Avoiding Toxicity and Overdosing

Because some vitamins can be toxic if taken at extremely high doses and/or in the wrong form, learning how they are formed, how they act, and how they are cleared from the body can help in understanding how best to ensure the safe use of supplements. Vitamin A, for example, offers an excellent lesson in how excess intake, particularly in the wrong form, can have toxic consequences.

Vitamin A is actually a hormone, which the body forms from the beta-carotene found in bright orange carrots, pumpkin, or squash. Because the body will only convert the orange pigment betacarotene to vitamin A when it is needed, if you eat too many orange plant foods, your skin might turn orange, but you will not be subject to vitamin A toxicity.

By contrast, vitamin A toxicity is possible when vitamin A or vitamin A containing foods are eaten directly. For example, eskimos, who eat whale liver that is rich in stored vitamin A, could develop health issues from vitamin A poisoning, such as liver disease or bone disease.

Toxicity occurs with intakes of greater than 25,000 international units (IU) of vitamin A, which is only five times the required amount or RDA of 5,000 IU. At one time, multivitamins could contain up to 10,000 IU of vitamin A. However, after reports from Harvard population scientists that nurses taking more than 8,500 IU of vitamin A from foods and supplements had evidence of bone disease, the vitamin industry reduced or eliminated vitamin A from multivitamins and replaced half or all with beta-carotene, which could be safely converted to vitamin A without leading to vitamin A toxicity.

Of note, although beta-carotene is safe when ingested from foods, some have questioned its safety when taken in mega-doses as supplements. After noting a lower incidence of lung cancer in people who had high levels of beta-carotene in their blood, investigators at the National Cancer Institute gave beta carotene supplements to smokers in an effort to prevent the development of lung cancer. The study was stopped prematurely because the subjects actually showed an increased risk of lung cancer. However, the dose was so high that subjects were, in effect, given six times the amount of betacarotene found in a healthy diet. Thus, although the study results led to questions of whether excessive intake of beta-carotene is safe in smokers, the vitamin remains safe when ingested from foods or when taken at standard doses in multivitamins.