RDAs are broad enough to prevent vitamin deficiencies and avoid the side effects associated with very large doses of some vitamins. If your diet doesn’t meet these guidelines, or if you take very large amounts of vitamins as supplements, you may be in for trouble.

The good news is that vitamin deficiencies are rare among people who have access to a wide variety of foods and know how to put together a balanced diet. For example, the only people likely to experience a vitamin E deficiency are premature and/or low–birth weight infants and people with a metabolic disorder that keeps them from absorbing fat.

A healthy adult may go as long as 10 years on a vitamin E–deficient diet without developing any signs of a problem. Aha, you say, but what’s this subclinical deficiency I hear so much about? Nutritionists use the term subclinical deficiency to describe a nutritional deficit not yet far enough advanced to produce obvious symptoms.

In lay terms, however, the phrase has become a handy explanation for common but hard-to-pin-down symptoms such as fatigue, irritability, nervousness, emotional depression, allergies, and insomnia. And it’s a dandy way to increase the sale of nutritional supplements. Simply put, the RDAs protect you against deficiency.

If your odd symptoms linger even after you take reasonable amounts of vitamin supplements, probably something other than a lack of any one vitamin is to blame. Don’t wait until your patience or your bank account has been exhausted to find out. Get a second opinion as soon as you can.

Here lists the symptoms of various vitamin deficiencies:

• Vitamin A - Poor night vision; dry, rough, or cracked skin; dry mucous membranes including the inside of the eye; slow wound healing; nerve damage; reduced ability to taste, hear, and smell; inability to perspire; reduced resistance to respiratory infections.

• Vitamin D - In children: rickets (weak muscles, delayed tooth development, and soft bones, all caused by the inability to absorb minerals without vitamin D) In adults: osteomalacia (soft, porous bones that fracture easily)

• Vitamin E - Inability to absorb fat.

• Vitamin K - Blood fails to clot.

• Vitamin C - Scurvy (bleeding gums; tooth loss; nosebleeds; bruising; painful or swollen joints; shortness of breath; increased susceptibility to infection; slow wound healing; muscle pains; skin rashes).

• Thiamin (vitamin B1) - Poor appetite; unintended weight loss; upset stomach; gastric upset (nausea, vomiting); mental depression; an inability to concentrate.

• Riboflavin (vitamin B2) - Inflamed mucous membranes, including cracked lips, sore tongue and mouth, burning eyes; skin rashes; anemia.

• Niacin - Pellagra (diarrhea; inflamed skin and mucous membranes; mental confusion and/or dementia).

• Vitamin B6 - Anemia; convulsions similar to epileptic seizures; skin rashes; upset stomach; nerve damage (in infants)

• Folate - Anemia (immature red blood cells).

• Vitamin B12 - Pernicious anemia (destruction of red blood cells, nerve damage, increased risk of stomach cancer attributed to damaged stomach tissue, neurological/psychiatric symptoms attributed to nerve cell damage)

• Biotin - Loss of appetite; upset stomach; pale, dry, scaly skin; hair loss; emotional depression; skin rashes (in infants younger than 6 months).

Can you get too much of a good thing? Darn right, you can. Some vitamins are toxic when taken in the very large amounts popularly known as megadoses. How much is a megadose? Nobody knows for sure. The general consensus, however, is that a megadose is several times the RDA, but the term is so vague that it’s in neither my medical dictionary nor the dictionary on my computer.

• Megadoses of vitamin A (as retinol) may cause symptoms that make you think you have a brain tumor. Taken by a pregnant woman, megadoses of vitamin A may damage the fetus.
• Megadoses of vitamin D may cause kidney stones and hard lumps of calcium in soft tissue (muscles and organs).
• Megadoses of niacin (sometimes used to lower cholesterol levels) can damage liver tissue.
• Megadoses of vitamin B6 can cause (temporary) damage to nerves in arms and legs, fingers, and toes.

But here’s an interesting fact: With one exception, the likeliest way to get a megadose of vitamins is to take supplements. It’s pretty much impossible for you to cram down enough food to overdose on vitamins D, E, K, C, and all the Bs.

Did you notice the exception? Right: vitamin A. Liver and fish liver oils are concentrated sources of preformed vitamin A (retinol), the potentially toxic form of vitamin A. Liver contains so much retinol that early 20th century explorers to the South Pole made themselves sick on seal and whale liver.

Cases of vitamin A toxicity also have been reported among children given daily servings of chicken liver. (See Table 10-3 for more on vitamin A toxicity, this time from supplements.) On the other hand, even very large doses of vitamin E, vitamin K, thiamin (vitamin B1), riboflavin (vitamin B2), folate, vitamin B12, biotin, and pantothenic acid appear safe for human beings. Lists below the effects of vitamin overdoses.

• Vitamin A - 15,000 to 25,000 IU retinol a day for adults (2,000 IU or more for children) may lead to liver damage, headache, vomiting, abnormal vision, constipation, hair loss, loss of appetite, low-grade fever, bone pain, sleep disorders, and dry skin and mucous membranes. A pregnant woman who takes more than 10,000 IU a day doubles her risk of giving birth to a child with birth defects.

• Vitamin D - 2,000 IU a day can cause irreversible damage to kidneys and heart. Smaller doses may cause muscle weakness, headache, nausea, vomiting, high blood pressure, retarded physical growth, and mental retardation in children, and fetal abnormalities.

• Vitamin E - Large amounts (more than 400 to 800 IU a day) may cause upset stomach or dizziness.

• Vitamin C - 1,000 mg or higher may cause upset stomach, diarrhea, or constipation.

• Niacin - Doses higher than the RDA raise the production of liver enzymes and blood levels of sugar and uric acid, leading to liver damage and an increased risk of diabetes and gout.

• Vitamin B6 - Continued use of 50 mg or more a day may damage nerves in arms, legs, hands, and feet. Some experts say the damage is likely to be temporary; others say that it may be permanent.

• Choline - Very high doses (14 to 37 times the adequate amount) have been linked to vomiting, salivation, sweating, low blood pressure, and — ugh! — fishy body odor.

You may not have to go sky-high on vitamin A to run into trouble. In January 2003, new data from a long-running (30-year) study at University Hospital in Uppsala (Sweden) suggested that taking a multivitamin with normal amounts of vitamin A may weaken bones and raise the risk of hip fractures by as much as 700 percent, a conclusion supported by data released in 2004 from the long-running Nurses’ Health Study.

A high blood level of retinol — from large amounts of vitamin A from food or supplements — apparently inhibits special cells that usually make new bone, revs up cells that destroy bone, and interferes with vitamin D’s ability to help you absorb calcium.

Of course, confirming studies are needed, but you can bet the debate about lowering the amount of A in your favorite supplement will be vigorous. The new recommendations for vitamin A are 700 RE/2,300 IU of vitamin A for women and 900 RE/3,000 IU for men, but many popular multivitamins still contain 750–1500 RE/2,500–5,000 IU.