Are Too Many Vitamins Harmful?

Vitamins help your body perform a variety of physiological functions that are essential for good health. There are a total of 13 vitamins and each one must be consumed daily to maintain proper body function. Too much of a certain vitamin, however, can lead to a vitamin toxicity, which can have negative health effects.

Water-Soluble vs. Fat-Soluble

Vitamins are classified as either water-soluble or fat-soluble based on how your body absorbs them. Water-soluble vitamins dissolve in water when they enter your body and travel directly into your bloodstream. The water-soluble vitamins include the B vitamins and vitamin C. Fat-soluble vitamins need dietary fat in order to be absorbed. The fat-soluble vitamins, which include A, D, E and K, travel through the digestive system bound to fat and then enter your lymphatic system. After the fat-soluble vitamins circulate through your lymphatic system, they finally move into your bloodstream.

Because water-soluble vitamins are dissolved in water, excess amounts are carried to the kidneys, where they are excreted through your urine. The body has the ability to store excess amounts of fat-soluble vitamins. Because of this ability, fat-soluble vitamins have more of a potential to cause toxicity than water-soluble vitamins.

Most Common Toxicities

The most common toxicities are those from the fat-soluble vitamins A, D and E. Vitamin K has not been shown to have any toxic effects in large amounts. Four of the nine water-soluble vitamins also have a potential for toxicity, although it is not as high. The water-soluble vitamins that may cause a toxicity include niacin, vitamin B6, folate and vitamin C.

Symptoms

Symptoms of a vitamin toxicity vary based on which vitamin is causing the toxicity. Different vitamins affect different body systems. A vitamin A toxicity, also referred to as hypervitaminosis A, causes nausea, vomiting, headaches, dizziness and blurred vision. If left untreated, hypervitaminosis A can lead to serious liver damage. Vitamin D toxicity manifests as loss of appetite, nausea, vomiting and constipation. Excess amounts of vitamin E can lead to increased risk of bleeding and hemorrhaging.

Niacin toxicities cause flushing of the face and chest, nausea and vomiting. Too much niacin can also lead to high blood sugar and liver damage. Too much vitamin B6 can cause nerve damage. A folate toxicity does not have symptoms, but it can mask the symptoms of vitamin B12 deficiency, according to "Nutrition and You" by Joan Salge Blake. Vitamin C toxicity manifests as nausea, abdominal cramps and diarrhea.

Tolerable Upper Intake Levels

In order to avoid toxic effects caused by excessive vitamin intake, the Food and Nutrition Board has set upper tolerable intake levels for the vitamins that have toxic potential. The upper tolerable intake level is the highest amount of a specific vitamin that can be consumed without causing any negative health effects. The tolerable upper intake levels for vitamin A, vitamin D and vitamin E are 3,000 mcg, 50 mcg and 1,000 mg, respectively. Tolerable upper intake levels for the water-soluble vitamins niacin, vitamin B6, folate and vitamin C are set at 35 mg, 100 mg, 1000 mcg and 2,000 mg, respectively.

Considerations

Most vitamin toxicities occur as a result of excess vitamin supplementation, rather than consuming large amounts of the vitamin in your diet. When choosing a supplement, it is important to read the labels and make sure that the vitamin amounts do not exceed the upper tolerable intake levels. It may be helpful to work with your doctor or dietitian to determine which types of vitamins are right for you.