The term "vitamin A" makes it sound like there is one particular nutrient called "vitamin A," but that is not true. Vitamin A is a broad group of related nutrients. Each of these nutrients provides us with health benefits, but these benefits may be quite different and they may be provided in different ways.
There are two basic forms of vitamin A: retinoids (found in animal foods) and carotenoids (found in plant foods). These two forms aren't just chemically different - they also provide us with different types of health benefits. There are some specific immune, inflammatory, genetic, and reproductive-related benefits of vitamin A that can only be obtained from the retinoid forms of the vitamin. These retinoid forms can be especially important with respect to pregnancy and childbirth, infancy, childhood growth, night vision, red blood cell production, and resistance to infectious disease. Yet even if we are not faced with any of these special conditions, each of us needs retinoid forms of vitamin A.
Like the retinoid forms of vitamin A, the carotenoid forms also provide us with unique health benefits. Most carotenoid forms of vitamin A function as antioxidant and anti-inflammatory nutrients. Sometimes specific carotenoids have a special role to play in the protection of our health. For example, the only carotenoids found inside the retina of the human eye are the xanthophylls lutein and zeaxanthin. Anyone needing to focus on vitamin A benefits related to eye health (for example, prevention of age-related macular degeneration) would need to develop a meal plan that not only included foods that were rich in vitamin A, but more specifically, rich in these two specific carotenoid forms of the vitamin. (Spinach, kale, and Swiss chard would be examples of foods that are rich in lutein and zeaxanthin.)
At first glance, it looks like we need to eat both animal and plant foods in order to get both retinoid and carotenoid forms of vitamin A. In some instances, that is true. However, in some other instances, it is not. In the bodies of many individuals, carotenoid forms of vitamin A can be effectively converted into retinoid forms, therefore providing the physiological functionality noted above. Alpha-carotene, beta-carotene, and beta-cryptoxanthin are three carotenoid forms of vitamin A that can be converted by our body into retinoid forms under certain conditions.
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