Basic Description Role in Health Support Summary of Food Sources Nutrient Rating Chart Impact of Cooking, Storage and Processing Risk of Dietary Deficiency Other Circumstances that Might Contribute to Deficiency

Relationship with Other Nutrients Risk of Dietary Toxicity Disease Checklist Public Health Recommendations References

Basic Description

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.

Vitamin A-Related Nutrients

Retinoids (found in animal foods)	Carotenoids (found in plant foods)
retinol	Carotenes	Xanthophylls
retinal	alpha-carotene*	astaxanthin
retinoic acid	beta-carotene*	beta-cryptoxanthin*
retinyl esters	gamma-carotene*	canthaxanthin
	delta-carotene	fucoxanthin
	epsilon-carotene	lutein
	zeta-carotene	neoxanthin
		violaxanthin
		zeaxanthin

*Once a food has been consumed, these carotenoid forms of vitamin A may be converted by the body into retinoid forms under certain conditions.

Retinoids

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.

Carotenoids

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.

Conversion of Carotenoids to Retinoids

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.

We created the chart below to help explain how all of these factors are interrelated.

Top 25 Vitamin A-Containing WHFoods Retinol and Carotenoid Content*

	Food	Total for All Forms (mcg RAE)	Retinol (mcg RE)	Total Carotenoids (mcg RE)	Beta-carotene (mcg)	Lutein & Zeaxanthin (mcg)	Lycopene (mcg)
1	Sweet potato	1922	0	3844	23018	0	0
2	Carrots	1019	0	2038	10108	312	1
3	Spinach	943	0	1887	11318	20354	0
4	Kale	885	0	1771	10625	23720	0
5	Mustard greens	865	0	1732	10360	8347	0
6	Collard greens	722	0	1444	8575	11774	0
7	Turnip greens	549	0	1098	6588	12154	0
8	Swiss chard	536	0	1072	6391	19276	0
9	Winter squash	535	0	1071	5726	2901	0
10	Romaine lettuce	409	0	819	4912	2173	0
11	Bok choy	361	0	722	4333	65	0
12	Cantaloupe	271	0	541	3232	42	0
13	Bell peppers	144	0	288	1494	47	0
14	Parsley	128	0	256	1536	1691	0
15	Broccoli	121	0	241	1449	1685	0
16	Asparagus	91	0	181	1087	1388	54
17	Sea vegetables	81	- -	- -	973	- -	- -
18	Chili peppers	80	0	160	810	17	1
19	Tomatoes	75	0	150	808	221	4631
20	Basil	56	0	112	666	1198	0
21	Papaya	131	0	262	756	246	5045
22	Shrimp	102	102	102	0	0	0
23	Eggs	75	74	75	5.5	176	0
24	Brussels sprouts	61	0	121	725	2012	0
25	Grapefruit (pink/red)	59	0	119	706	8	1453
	Food	Total for All Forms (mcg RAE)	Retinol (mcg RE)	Total Carotenoids (mcg RE)	Beta-carotene (mcg)	Lutein & Zeaxanthin	Lycopene (mcg)

* All values are listed per serving size as identified on our website. Due to unavailability of data, content of many other specific carotenoids (for example, alpha-carotene) is not presented. mcg RAE is microgram retinol activity equivalents. mcg RE is microgram retinol equivalents. mcg is micrograms.

To better understand this chart, it's helpful to start with the third column from the left that is labeled "Total for all forms (mcg RAE)." In this column, you will notice that the numbers drop lower and lower as you move from the first food (Sweet potatoes) and proceed down the list. This column tells you that if you are seeking foods that will provide you with the greatest potential amount of vitamin A in its retinoid form, you are likely to get the best results by choosing Food 1 (Sweet potatoes) versus Food 2 (Carrots), Food 2 (Carrots) versus Food 3 (Spinach), and so on down the list.

Now take a look over at the fourth column in the chart, which is labeled, "Retinol mcg RE." This column tells you that none of the Top 25 Vitamin A-containing foods on our website contain vitamin A in its retinoid form! So how can these foods provide you with the greatest potential for vitamin A in its retinoid form when they don't actually contain any retinoids? The answer lies in their unusually high carotenoid content. Provided that your body has the ability to effectively convert carotenoids into retinoids, you'll actually end up with more retinoid forms of vitamin A by eating any of these Top 25 Vitamin A foods, even though all of these are plant foods that do not directly provide any Vitamin A in retinoid form.

We created a second chart below to show you how the animal-derived foods featured on the website ranked as sources of Vitamin A.

