Have You Ever Wondered 124

. . . why a nutrient supplement label may list the percent of vitamin A from beta carotene? The supplement may contain beta carotene but not vitamin A itself. However, the body converts beta carotene to vitamin A. . . . if ridges or white marks on your fingernails suggest a vitamin deficiency? No, but it's a common misconception. Instead, they're often caused by a slight injury to the nail. Although they may have other causes, too, a nutrient deficiency isn't one of them.

Appearance-conscious teens often hear that taking gelatin pills strengthens nails, but there's no quick nutritional cure for nails that break and split. Fingernails are mainly dead protein cells that get their strength from amino acids. Gelatin doesn't contain these amino acids.

. . . if supplements with "phytonutrients" are a good choice? Fromphyto, Greek for plant, these botanical substances are extracted from vegetables and other plant foods. There's not enough scientific evidence to know if supplement manufacturers have picked the right active substance from plant sources for any benefit.

Plants have thousands of phytonutrients. Science hasn't yet revealed which one, if any, or what amount in a supplement might offer any health benefits. Better advice: Get "phytos" from food. Any health-promoting benefit might come from the interaction of many phytonutrients provided naturally in food.

. . . if dietary supplements can protect against biological threats? No, although some supplement promoters may make this claim. According to the Centers for Disease Control and Prevention and the U.S. Food and Drug Administration (FDA), no current and credible scientific evidence suggests that supplements on the market today offer protection from or treatment for biological contaminants such as anthrax, SARS, or bird (avian) flu.

Likewise, FDA advises against taking antibiotics for protection from foodborne illnesses caused by bacteria, unless prescribed by a doctor. Antibiotics can't protect against viruses or chemicals that contaminate food.

example, taking extra vitamin B6 has been suggested to help relieve premenstrual tension. Yet there's limited evidence to support large vitamin B6 doses for relief of premenstrual syndrome (PMS). Many women have viewed large vitamin B6 doses as harmless, since they are water-soluble. Instead, they may cause irreversible nerve damage when taken in very large doses above the Tolerable Upper Intake Level (UL): 500 to 5,000 mg vitamin B6 per day.

As other examples, very high doses of vitamin C can cause diarrhea and nausea. Liver damage may be caused by high doses of niacin (as time-released nicotinic acid); sometimes a physician will prescribe high doses of niacin to help lower an elevated blood cholesterol level. Excessive amounts of folic acid can hide symptoms of pernicious anemia, so the disease gets worse without being detected.

Children are more vulnerable to overdoses of vitamins and minerals than adults. In fact, excessive iron—perhaps from iron supplements intended for their mother—can be fatal to children.

The way your body handles large nutrient doses from dietary supplements depends on many factors. Your body size, supplement dose (amount and fre quency), and how long you take them influence whether a megadose will be toxic for you.

See the Appendices for the Tolerable Upper Intake Level (UL) for many nutrients. The UL is the maximum amount that appears safe for most healthy people. Consuming more may increase some health risks. See chapter 4 for more about vitamins and minerals that may be sold in supplement form.

Nutrient-Nutrient Interactions. High doses of some nutrients may result in deficiencies of others. For example: high calcium intake may inhibit the absorption of iron and other trace nutrients. High doses of vitamin E can interfere with the action of vitamin K and make anticoagulant drugs such as Coumadin (warfarin) more powerful.

Even low levels of dietary supplements may contribute to health problems for some people. For example, those at risk for hemochromatosis need to be careful of taking extra iron. Folic acid can mask a vitamin B12 deficiency, which may cause neurological damage. And zinc supplements in excess of the UL can decrease levels of "good" cholesterol (high-density lipoprotein blood cholesterol), impair immunity, and reduce copper status.

Benefits. Emerging research is exploring the link between higher levels of some antioxidant nutrients (vitamin C, vitamin E, selenium) and omega-3s with reduced risk for some health problems. However, the jury's still out. Currently available research doesn't show that levels higher than the Recommended Dietary Allowance are effective in cancer or heart disease prevention. Until more is known, be cautious about taking supplements to protect against disease. See "Antioxidant Nutrients: Enough, or Too Much?" in this chapter.

Except for rare medical conditions, few people need more than 100 percent of their Recommended Dietary Allowances (RDAs) of any nutrient. Large mineral or vitamin doses are prescribed only for certain medically diagnosed health problems. Even then, their use should be monitored carefully by a doctor.

