Micronutrients: Vitamin A


This post is dedicated to my good friend since high school Angela Mak who wanted to learn more about the different types of vitamins and their various functions.

For starters, let’s learn about the differences between macro and micronutrients. Generally speaking, nutrients are substances needed by the body for proper growth and development. Our bodies can’t produce nutrients ourselves so we need to eat in order to meet our nutrient requirements.  You can probably tell from the prefixes that “macro” is on a larger scale than “micro”. Similarly, macronutrients such as carbohydrates and proteins are needed in larger quantities compared to micronutrients. For example, adults should have at least 200-300 grams of carbohydrates a day versus the 700-900 micrograms of vitamin A    (285, 000 times less!).

Wait…700 micrograms is not a lot so why is vitamin A important?

First, some background information about vitamin A. There are multiple sources of vitamin A. You can get vitamin A from meat (in the form of retinal esters) or from plants (as beta carotene). The best plant sources come from orange and red plants like carrots. Vegetarians have a harder time meeting vitamin A requirements because beta carotene from plants has to be converted before the body can use it. This conversion is not 100% efficient and you need 12 times more beta carotene than retinal esters.

Vitamin A’s primary role is for eyesight and immune protection. Vitamin A works with a protein in the eye called “rhodopsin” to detect light and send signals to the brain. Therefore people deficient in vitamin A can suffer from a condition known as “night time blindness”. Vitamin A also keeps the membranes around your nose, lungs and skin healthy, helping to form mucus. Lacking vitamin A will cause the membrane to dry and crack leading to infections. An extreme case of vitamin A deficiency is “Xerophthalmia” when the eye gets too dry and fails to make tears.

Does this mean I should eat a lot of meat to get perfect vision?

Unfortunately that’s not quite how it works. Too much vitamin A is toxic because vitamin A is fat soluble. It can be stored in the body as opposed to water soluble vitamins that can be excreted in urine. Vitamin A is kept in the liver and liver cells can only hold a certain amount of vitamin A. Continually consuming high amounts of vitamin A causes liver failure and eventual death. In 1597, Dutch officers became severely ill from eating polar bear liver. They experienced vitamin A toxicity, recalling that animal foods are generally higher in vitamin A and because they were eating liver which is the polar bear’s primary storage site of vitamin A. If you are curious, adults should take no more than 2000-3000 micrograms of vitamin A (the kind found in meat).

Vegetarians on the other hand don’t have to worry about vitamin A toxicity. The only thing that happens is that as more and more beta carotene is stored (beta carotene is an red-orange pigment which gives carrots their colour), the orange becomes more visible in the skin. In other words, you turn orange. Guess the people that told you that eating too many carrots turns you orange weren’t lying after all!



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