintain a healthy reproductive system, nerves, and muscles. You get vitamin E from tocopherols and tocotrienols, two families of naturally occurring chemicals in vegetable oils, nuts, whole grains, and green leafy vegetables — your best natural sources of vitamin E.
Tocopherols, the more important source, have two sterling characteristics: They're anticoagulants and antioxidants that reduce blood's ability to clot, thus reducing the risk of clot-related stroke and heart attack. Antioxidants prevent free radicals (incomplete pieces of molecules) from hooking up with other molecules or fragments of molecules to form toxic substances that can attack tissues in your body. In fact, nutrition scientists at Purdue University released a study showing that vitamin E promotes bone growth by stopping free radicals from reacting with polyunsaturated fatty acids (see Chapter 7 for information on fats) to create molecules that interfere with the formation of new bone cells.
But some claims about E's heart health benefits are now considered iffy. True, a recent clinical trial at Cambridge University in England showed that taking 8oo IU (International Units) of vitamin E, two times the RDA, may reduce the risk of nonfatal heart attacks for people who already have heart disease. And, yes, the federal Women's Health Study found that older women taking 6oo IU vitamin E per day had a lower risk of heart attack and a lower risk of death from heart disease. But the Heart Outcomes Prevention Evaluation (HOPE) study showed no such benefits. In fact, people taking 4oo IU per day vitamin E were more likely to develop heart failure. No one (and no study) has found similar problems among those taking less vitamin E, say ioo IU/day. Whew.
The best sources of vitamin E are vegetables, oils, nuts, and seeds. The RDA is expressed as milligrams a-tocopherol equivalents (abbreviated as a-TE).
Vitamin K is a group of chemicals that your body uses to make specialized proteins found in blood plasma (the clear fluid in blood), such as prothrom-bin, the protein chiefly responsible for blood clotting. You also need vitamin K to make bone and kidney tissues. Like vitamin D, vitamin K is essential for healthy bones. Vitamin D increases calcium absorption; vitamin K activates at least three different proteins that take part in forming new bone cells. For example, a report on 888 men and women from the long-running Framingham (Massachusetts) Heart Study shows that those who consumed the least vitamin K each day had the highest incidence of broken bones. The same was true for a 1999 analysis of data from the Nurses' Health Study.
Vitamin K is found in dark green leafy vegetables (broccoli, cabbage, kale, lettuce, spinach, and turnip greens), cheese, liver, cereals, and fruits, but most of what you need comes from resident colonies of friendly bacteria in your intestines, an assembly line of busy bugs churning out the vitamin day and night.
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