Vitamin A Ranking for WH Animal Foods

	Food	Total for All Forms (mcg RAE)	Retinol (mcg RE)	Total Carotenoids (mcg RE)	Beta-carotene (mcg)	Lutein & Zeaxanthin (mcg)	Lycopene (mcg)
22	Shrimp	102	102	0	0	0	0
23	Eggs	75	74	1	5.5	176	0
26	Cow's milk	56	55	2	8	0	0
33	Cheese	77	73	4	12	0	0
34	Yogurt	67	66	2	12	0	0
35	Salmon	58	58	0	0	0	0
37	Sardines	29	29	0	0	0	0
50	Chicken	7	7	0	0	0	0
61	Turkey	3	3	0	0	0	0
39	Tuna	25	25	0	0	0	0
66	Cod	2	2	0	0	0	0
67	Scallops	2	2	0	0	0	0
not in Top 100	Beef	0	0	0	0	0	0
Not in Top 100	Lamb	0	0	0	0	0	0
	Food	Total for All Forms (mcg RAE)	Retinol (mcg RE)	Total Carotenoids (mcg RE)	Beta-carotene (mcg)	Lutein & Zeaxanthin	Lycopene (mcg)

* All values are listed per serving size as identified on our website. Due to unavailability of data, content of many other specific carotenoids (for example, alpha-carotene) is not presented. mcg RAE is microgram retinol activity equivalents. mcg RE is microgram retinol equivalents. mcg is micrograms.

In this second chart, you'll notice that even though most of our animal foods contain vitamin A in its retinoid form (as shown by those numbers in the fourth column which is labeled, "Retinol (mcg RE)," their carotenoid content is very low (or absent), giving our body very little to work with if it wanted to convert carotenoids into retinoids. The only exception here would be eggs and their relatively high content of the carotenoids lutein and zeaxanthin. Since neither lutein nor zeaxanthin can be converted by the body into retinoids, however, the presence of these carotenoids does not help eggs move up on our ranking list (although they still do not do too badly at number 37 out of more than 125 foods).

We'd like to end this description with four key take-away points:

• 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.

Role in Health Support

How does vitamin A support health?

Retinoid Forms

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.

Vision Support

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.

Support of the Immune and Inflammatory Systems

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 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.

Cell Growth Support

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.

Other Roles for Vitamin A

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).

Carotenoid Forms

Preventing Vitamin A Deficiency

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.

Antioxidant, Anti-Inflammatory, and Immune-Enhancing Activity

In recent years, carotenoids have received a large amount of research attention as potential anti-cancer 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).

Promoting Proper Cell Communication

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.

Summary of Food Sources

What foods provide vitamin A?

Retinoids forms of vitamin A are provided by animal foods, including the following WHFoods: cow's milk, shrimp, eggs, salmon, halibut, cheese, yogurt, scallops, sardines, tuna, cod, and chicken.

Carotenoid forms of vitamin A are provided by most of the fruits and vegetables on our WHFoods list. Please see the Basic Description section above for a list of the Top 25 plant foods rich in carotenoids.

Nutrient Rating Chart

Introduction to Nutrient Rating System Chart

In order to better help you identify foods that feature a high concentration of nutrients for the calories they contain, we created a Food Rating System. This system allows us to highlight the foods that are especially rich in particular nutrients. The following chart shows the World's Healthiest Foods that are either an excellent, very good, or good source of vitamin A. Next to each food name, you'll find the serving size we used to calculate the food's nutrient composition, the calories contained in the serving, the amount of vitamin A contained in one serving size of the food, the percent Daily Value (DV%) that this amount represents, the nutrient density that we calculated for this food and nutrient, and the rating we established in our rating system. For most of our nutrient ratings, we adopted the government standards for food labeling that are found in the U.S. Food and Drug Administration's "Reference Values for Nutrition Labeling." Read more background information and details of our rating system.

Impact of Cooking, Storage and Processing

How do cooking, storage, or processing affect vitamin A?

Retinoid Forms

Preformed vitamin A is relatively stable in the animal foods that contain it. Ordinary handling, storage, and cooking methods for these foods will usually be sufficient to preserve the content of preformed vitamin A. (Most preformed vitamin A is found in the form of retinyl esters.)

In the case of milk, preformed vitamin A in the form of retinyl palmitate has been added to the milk based on U.S. Food and Drug Administration (FDA) fortification requirements. The vitamin A in fortified milk can be damaged by sunlight. In research studies, about 8-31% of the retinyl palmitate in fortified milk is lost following one day of sunlight exposure. As long as milk is stored in the refrigerator in an opaque container (either darkly tinted green or brown glass, or thick solid-color plastic, or waxed cardboard), loss of retinyl palmitate from light exposure should be negligible. (Due to the light sensitivity of retinyl palmitate, we do not recommend packaging in clear glass or clear plastic.)

Carotenoid Forms

The jury is still out regarding the impact of cooking and processing on carotenoids in food. High-carotenoid foods like carrots typically have the vast majority of their carotenoids occurring in all-trans form. While this form can provide excellent health support, it is not as readily available to the bloodstream or to our cells as another form called the cis form. The cooking of a plant food decreases the total amount of all-trans carotenoids found in the food, but it also increases conversion of many all-trans carotenoids into their more available cis form. As a net result, some studies show better support of carotenoid levels in the blood after consumption of a cooked plant food product (like tomato paste) than a non-cooked plant food product (like fresh tomatoes). In the case of tomatoes, the carotenoid of greatest interest has been lycopene, and several studies have shown better support of blood lycopene levels from cooked and processed tomato products versus fresh tomatoes. However, other factors may have played an important role here, including the breaking apart of cells in the tomato during processing. The crushing of the tomato cells may have made the cell contents more readily available for digestion and metabolism and thereby improved blood levels of tomato constituents, including lycopene. If this sequence of events played a key role, it gives all of us a very good reason to eat slowly and do an outstanding job chewing our food (including tomatoes). Thorough chewing could accomplish the same result as industrial processing, i.e., breaking open of most tomato cells and providing us with easier access to their nutrients.

