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. Here is a summary chart showing basic relationships between the forms of vitamin A.
As you can see in the chart above, 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 in 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.
We use this phrase—”under certain conditions”—to refer to the fact that the bodies of many individuals may not be well equipped to convert carotenoid forms of vitamin A into retinoid forms. Many different factors can contribute to problems with this conversion, including: a person’s inherited genetic tendencies, digestive problems, bacterial imbalances in the digestive tract, excessive use of alcohol, excessive exposure to toxic chemicals, imbalanced intake of vitamin A and vitamin D as a result of high-dose supplementation, and the use of certain over-the-counter and/or prescription medications. So there is a need for caution here. If you are a person who avoids animal foods and you are trying to obtain more retinoid forms of vitamin A by consuming plant foods that are high in carotenoids, you might get a very large amount of carotenoids yet still be unable to convert these carotenoid forms of vitamin A into the retinoid form that is also required by the body for proper physiological functioning.
If you recognize some of the problem factors in the list above as potentially affecting your own body’s ability to convert carotenoid forms of vitamin A into retinoid forms, we recommend that you consult with a healthcare provider to determine possible helpful steps. A healthcare provider with experience in this area may be able to help you improve your digestion, reduce the impact of medications, lessen your toxic exposure, and balance the amounts of vitamins A and D in any supplements that you are taking. Also, while still expensive in the healthcare marketplace, some forms of lab testing—including genetic testing—may be available to help you determine potential vitamin A-related problems.
Two additional important points: (1) if a person’s body is effectively able to convert carotenoids into retinoids, beta-carotene is the best carotenoid for the body to work with, since in comparison to alpha-carotene or beta-cryptoxanthin, it takes only half as much beta-carotene for the body to create the same amount of retinol; and (2) if a person’s body is effectively able to convert carotenoids into retinoids, there may be some advantages to letting it do so (rather than trying to directly obtain high levels of retinol from food). Allowing the body to decide about the degree of conversion may provide more optimal regulation of both carotenoid and retinoid levels.
Researchers have developed a system for evaluating the degree to which carotenoid forms of vitamin A can be converted into retinoid forms. This system is based on units of measurement called retinol activity equivalents (RAE) and retinol equivalents (RE). RAE and RE are yardsticks for measuring the retinoid-converting potential of carotenoid-containing foods. The higher the RAE or RE, the greater the potential for conversion of carotenoids into retinoids.
There are two basic forms of vitamin A (retinoids and carotenoids) and both forms provide unique health benefits. If your body is able to effectively convert carotenoids into retinoids, you don’t have to eat animal foods in order to obtain retinoid forms of vitamin A that are essential for health. If your body is unable to do this conversion effectively, you’ll either need to include animal foods in your meal plan or obtain retinoid forms of vitamin A through dietary supplements. Many factors can compromise the body’s ability to convert carotenoids into retinoids, including: genetic tendencies, digestive problems, bacterial imbalances in the gut, excessive alcohol use, excessive exposure to toxins, imbalanced intake of vitamin A and vitamin D in supplement form, and use of over-the-counter and/or prescription medications. Some Vitamin A health benefits will require you to eat foods that are rich in specific carotenoids. A great example is eye health and the unique role of two specific carotenoids (lutein and zeaxanthin) in the health of our eyes.
While vitamin A is best known for its vital role in vision, the retinoid forms of this vitamin also participate in physiological activities related to the immune system, inflammatory system, maintenance of epithelial and mucosal tissues, growth, reproduction, bone development, creation of red blood cells, and production of spermatozoa (male reproductive cells). In food, retinoid forms of vitamin A typically appear as retinyl esters. The body is typically able to convert these retinyl esters into metabolically active forms of vitamin A including retinol, retinal, and retinoic acid.
The human retina contains four kinds of photopigments that store vitamin A compounds. One of these pigments, called rhodopsin, is located in the rod cells of the retina. Rhodopsin allows the rod cells to detect small amounts of light, and, thus, plays a fundamental role in the adaptation of the eye to low-light conditions and night vision.
