If you’ve looked at our vitamin A page, you might have noticed something a little confusing. Near the top of the page we have a chart showing the recommended daily values of this nutrient, measured in micrograms (μg). But when we come to list the foods which are highest in vitamin A, we switch to International Units (IU). What’s going on?
There’s a very good reason for this, but it’s a little complicated to explain. We measure vit A in IU as opposed to (milli/micro)grams because there are different forms of this vitamin. For example, one of the animal forms is retinol. 1 IU is the biological equivalent of 0.3 μg retinol. So far, simple enough.
But there are also plant forms such as beta-carotene, known as provitamins, which means it can be converted into useable vitamin A within the body. 1IU is the equivalent of 0.6 μg beta-carotene. These are two different forms of vitamin A, but they have different levels of biological activity, meaning you need more beta-carotene to have the same effect as a smaller amount of retinol. So rather than saying you need however many micrograms of vitamin A per day, we use international units.
This might help explain one of the best known ‘old wives tales’. that of carrots helping you see in the dark. The first form of vitamin A I mentioned, retinol, has a clue in it’s name. It’s very close to the word ‘retina’, the light-sensitive layer of the eye. No surprise then, to learn that this vitamin is vital for maintaining good eyesight. And of course, one of the best sources of it is… carrots, with just one carrot a day providing all the vitamin A you need!
The daily value for the average adult for this vitamin is 5000 IU. Other great sources include butternut squash, kale, sweet potato, and liverwurst.