Health Encyclopedia

Vitamins: Too Much of a Good Thing?

A balanced diet is your best source for essential vitamins and minerals, but sometimes a dietary supplement is necessary. Your health care provider can help you determine what kind of supplement you need.

Beyond that, higher doses of supplements may be harmful, says the Office of Dietary Supplements (ODS). If you make a habit of taking large, or "mega-doses," of nutrients, you could risk serious or even fatal illness. Always check with your health care provider before taking a dietary supplement, particularly one that contains more than the Recommended Daily Allowance (RDA).

Most vitamins and minerals are provided adequately by foods in a well-balanced diet.

Water-soluble vs. fat-soluble

Vitamins are classified as fat-soluble or water-soluble. Fat-soluble vitamins (vitamins A, D, E, and K) are stored in the liver and used slowly by the body. Water-soluble vitamins (vitamins C and B complex) are not stored in the body; when your body has all it needs, it eliminates any extra amount in your urine.

Your body makes three of the vitamins you need: D, K, and the B vitamin biotin. The amount of vitamins and minerals, as well as other nutrients, that you should have each day is called the RDA.

Here's a look at the essential vitamins and minerals your body needs. The RDAs are from the ODS. You can read more about vitamins and RDAs at the ODS website.

Vitamin A

Vitamin A is a fat-soluble vitamin. It plays an important role in good night vision, bone growth, reproduction, and cell division. It maintains the surface linings of your eyes and your respiratory, urinary, and intestinal tracts. It helps your body regulate its immune system and may help prevent bacteria and viruses from entering your body by keeping your skin and mucus membranes healthy.

Good sources of vitamin A include whole eggs, whole milk, fish liver oil, and liver. Your body can also make vitamin A from substances called carotenoids found in dark red, dark green, and yellow vegetables. Most fat-free and dried nonfat milk are fortified with vitamin A to replace what was lost when the fat was removed. Fortified breakfast cereals and other fortified foods also provide vitamin A.

How much: For teen boys and men, the RDA is 900 micrograms (mcg); for teen girls and women, 700 mcg. (Women who are pregnant need 750 mcg and those who are breastfeeding need 1,300 mcg.) Children need less: Youngsters ages 1 to 3 need 300 mcg; ages 4 to 8 need 400 mcg; and ages 9 to 13 need 600 mcg. Vitamin A deficiencies rarely occur in this country.

The risks: In very large doses, such as those found in supplements, it can reduce bone-mineral density, leading to osteoporosis. Large doses also can cause bone and joint pain; liver and nerve abnormalities; hair loss; blurred vision; and birth defects, if taken by a pregnant woman. People who drink heavily should not take vitamin A supplements because of increased risk for liver damage. People who smoke should not take vitamin A supplements because of increased risk for lung cancer. The upper limits for daily vitamin A intake by age are adults, 3,000 mcg; teens, 2,800 mcg; children ages 9 to 13, 1,700 mcg; children ages 4 to 8, 900 mcg; children ages 1 to 3, 600 mcg; and infants up to a year, 600 mcg.

Vitamin B6 (Pyridoxine)

Vitamin B6 is a water-soluble vitamin that performs a wide variety of functions in your body. It is needed for more than 100 enzymes involved in using protein. It is essential for making red blood cells and for making hemoglobin. It helps the nervous and immune systems function efficiently. It is needed to make neurotransmitters such as serotonin and dopamine (substances that are required for normal nerve cell communication). Vitamin B6 is needed by your body to convert the amino acid tryptophan into the vitamin niacin. B6 can help maintain your blood sugar in a normal range by helping to convert stored carbohydrate to glucose when you have not eaten enough food.

Good sources of vitamin B6 are fortified cereals, beans, meat, poultry, fish, and some fruits and vegetables.

How much: The RDA for adult men and women up to age 50 is 1.3 mg. (Women who are pregnant need 1.9 mg and those who are breastfeeding need 2.0 mg.) For men ages 51 and older, the RDA is 1.7 mg; for women in that same age group, the RDA is 1.5 mg. Although inadequate intake of vitamin B6 is rare in this country, alcoholics and older adults are more likely to have inadequate vitamin B6 because they may have limited variety in their diet. Alcohol also promotes the destruction and loss of vitamin B6 from the body.

The risks: Daily supplemental doses of more than 100 mg of B6 may cause nerve damage to your arms and legs. This nerve damage, called neuropathy, is usually not caused by consuming too much in your diet.

Vitamin D

Vitamin D is a fat-soluble vitamin. Its main job in your body is to maintain normal levels of calcium and phosphorus. Vitamin D helps your body absorb calcium, keeping your bones strong. Working with several other vitamins, minerals, and hormones, vitamin D helps bone mineralization. A deficiency in vitamin D can cause the bones to lose calcium and become more susceptible to fracture. This vitamin prevents two bone-weakening diseases: rickets in children and osteomalacia, which causes muscle weakness and weak bones in adults.

