Some vitamins are toxic when taken in the very large amounts known as megadoses. It’s nearly impossible for your diet to provide an overdose of vitamins D, E, K, C, and all the Bs.
Vitamin A is the lone exception. Liver and fish liver oils are concentrated sources of preformed vitamin A (retinol). 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.
However, even very large doses of vitamins E, K, B1, B2, B12, and biotin, and pantothenic acid appear safe for human beings.
What constitutes a megadose? The general consensus is that a megadose is several times the Recommended Daily Allowances (RDAs), which is vague at best. Here are some consequences of vitamin megadoses:
| Vitamin | Overdose and Possible Effect |
|---|---|
| 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.