Beta-carotene is a yellow pigment that is found in many yellow, orange, and dark-green vegetables. For human beings and many other mammals, beta-carotene is a provitamin of vitamin A. This means that beta-carotene does not have vitamin A effects until the body converts it to retinol. Human beings can convert beta-carotene to retinol. Thus, human beings can get vitamin A from the beta-carotene in fruits and vegetables. In contrast, cats cannot convert beta-carotene to retinol. Retinol is found only in meat and other animal products, such as egg yolk. For this reason, cats cannot survive on a purely plant-based diet. If you want to make a purely plant-based (vegan) cat food, you must add the nutrients, such as retinol, that normally come only from animal sources.
Cats must get their vitamin A in the form of retinol or related compounds (such as retinyl palmitate). In contrast, it is better for human beings to get their vitamin A in the form of beta-carotene. The human body converts beta-carotene to retinol on an as-needed basis. If you eat a huge amount of the fruits and vegetables that contain beta-carotene, some of the extra beta-carotene might build up in your skin. As a result, you will get a healthy golden glow that is more attractive than a suntan. In contrast, if you overdose on retinol, either from taking supplements or from eating polar bear liver, you will get a potentially fatal swelling of the brain. This condition is called pseudotumor cerebri (which literally means fake tumor of the brain). If the brain swelling damages the nerves that connect the eyes to the brain, the result can be permanent blindness.
Human beings should get their vitamin A in the form of beta-carotene. And they should get their beta-carotene from fruits and vegetables, rather than from pills. People who eat a lot of fruits and vegetables tend to have better health, including lower rates of cancer. You cannot get the same effect by taking the vitamins in pill form. In fact, the vitamin pills might actually increase the risk of cancer.
In the 1980s, the National Cancer Institute launched a major study called the Carotene and Retinol Efficacy Trial (CARET). The purpose of the study was to see whether pills containing beta-carotene and retinol (in the form of retinyl palmitate) could reduce the risk of cancer in people who were at high risk for lung cancer. The study was stopped early because the cancer rate turned out to be higher in the people who got the vitamin A pills than in people who got a placebo.
Photo by mattbuck4950 