Retinal, the aldehyde form of the vitamin, participates in the synthesis of rhodopsin, and in the series of chemical reactions that causes visual excitation, which is triggered by light striking the rod cells. The remaining three pigments, collectively known as iodopsins, are found in the cone cells of the retina and are responsible for day vision.
Throughout the body, but particularly in our digestive tract, vitamin A plays a key role in support of immune and inflammatory functions. Our digestive tract can get exposed on a daily basis to potentially unwanted substances (like pesticide residues in food), as well as unwanted micro-organisms (like certain kinds of bacteria). Our immune and inflammatory systems are designed to help prevent us from being harmed by these events.
For example, in order to help neutralize unwanted bacteria and other micro-organisms, our immune system has the ability to make and release antibodies that can block their activity. Our immune and inflammatory systems also have “braking” function that prevents them overreacting. Recent research has shown that vitamin A plays a key role in both of these protective processes. Scientists now know that the T cell and B cells of the immune system cannot be correctly synthesized without vitamin A, nor can immune responses be effectively activated without participation of vitamin A. Interestingly, whenever we undergo an increase in whole body inflammation, our cells also increase their conversion of vitamin A in its retinol form into a second form called retinoic acid. This conversion required the participation of two enzymes (alcohol dehydrogenase and retinaldehyde dehydrogenase). The inability of our cells to make this vitamin A conversion is now believed to be a risk factor for increased susceptibility to infection, as well as for poor response to vaccination.
Researchers believe that vitamin A may be equally important for our immune and inflammatory “braking” system, in which our cells are prevented from becoming overreactive. Since some aspects of food allergy can be related to our immune system’s overreaction to food proteins, optimal intake of vitamin A may turn out to be important for lowering risk of certain types of food allergy.
Vitamin A is required for normal cell growth and development. Although the mechanisms by which vitamin A promotes cell growth and development are not yet fully understood, it is known that retinoic acid is necessary for the synthesis of many glycoproteins, which control cellular adhesion (the ability of cells to attach to one another), cell growth, and cell differentiation. For example, the production of red blood cells in our bone marrow (through a process called hematopoiesis) is a process that is known to require vitamin A in the form of retinoid acid. As described in the previous paragraph, retinoic acid can be synthesized in our cells from the retinyl esters found in food, and it takes two enzymes (alcohol dehydrogenase and retinaldehyde dehydrogenase) in order for this synthesis to occur. Researchers are actively investigating the link between this enzyme system and cell growth and believe that problems with synthesis of retinoic acid may hold the key for understanding a wide range of problems related to human growth and development.
It is also known that vitamin A is essential for reproductive processes in both males and females and plays a role in normal bone metabolism. In addition, some of the most cutting-edge research in the field of genetics has been examining the role of vitamin A (in the form of retinoic acid) in regulating genetic events. Vitamin A is also known to be required for proper production of sperm (through a process called spermatogenesis).
Until late in the 20th century, the functions of carotenoids were discussed only in terms of their potential to act in the same way as retinoids. From among the more than 600 carotenoids known to exist in plant foods, only three carotenoids – beta-carotene, alpha-carotene, and beta-cryptoxanthin – were designated as “provitamin A” carotenoids that could be converted by the body (under the right circumstances) into retinoids. Intake of these three carotenoids is still regarded as extremely important in preventing deficiency of vitamin A in its retinoid forms.
In recent years, carotenoids have received a large amount of research attention as potential anticancer and anti-aging compounds. These potential functions of carotenoids are closely related to their antioxidant and anti-inflammatory activity. Importantly, virtually all carotenoids provide antioxidant and anti-inflammatory benefits (even though it’s only a handful of carotenoids that can be converted into retinoids).
In addition to their antioxidant and immune-enhancing activity, carotenoids have shown the ability to stimulate cell-to-cell communication. Researchers now believe that poor communication between cells may be one of the causes of the overgrowth of cells, a condition that eventually leads to cancer. By promoting proper communication between cells, carotenoids may play a direct role in cancer prevention.
It is also believed that carotenoids participate in female reproduction. Although the exact function of carotenoids in female reproduction has not yet been identified, it is known that the corpus luteum contains a very high level of beta-carotene, suggesting that this nutrient plays an important role in reproductive processes.