Vitamins are measured according to the metric system, which is internationally standardized. In addition, vitamin amounts vary drastically, from 15 mcg daily for vitamin D to 90 mg daily for vitamin C. Understanding the jargon on the the label of your favorite dietary supplement or multivitamin can help you ensure you are maintaining healthy levels of all important vitamins.
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Perhaps the most confusing of the vitamin measurement units, the International Unit, or IU, is used by many supplement manufacturers as a standard measurement for vitamin recommendations. According to the University of North Carolina, the IU is an internationally standardized measurement of the biological effect of a substance when the dose administered is equal to 1 IU. This can be confusing for consumers, as the IU has no direct conversion to grams and milligrams. For example, one IU for vitamin A is equal to 0.3 mcg, but for vitamin D, one IU is equal to 25 nanograms.
A microgram -- mcg -- is an extremely small unit of measurement equal to one-millionth of 1 g or one-thousandth of 1 mg. For example, the average adult needs 15 mcg of vitamin D daily. This is equal to 0.015 mg and 0.000015 g. According to the Food and Nutrition Board of the Institute of Medicine, your recommended dietary allowance of vitamin A, vitamin D, vitamin K, folate, vitamin B-12, and biotin is measured in micrograms as the amount you need is small.
A milligram, abbreviated mg, is one-thousandth of 1 g and is equal to 1,000 mcg. For example, the average adult male needs 90 mg of vitamin C daily. This is equal to 90,000 mcg and 0.09 g. Your recommended dietary allowance of vitamin C, vitamin E, thiamin, riboflavin, niacin, vitamin B-6 and pantothenic acid are all measured in milligrams per day.
A gram, abbreviated g, is a larger unit of measurement and is used commonly in the metric system. For example, the metric equivalent of one serving of any type of food is usually 100 g. A gram is equal to 1,000,000 mcg and 1,000 mg. Since you only need small amounts of vitamins, vitamins are rarely measured in grams. However, macronutrients like protein and fiber are usually measured in grams, so you will find it helpful to be familiar with all the measurements, from micrograms up to grams.