When examining supplement labels or medication dosages, many individuals encounter both International Units (IU) and milligrams (mg), often leading to confusion. It is important to understand that these two units are not interchangeable and measure distinct properties of a substance. This difference is fundamental to correctly interpreting product information and ensuring appropriate intake.
What Milligrams Measure
A milligram (mg) is a standard unit of mass within the metric system. It represents one-thousandth of a gram, making it suitable for measuring very small quantities of substances. For instance, a small amount of food, specific chemical ingredients, or many medications are precisely quantified in milligrams.
What International Units Measure
In contrast, an International Unit (IU) quantifies the biological activity or effect of a substance, rather than its mass. This measurement is particularly relevant for substances like vitamins (e.g., A, D, E), hormones, and certain biologics where the biological effect can vary significantly depending on the specific form or purity of the substance. The World Health Organization’s Expert Committee on Biological Standardization defines the IU for each substance, establishing a reference preparation and a specific biological assay. This standardization ensures that different preparations of a substance with the same biological effect will contain the same number of IUs, regardless of their varying mass.
Why Both Units Exist and How They Relate
The existence of both units stems from the need to measure different characteristics: mass versus biological impact. There is no universal conversion factor between IU and milligrams because the biological activity represented by one IU is specific to each substance. For example, 1 IU of Vitamin D is approximately 0.025 micrograms (mcg), which translates to 0.000025 milligrams. In comparison, 1 IU of natural Vitamin E (d-alpha-tocopherol) corresponds to roughly 0.67 milligrams, while synthetic Vitamin E (dl-alpha-tocopherol) is about 0.9 milligrams per IU. For Vitamin A, 1 IU is equivalent to about 0.3 micrograms of retinol or 0.6 micrograms of beta-carotene, highlighting the variability even within different forms of the same vitamin.
Due to this complexity, the U.S. Food and Drug Administration (FDA) has transitioned some labeling from IU to metric units like milligrams or micrograms for certain vitamins, aiming to reduce consumer confusion. Consumers should always carefully check product labels for the stated units and consult healthcare professionals for dosage guidance, especially when conversion may be necessary.