Human beings can get vitamin A from eating vegetables. Many vegetables contain beta-carotene. Our body can convert beta-carotene to the active form of vitamin A. In contrast, cats need to get the active form of vitamin A from their food. This form of vitamin A is found only in animals, not in plants.
What Is Beta-Carotene?
Many fruits and vegetables are bright yellow, orange, or red. These colors come from pigments called carotenoids. One example is beta-carotene. Carotenoids do several important things in the plant. They help to collect light, which is used in photosynthesis. They also act as antioxidants. Antioxidants help to protect the plant against some of the dangerous by-products of photosynthesis.
Human beings and many other mammals can convert beta-carotene to retinol. Retinol is the active form of vitamin A. Thus, human beings can easily get their vitamin A from eating vegetables. A cat cannot.
Cats Need Retinol
A cat cannot convert beta-carotene to retinol. Retinol is found only in meat and other animal products, such as egg yolk. Thus, cats cannot survive on a purely plant-based diet. That is why cats are called obligate carnivores. To make a vegan cat food, you must add the nutrients, such as retinol, that normally come only from animal sources.
Cats Don’t Need Vitamin C
Even if a cat eats no fresh fruit or vegetables, it will never get scurvy. All plants and most animals can make their own supply of ascorbic acid. Human beings and guinea pigs cannot. If we don’t get ascorbic acid (vitamin C) from our food, we will die of scurvy.
Human Beings Should Eat Vegetables
Cats must get their vitamin A in the form of retinol or related compounds (such as retinyl palmitate). In contrast, human beings should get their vitamin A in the form of beta-carotene.
Your body has ways to limit the amount of beta-carotene it absorbs from your food. If you eat a lot of fruits and vegetables, a little bit of extra beta-carotene might build up in your skin. The result is a healthy golden glow that is more attractive than a suntan. Your body will convert beta-carotene to retinol only on an as-needed basis. Thus, you cannot get a vitamin A overdose from eating too many carrots.
Even if you binge on carrot juice, you will merely turn yellow. But if you overdose on retinols, you might go blind or even die. An overdose of retinols can cause a dangerous swelling of the brain. Doctors call this condition pseudotumor cerebri (fake tumor of the brain). This swelling can destroy the nerves that lead from the eye to the brain. Pseudotumor cerebri has happened to people who overdosed on vitamin A supplements. It has also happened to people who ate polar bear liver.
Food, Not Pills!
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 vitamin pills. In fact, the vitamin pills might actually increase the risk of cancer.
Beta-Carotene, Vitamin A, and Lung 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 beta-carotene supplements would protect people against lung cancer. The study recruited cigarette smokers and people with asbestos in their lungs. These people were given either a vitamin pill or a placebo. The vitamin pill contained beta-carotene and retinol (in the form of retinyl palmitate). The study was stopped early for safety reasons. The pills increased the risk of getting lung cancer.
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