Vitamins are classified into two categories, water-soluble and fat-soluble, based on how your body absorbs them. There are a total of nine water-soluble vitamins: thiamin, riboflavin, niacin, folate, vitamin B6, vitamin B12, vitamin C, pantothenic acid and biotin. Although most of the water-soluble vitamins are not toxic, some of them have the potential for toxicity.
When you consume foods that contain water-soluble vitamins, the foods are digested and the vitamins are dissolved in water in your body. Once the vitamins are dissolved, they enter the bloodstream. The vitamins that your body needs are absorbed in the small intestine and excess amounts remain in the bloodstream, where they travel to your kidneys. The excess amounts are usually excreted in your urine.
Method of Toxicity
Of the nine water-soluble vitamins, only four have been shown to have the potential for toxicity: niacin, vitamin B-6, folate and vitamin C.
Niacin and vitamin C are not known to be toxic when consumed in large amounts through the food you eat. Their toxic potential is related to intake from supplementation only. Vitamin B-6 can be toxic when consumed in large amounts, although in “Nutrition and You,” Joan Salge Blake states it would be very difficult to eat excessive amounts of the vitamin. The natural form of folate that is found in foods does not have toxic potential. Folic acid in supplements or in fortified foods, however, can be toxic when consumed in large amounts.
Niacin toxicity is usually characterized by flushing, a reddish coloring in the face, arms and chest. In addition, too much niacin can cause nausea and vomiting and raise blood glucose levels. If niacin levels are not corrected, this can lead to liver damage. Vitamin B-6 toxicity can cause nerve damage. Consuming too much folate does not cause any symptoms, but it can mask the symptoms of a B-12 deficiency, which can eventually lead to anemia. Excessive vitamin C intake can cause nausea, stomach cramps and diarrhea. Excess vitamin C can also lead to the development of kidney stones, Salge Blake notes.
Upper Tolerable Intake Levels
To avoid a vitamin toxicity, the Food and Nutrition Board has set an upper tolerable intake level, or UL, for each of the four water-soluble vitamins. The upper tolerable intake level is the highest amount of the vitamin that you can consume on a daily basis without causing any negative side effects.
The upper tolerable intake levels for niacin, vitamin B-6, folate and vitamin C are 35 milligrams, 100 milligrams, 1,000 micrograms and 2,000 milligrams, respectively.
Although some of the water-soluble vitamins have the potential for toxicity, you are more likely to experience deficiencies instead. This is because the body cannot store excess amounts of water-soluble vitamins and excretes the surplus in your urine.