Your body makes vitamin D when your skin is exposed to ultraviolet rays from the sun. The amount of exposure depends on where you live, the time of year, the time of day, cloud cover, and whether you wear sunscreen. Generally, 15 minutes of sun exposure, without sunscreen, several times a week is sufficient for your body to make the vitamin D it needs. Sunscreen with a sun protection factor of 8 or greater will block the rays needed for making vitamin D. Experts say, however, that it's important to wear sunscreen whenever you will be in the sun for longer than 10 to 15 minutes.

You can boost your vitamin D supply by getting enough in your diet. Good food sources for vitamin D are those fortified with this vitamin, plus fatty fish and fish oils. Milk is one of the major fortified foods; one cup of milk provides about a fourth of the vitamin D an adult requires. Other milk products, such as cheese, yogurt, and ice cream, usually are not fortified.

How much: Assuming minimal sun exposure, the 2010 Dietary Guidelines for Americans recommend that adult men and women younger than 70 get 600 IU a day. Over age 70, men and women should get 800 IU a day.

The risks: Too much vitamin D can cause nausea, vomiting, poor appetite, constipation, weakness, and weight loss. An excess of this vitamin can also raise the level of calcium in the blood, which can cause mental confusion and abnormal heart rhythms. Unless you consume large quantities of cod liver oil, it is unlikely that you will get too much vitamin D from your diet. Vitamin D toxicity comes from supplements. The Institute of Medicine says that the safe limit for vitamin D intake is 1,000 to 1,500 IU/day for infants, 2,500 to 3,000 IU/day for children 1–8 years, and 4,000 IU/day for children 9 years and older and adults.

Some experts worry that people who take vitamin D supplements and consume foods like beverages and cereals with added calcium may develop calcification in heart tissue.

Vitamin E

Vitamin E is a fat-soluble vitamin that exists in eight different forms. It has a role in the immune system and other metabolic processes. The most active form of vitamin E in humans is alpha-tocopherol. Alpha-tocopherol is a powerful antioxidant. Antioxidants protect your cells against the effects of free radicals, which are potentially damaging by-products of energy metabolism. Free radicals can damage cells and may contribute to the development of cardiovascular disease and cancer. Researchers are trying to determine whether vitamin E might help prevent or delay the development of those chronic diseases.

Vitamin E is found in vegetable oils; nuts; green leafy vegetables; and fortified cereals. The Dietary Guidelines for Americans state that most Americans should increase their consumption of foods rich in vitamin E. An actual deficiency of vitamin E, however, is rare.

How much: Males and females ages 14 and older should get 15 mg of vitamin E a day. (Women who are breastfeeding need 19 mg.) Children ages 9 to 13 should get 11 mg a day. For ages 4 to 8, the RDA is 7 mg; for ages 1 to 3, the RDA is 6 mg.

People at risk for vitamin E deficiency are those with fat malabsorption (Crohn’s disease, cystic fibrosis, sprue); low-birth-weight, premature infants; and people with certain rare genetic disorders.  Large doses of supplemental vitamin E have been touted to prevent heart disease, cancer, and cataracts, but with little evidence. 

The risks: Excessive intake of vitamin E may actually increase the risk for heart failure, especially in people with cardiovascular disease. Vitamin E can also act as an anticoagulant and in high doses may increase the risk for bleeding problems. 

Niacin (Vitamin B3)

Niacin, also called nicotinic acid, is a water-soluble vitamin made in your body from the amino acid tryptophan. It helps your body make energy from carbohydrates and bolsters the health of your skin, nervous system, and digestive tract. In higher doses, niacin can raise HDL ("good') cholesterol.

How much: The RDA for males 14 years and older is 16 mg. Females of the same age group need 14 mg. (Women who are pregnant need 8 mg and those who are breastfeeding need 17 mg). Children ages 9 to 13 years old need 12 mg. Those ages 4 to 8 require 8 mg; ages 1 to 3 need 6 mg. 

The risks: Very high doses (3 to 9 grams a day) can cause severe liver damage. A reversible but unpleasant skin condition called "niacin flush" can occur at daily doses as low as 50 mg. Gastrointestinal side effects such as nausea, vomiting, and abdominal pain can take place with doses greater than 1,000 mg per day. That's especially true with slow-release products.

Iron

Iron is an essential mineral in your body. It plays a crucial role in both oxygen transport and use. Almost two-thirds of the iron in your body is in the protein hemoglobin, found in red blood cells. The rest of the iron in your body is stored until needed. Adult men and postmenopausal women lose little iron from their bodies. Women with heavy menstrual periods can lose a significant amount of iron, according to the National Institutes of Health (NIH).

Good sources of iron include meat, fish, poultry, lentils, beans, flours, cereals, and grain products fortified with iron.

How much: The RDA for men 19 and older is 8 mg. For women ages 19 to 50, it is 18 mg; after age 50 (after menopause), it drops to 8 mg. (Women who are pregnant need 27 mg and those who are breastfeeding need 9 to 10 mg.) For children, the RDA is 11 mg for ages 6 to 12 months, 7 mg for ages 1 to 3 years, 10 mg for ages 4 to 8, and 8 mg for ages 9 to 13. For teens 14 to 18, the RDA is 11 mg for males and 15 mg for females.

