Understanding the difference: IU, mg, mcg

Knowing Your Dosage

The human body requires a variety of essential vitamins. Although a healthy diet should provide most of the vitamins that we need, taking vitamin supplements helps to fill in for dietary shortcomings or aid in enhancing well-being. Vitamins and supplements are measured in many forms which include milligrams (mg), micrograms (mcg) or International Units (IU). How do we derive a dosage from these?

Milligram and Microgram

A gram is a metric measurement of weight. The gram is conveniently used for everyday products. For comparison purposes, a teaspoon is precisely 4.2 grams, a tablespoon is 15 grams and one cup is 128 grams. Milli- is a prefix meaning one thousandth. Therefore, a milligram is one thousandth of a gram, and a gram is one thousand milligrams (1g = 1000mg). Whereas Micro- is a prefix meaning one millionth. Therefore, a microgram is one millionth of a gram, and a gram is one million micrograms (1g = 1 000 000mcg).

For example, if:
1mcg  x 1,000 = 1 mg and
1mcg  x 1,000,000 = 1g

Therefore, a Vitamin of 5mg (milligram) dosage is stronger than a Vitamin of 5mcg (microgram) dosage.

How to Convert Your Dosages?

  1. To convert from micrograms to milligrams, divide by 1,000.
    (For example, 100 mg of vitamin C equals 100,000 mcg, or µg.)
  2. To convert from milligrams to micrograms, times by 1,000.
    (For example, 100 mg of vitamin C equals 100,000 mcg or µg.)

International Units (IU)

Milligrams and micrograms indicate the amount of mass in a sample, but International Units indicate the level of biological activity of a substance specific to each substance.

One international unit, or IU, is a measurement of the biological effect that a specific substance has on the body.
The IU value for specific formulations of vitamins has been determined by an International agreement among a great number of scientific organizations. For each type of vitamin preparation, there is a unique IU value based on the method used for formulating that vitamin.

The formulating methods include:

  • Water-soluble Vitamins
  • Fat-soluble Vitamins

The International Unit will be indicated in a Vitamin or Supplements RDA (Recommended Daily Allowance) or Dosage & Directions of Use.

For example, if the RDA of a Vitamin for adults ranges from 600-800 International Units (IU), then the packaging would indicate the 40 IU equals one microgram (1mcg or µg). Thus we can calculate that if:

one mcg = 40 International Unit
600-800 International Unit = RDA
600/40 = 15 & 800/40 = 20

Therefore The RDA is equal to 15-20 micrograms.

Always remember that ‘more’ isn’t necessarily better in nutrition. Many nutrients are only required in microgram amounts and taking larger amounts could be unnecessary or even detrimental to health. Now that you know how to, converting between these units will help clarify the amount of vitamin needed and aid in ensuring you reach optimum health.