Vitamin A

What Is Vitamin A?

Vitamin A is not one compound but a family. Retinol and its stored form, retinyl esters, come ready-made in animal foods such as liver, dairy, eggs and oily fish. Plants supply provitamin A carotenoids, mainly beta-carotene, which the body converts into retinol when it needs more.

Why Vitamin A Is Counted in µg RAE

Because the two families are not worth the same, vitamin A is counted in retinol activity equivalents (RAE). 1 µg of retinol is 1 µg RAE. It takes 12 µg of beta-carotene from food, or 24 µg of alpha-carotene or beta-cryptoxanthin, to make the same 1 µg. Beta-carotene in a supplement converts better, at 2 µg to 1 µg RAE.

So the 835 µg RAE per 100 g listed for raw carrots already allows for the conversion: the carrot holds many times that weight of beta-carotene and alpha-carotene. Carotenoids are also absorbed with fat, so a carrot eaten with some oil or cheese gives more than one eaten on its own.

What Vitamin A Does

In the eye, a form of vitamin A called retinal joins a protein to make rhodopsin, the light-sensing pigment in the rods of the retina. Rods are what the eye uses to see in dim light, which is why night blindness is an early sign of running short. Vitamin A also keeps the cornea and the conjunctiva, the membrane over the eye, working normally.

Outside the eye, vitamin A switches genes on and off as cells grow and specialize. That is how it supports the immune system, reproduction, and the normal formation and upkeep of the heart, lungs, eyes and other organs.

Vitamin A in a Day of Food Without Liver

One ordinary day from foods in the Caltide food library, in usual portions, with no supplements and no organ meat.

One food carries the day: sweet potato alone gives 56% of the total. Without it, the rest of the day falls short of the 700 to 900 µg adults need.

Why Liver Is So High in Vitamin A

Liver is where animals store vitamin A, so eating liver means eating that store. The NIH lists 6,582 µg RAE in 85 g of pan-fried beef liver, more than seven times the 900 µg men need a day and over twice the 3,000 µg adult upper limit, in a single serving. An occasional portion is not a problem for most adults; liver every day, or liver plus a retinol supplement, can be.

Who Is Most Likely to Be Low in Vitamin A

Deficiency is rare in Europe and North America but common in low-income countries, where the WHO counts it as a leading cause of preventable blindness in children. It also makes infections such as measles and diarrhea more severe and more often fatal. In richer countries, the people most at risk are premature infants and people with cystic fibrosis or other conditions that block fat absorption.

How Vitamin A Works With Other Nutrients

Vitamin A, like vitamin D, dissolves in fat, so very low-fat diets and fat-blocking drugs reduce how much is absorbed. Beta-carotene has its own page, because it also matters as a pigment and in supplement trials, not only as a source of vitamin A.