The risks: Because almost no iron is excreted from the body of children, men, and nonmenstruating women, getting too much iron can be dangerous or even fatal. This is especially true for children. Five or six high-potency iron tablets can be fatal to a child weighing 22 pounds, according to the NIH. One to 3 grams can be fatal to a child under age 6; lower doses can cause severe vomiting and diarrhea. Keep all iron supplements tightly capped and out of children's reach. Treat any excess iron ingestion as a poisoning emergency. Symptoms of excess iron in adults includes constipation, nausea, vomiting, and diarrhea.

People with hemochromatosis, a condition in which more iron is absorbed and stored than normal, should NOT take iron supplements. Most cases of hemochromatosis are genetic, but people with it often don't know they have it until after age 40, when symptoms such as joint pain and fatigue first appear. The excess iron is stored in body organs, which are damaged by the overload. Most multiple vitamins for adults are no longer formulated with iron to reduce the risk for iron accumulation. Prenatal vitamins, however, do contain iron. 

Magnesium

Magnesium is another essential mineral. It helps with more than 300 chemical reactions in the body. It maintains normal muscle and nerve function, keeps heart rhythm steady and bones strong, helps in making proteins, helps regulate blood sugar levels, promotes normal blood pressure, and helps in energy metabolism. About 50 percent of the magnesium in your body is stored in the cells of your tissues and organs; about half combines with calcium and phosphorus in your bones; and about 1 percent is found in blood.

Good sources of magnesium are green leafy vegetables, nuts, seeds, and some whole-grain foods. Tap water that is “hard” (meaning it contains more minerals) can be a source of magnesium, but the amount varies according to the water supply.

How much: The RDA for magnesium for men ages 19 to 30 is 400 mg; men 31 and older should get 420 mg. Women ages 19 to 30 should get 310 mg; women 31 and older should get 320 mg. (Women who are pregnant or breastfeeding should get more.) The RDA for teens 14 to 18 is 410 mg for boys and 360 mg for girls. The RDA for children ages 9 to 13 is 240 mg; for children ages 4 to 8, it is 130 mg; for children ages 1 to 3, it is 80 mg. 

The risks: The amount of magnesium in your diet does not pose any health risk, but magnesium supplements, often found in laxatives and antacids, may lead to diarrhea. Very high doses of magnesium can cause kidney failure.

Selenium

Selenium is an essential trace mineral, required in only small amounts. It helps protect cells against the effects of free radicals, according to the NIH. It also helps keep the immune system and thyroid gland functioning.

Good sources of selenium are meats, breads, Brazil nuts, and walnuts. In general, selenium is found in the soil, and makes its way into our diet through plants that grow in selenium-rich soil or through animals that feed on plants grown in that soil.

How much: The RDA for males and females 14 years and older is 55 mcg. (Women who are pregnant need 60 mcg and those who are breastfeeding need 70 mcg.) Children ages 9 to 13 years old need 40 mcg; for ages 4 to 8 years, it is 30 mcg; for ages 1 to 3 years it is 20 mcg. 

The risks: It is unlikely that you will get too much selenium in your diet, even if you take a supplement. High levels of selenium in the blood can lead to a condition called selenosis, according to the NIH. Symptoms of selenosis include gastrointestinal upsets, hair loss, white blotchy nails, and mild nerve damage.

Zinc

Zinc is an essential mineral found in almost every cell in your body, according to the NIH. Zinc stimulates the activity of about 100 enzymes. It helps maintain your immune system and your senses of taste and smell; it helps wounds heal and helps your cells make DNA. It's also important for growth and development of children, from pregnancy through adolescence.

Good sources of zinc include oysters, red meat, poultry, beans, nuts, whole grains, fortified breakfast cereals, and dairy products. A person with a diet high in animal protein will absorb more zinc than a person who eats mostly plant protein. That's because substances called phytates found in breads, cereals, and beans decrease zinc absorption, according to the NIH.

How much: The RDA for teen boys and men is 11 mg; for women, it's 8 mg. (Women who are pregnant need 11 to 12 mg and those who are breastfeeding need 12 to 13 mg.) The RDA for children is 3 mg for ages 6 months to 3 years; 5 mg for ages 4 to 8 years; and 8 mg for ages 9 to 13. Teenage girls need 9 mg.

The risks: Consuming too much zinc—more than 150 mg—can interfere with the functioning of copper and iron in the body. Too much zinc also interferes with immune system function and reduces the level of HDL cholesterol. The safe upper limit of zinc intake for adults, according to the National Academy of Sciences, is 40 mg. For children, the safe upper limits are 4 mg for ages newborn to 6 months; 5 mg for ages 7 months to a year; 7 mg for ages 1 to 3 years; 12 mg for ages 4 to 8; 23 mg for ages 8 to 13; and 34 mg for ages 14 to 18.