That said, can you overdose on vitamins or minerals naturally occurring in food? That's highly unlikely. As we mentioned, taking very high doses of dietary supplements—or taking too many, too often—can be dangerous. The vitamin and mineral content of food is much more balanced. In amounts normally consumed, even if you enjoy extra helpings, you won't consume toxic levels of nutrients. So eat a variety of foods—and enjoy! Note: Nutrient amounts can add up if you consume a lot of highly fortified foods.

Other Cautions

You may take dietary supplements for potential health benefits. It's not uncommon for people diagnosed with cancer, AIDS, or other life-threatening health problems, who are desperate for a cure, to put their hopes and healthcare dollars in alternative treatments, including dietary supplements. However, supplements may offer a false sense of security—and a serious problem if you neglect well-proven approaches to health or delay medical attention.

If you choose to take a dietary supplement, ask your healthcare provider. See "Guidelines for Supplement Use " in this chapter. Seek medical attention and proven treatment for health problems first. Even if you're healthy, get regular medical checkups, eat wisely, and live a healthful lifestyle rather than rely on the "security" of supplements.

What We Know About...

Calcium Supplements: A Bone Builder. For people of every age, food choices can supply an adequate amount of calcium. As an extra safeguard, many doctors also recommend calcium supplements, especially for menopausal and postmenopausal women and for women who simply don't consume enough calcium. The reason? To help stave off bone loss that comes with hormonal changes. If you're advised to take a calcium supplement, keep these pointers in mind:

• Read the label. Calcium in all over-the-counter supplements isn't the same. First, the calcium amount differs. Multivitamin/mineral supplements don't have as much calcium as calcium supplements do.

Calcium supplements are sold as compounds, such as calcium citrate and calcium carbonate. Their amounts and absorption of calcium differ; check the label. Aiding absorption, chewable and liquid calcium supplements dissolve even before entering the stomach. Calcium carbonate is absorbed best with food; calcium citrate can be taken any time.

  • Consider a calcium supplement or a multivitamin with vitamin D if you don't get enough from food. Vitamin D is essential for calcium absorption.
  • Avoid calcium supplements with dolomite, unrefined oyster shell, or bonemeal without a USP symbol. They might contain small amounts of hazardous contaminants: lead, arsenic, mercury, or cadmium. What's dolomite? A mineral compound found in marble and limestone.
  • Take calcium supplements as intended—as a supplement, not as your only important calcium source. Although calcium supplements may boost calcium intake, they don't provide other nutrients your bones and body need: vitamin D, magnesium, phosphorus, and boron. Milk, for example, provides vitamin D, a nutrient that helps deposit calcium in your bones.
  • If you take both calcium and iron supplements, take them at different times of the day. They'll each be better absorbed when taken on their own.
  • If you take two or three low-dose tablets daily, space them throughout the day for better absorption. Calcium in supplements is absorbed best in doses of 500 milligrams or less.
  • Follow the dosage advised by your healthcare provider. The Tolerable Upper Intake Level for calcium is 2,500 milligrams daily from food and supplements.
  • Drink plenty of fluids with calcium supplements to avoid constipation. The lactose and vitamin D in the milk help to enhance calcium absorption.
  • If you take medications or other supplements, ask your doctor or registered dietitian about interactions. For example, calcium and tetracycline bind; neither is adequately absorbed as a result. Calcium also inhibits magnesium, phosphorus, and zinc absorption.
  • If you don't drink milk and want an alternative to calcium pills, consider calcium-fortified juice or soy beverage. One cup of calcium-fortified juice or soy beverage can contain about 300 milligrams of calcium, the same amount as in a cup of milk, and provides vitamin C, folate, and other nutrients. Still, you need a vitamin D source to aid absorption; some calcium-fortified juices and soy beverages are also fortified with vitamin D.
  • Calcium supplements—to protect against osteoporosis (brittle-bone disease)—can't make up for your lifestyle choices or poor health habits. Regular weight-bearing physical activity is important for healthy bones. For healthy bones, avoid smoking, too.

Are calcium supplements right for everyone? For people with kidney damage or urinary tract stones, calcium supplements pose risks. If you have a history of kidney stones, take calcium supplements under your doctor's care. See "Osteoporosis: Reduce the Risks"in chapter 22.

Coral calcium? No evidence shows that it's better. People with shellfish allergies may react to coral calcium since it's from coral reefs. It also may contain lead, which can be dangerous. Refer to chapter 8, "Get the Lead Out."

Iron Supplements: Enhancing the Benefit. Physicians often advise iron supplements for premenopausal and pregnant women, and for some children and teens. If you're advised to take an iron supplement, remember:

  • Pick a better-absorbed form of iron (ferrous sulfate).
  • Check the dosage when choosing an iron supplement. Dosages of 15 to 30 milligrams per day are
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