Decoding the Mystery: Why are Vitamins A, D, and E Expressed as IU on Labels, But Not Vitamin K?

When it comes to understanding the labels on dietary supplements and food products, it can often feel like you need a degree in nutrition science. One common source of confusion is the use of International Units (IU) to express the amount of certain vitamins, such as A, D, and E. However, you may have noticed that Vitamin K is not expressed in IU. This article will delve into the reasons behind this discrepancy and help decode the mystery of vitamin labeling.

Understanding International Units (IU)

International Units are a type of measurement used in the pharmaceutical and supplement industries to quantify the amount of a particular substance, such as a vitamin, in a product. The purpose of using IU instead of weight (like milligrams or micrograms) is to account for the varying biological activity or potency of different substances. For example, one IU of Vitamin E does not weigh the same as one IU of Vitamin A because they have different biological activities.

Why Vitamins A, D, and E are Expressed in IU

Vitamins A, D, and E are fat-soluble vitamins, which means they are stored in the body’s fatty tissue and liver. These vitamins are expressed in IU on labels because their biological activity can vary. For instance, Vitamin E occurs in eight different forms, each with a different level of biological activity. Therefore, expressing Vitamin E in IU provides a standardized measurement that reflects its biological activity, regardless of the specific form present.

Why Vitamin K is Not Expressed in IU

Unlike Vitamins A, D, and E, Vitamin K is not expressed in IU on labels. This is because Vitamin K occurs in two natural forms, K1 and K2, which have the same level of biological activity. Therefore, there is no need to use IU to account for varying biological activities. Instead, Vitamin K is typically expressed in micrograms (mcg).