Vitamins are naturally occurring chemicals in foods that help your body function and grow. Different vitamins are responsible for different functions in the body, and some perform multiple functions. Ideally, you should get all of the vitamins you need from a healthy, balanced diet. When that is not possible, you can also get vitamins through supplementation. Vitamins come in two different forms -- fat-soluble and water-soluble -- and the type of vitamin determines the dosage and how much you need regularly.
Fat-Soluble Vitamins
Vitamins A, D, K and E are the fat-soluble vitamins. These vitamins dissolve in fat and your body stores these vitamins in your body fat and in fatty tissue, such as the liver. Additionally, your body makes its own supply of vitamin D from sunlight. Because your body either stores or makes these vitamins, you do not need to eat them every day, and neither do you need large doses of these vitamins. The only possible exception is vitamin D. Because your body relies on sunlight to make this vitamin, people who live in northern climates produce less vitamin D and must get their vitamin D through fortified foods, such as milk. Individuals who do not consume much dairy may need to take vitamin D supplements.
Water-Soluble
Vitamin C and the eight B vitamins are all water-soluble. The B vitamins, or B-complex, are thiamine, riboflavin, niacin, B-6, folate, B-12, biotin and pantothenic acid. These vitamins dissolve in water and your body does not store them. Instead, your kidneys filter any excess vitamins out of your body through urination. Because you do not store these vitamins, you need to consume them more regularly, and your body can tolerate higher dosages of these vitamins than the fat-soluble variety. Most of the water-soluble vitamins occur naturally in plants except vitamin B-12. B-12 only occurs naturally in animal products. People who do not eat animal products need to use B-12 supplements or eat foods with B-12 added.
Fat-Soluble Dosing
Vitamin dosing varies by vitamin and by the age of the individual. Dosing for vitamin A is between 300 and 400 mcg for children below age 9. For males over age 9 the dosing is 600 and 900 mcg, and 600 to 700 mcg for females. Pregnant women can have as much as 770 mcg, and lactating women 1,300 mcg. Dosing for vitamin D is 5 mcg for males and females from infancy to age 50; this includes pregnant and lactating women. After age 50, the dosage increases to between 10 and 15 mcg. The dosing for vitamin E is between 6 and 10.5 IU for infants and children under age 9. Males and females over age 9 need between 16.5 and 22.5 IU. Lactating women need 28.8 IU. The body produces vitamin K in the intestines and gets it from food sources, such as leafy green vegetables. The RDA for vitamin K is 120 mcg for males over age 19, and 90 mcg for females. No dosing information is available for younger individuals.
Water-Soluble Dosing
Dosing for Vitamin C is 90 mg for males over age 19 and 75 mg for females. Dosing for thiamine, riboflavin and niacin is 1.2 mg to 1.3 mg for males, and 1.1 to 1.3 mg for females. Males need 16 mg of niacin and females need 14 mg. Males and females both need 400 mcg of folate, 2.4 mcg of B-12, 5 mg of panthothenic acid and 30 mg of biotin.
Upper Limits
Taking more than the recommended dosage, especially of the fat-soluble vitamins, has the potential for negative side effects. These affects vary by vitamin and can include dizziness, nausea and vomiting, and neurological damage. Several vitamins have a safe upper limit, which is the most you can take without adverse effects. The upper limits vary by age, with younger people falling on the low end of the scale and older individuals and pregnant or lactating women on the higher end.
The safe upper limits for the fat-soluble vitamins are 600 to 3,000 mcg for vitamin A; 25 to 50 mg for vitamin D; and 200 to 1,000 mg for vitamin E. There is no established upper limit for vitamin K. Some water-soluble vitamins, such as B-12, do not have upper limits. The vitamins that do have upper limits are: Vitamin C at 2,000 mg; B-6 at 100 mg; niacin at 35 mg; and folate at 1,000 mcg.
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