Which Vitamins Are Bad For You?

Vitamins are a diverse group of nutrients that support a variety of functions in your body. Along with minerals, vitamins are classified as micronutrients, meaning you need to consume relatively small amounts of them to fulfill your needs. By definition, vitamins -- "vital amines" -- are essential for normal cellular function and optimal health. Thus, no vitamin is bad for you when it is consumed in appropriate amounts. However, some vitamins can have toxic effects when they are ingested in high doses. The Institute of Medicine has established tolerable upper intake levels, or ULs, for those vitamins that have been associated with toxicity symptoms.

Fat-Soluble Vitamins

Vitamins A, D, E and K are called "fat-soluble" vitamins because they are found in the oils and fats of the plants and animals that synthesize and consume them. These nutrients can be stored in your tissues for relatively long periods, so they can cause toxic symptoms if you take too much. Such symptoms include headache, abdominal pain, nausea, vomiting, diarrhea, sweating, flushing, dry skin, irritability, weakness and immune suppression. The UL for vitamin A ranges from 2,000 IU for toddlers to 10,000 IU for adults. The UL for vitamin D ranges from 1,000 IU for infants to 2,000 IU for adults. For vitamin E supplements, the UL is 1,500 IU. There is no UL for vitamin K.

B Vitamins

In general, the B vitamins are water-soluble and do not cause toxicity concerns. However, high doses of some B vitamins can cause problems, such as tingling extremities, flushing, stomach irritation, vomiting, weakness, increased salivation and unusual body odor. High doses of some B vitamins can also interfere with the functions of other nutrients. The Institute of Medicine has established ULs for four of the B vitamins -- niacin, pyridoxine, choline and folate. The UL for niacin, which applies specifically to supplements, is 35 mg for all adults. Although vitamin B-6, or pyridoxine, is usually well-tolerated in doses up to 200 mg daily, the IOM's UL for this nutrient is 100 mg for adults. For choline, the UL is 3.5 g daily, and for folate supplements the UL for adults is 1 mg.

Vitamin C

Ascorbic acid, or vitamin C, is another water-soluble vitamin that is readily eliminated in your urine, but high doses can cause diarrhea, nausea, urinary burning, skin sensitivity and breakage of red blood cells. According to Dr. Elson Haas, author of "Staying Healthy with Nutrition," most of these symptoms are only associated with doses of 5 to 10 g daily. Nevertheless, the Institute of Medicine has established a UL for vitamin C of 2,000 mg daily for adults.

Considerations

The Institute of Medicine has established tolerable upper limits for vitamins that cause toxic symptoms when they are ingested in high doses. Many of these upper limits are not based on toxicities, per se, but on interactions with other nutrients or a perceived need to prevent imbalances in vitamin intake. Some guidelines, such as those for vitamin C, do not necessarily coincide with real-world experience, for many individuals routinely take doses higher than the ULs without symptoms or side effects. If you have questions about your vitamin needs, talk to your physician or a nutritionist.