Retinoid and Carotenoid Forms

Preliminary research has shown an unwanted impact of food irradiation on both retinoid form and carotenoid form vitamin A. At least one researcher has found a decrease of about 12% in total vitamin A content of chicken feed following irradiation at 20 kGy (kilogray). In this same study, the decrease in beta-carotene following irradiation was approximately 25%. We would like to note that there is very little research at this point on the nutrient impact of commercial food irradiation, and that the jury is still out on the impact of this food practice on vitamin A quantity and/or quality.

Risk of Dietary Deficiency

What food practices might lead to a dietary deficiency of vitamin A?

Retinoid Forms

In the United States, about two-thirds of all vitamin A intake from food comes in the form of retinyl esters found in animal products. Butter, cheese, eggs, and organ meats like liver are among the top 10 sources of vitamin A for U.S. adults. Persons who seldom consume any of these foods may be a greater risk of vitamin A deficiency.

One plant-based group of products - the ready-to-eat cereals - are also found within the top 10 sources of vitamin A retinoids because these cereals have often been fortified with vitamin A in the form of retinyl palmitate. The vitamin A in fortified cereals is very likely to have been obtained from a synthetic, non-animal source. However, since it is possible for this vitamin A to have be derived from an animal source, persons who are trying to avoid animal products should contact the cereal manufacturer to make sure that the cereal's retinyl palmitate was produced synthetically. .

Outside of the Unites States, dietary deficiency of vitamin A in its retinoid form is common in many non-industrialized countries, and it is associated with the high incidence of blindness, viral infections, and child mortality.

Carotenoid Forms

Carrots, tomatoes, leafy greens, sweet potatoes, and margarines colored with beta-carotene are found in the top 10 provitamin A-containing foods in the U.S. However, since U.S. adults eat relatively small amounts of vegetables in general, foods containing provitamin A in carotenoid form only account for about one-third of all dietary intake of vitamin A.

U.S. adults who take vitamin A supplements have a much lower rate of deficiency than non-supplement users. However, even when vitamin A from food and supplements is added together, about one-third of all U.S. adults still fail to get the vitamin A they need.

If regular intake of green leafy vegetables or brightly orange- or red-colored vegetables like carrots, tomatoes, sweet potatoes, or red bell peppers in not part of your meal plan, your risk of carotenoid deficiency is increased. Similarly, if you do not regularly consume orange-, red- or pink-colored fruits like watermelon, cantaloupe, papaya, and pink grapefruit, you are also likely to have increased risk of carotenoid deficiency. If these types of vegetables and fruits are both missing from your routine diet, you'll need to add foods from these groups if you want to lower your chances of overall vitamin A deficiency.

One final deficiency note: recent research has shown a relationship between very low birthweight infants (LVBW) and deficient vitamin A intake by the mother. Since these studies looked at blood levels of retinol in the mothers rather than their food intake, we don't know whether low blood levels were the result of poor carotenoid intake from plant foods, poor retinoid intake from animal foods, or a combination of the two. If you're a woman who is considering pregnancy or who is already pregnant, it's especially important consume foods that are rich in vitamin A. These foods could be plant foods rich in provitamin A, animal foods rich in preformed vitamin A, or both.

Other Circumstances that Might Contribute to Deficiency

What other circumstances might contribute to a deficiency of vitamin A?

Both Retinoid and Carotenoid Forms

Since carotenoids and retinoids are fat-soluble nutrients, vitamin A deficiencies involving either carotenoids or retinoids may be caused by a diet that is extremely low in fat and/or the presence of medical conditions that cause a reduction in the ability to absorb dietary fat, such as pancreatic enzyme deficiency, Crohn's disease, celiac sprue, cystic fibrosis, surgical removal of part or all of the stomach, gall bladder disease, and liver disease.

In addition, chronic diarrhea caused by gastrointestinal infections and/or intestinal parasites may contribute to deficiency of vitamin A in either carotenoid or retinoid forms. Viral infections, including the measles, can decrease retinoid-form vitamin A. In addition, exposure to certain toxic chemicals (for example, polybrominated biphenyls and dioxins) can enhance the breakdown of retinoid-form vitamin A by the liver.

In this section about "Other Circumstances That Might Contribute to Deficiency," it's also important to remember that while carotenoid forms of vitamin A can be effectively converted into retinoid forms inside the bodies of many individuals, this conversion process does not always take place in the way that we would like. Many different factors can contribute to problems with conversion of carotenoids into retinoids. These factors include: a person's inherited genetic tendencies; digestive problems; bacterial imbalances in the digestive tract; excessive use of alcohol; smoking; 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.

Inadequate intake of protein can also contributes to retinoid-form vitamin A deficiency (see further explanation in the section on Nutrient Interactions).

Relationship with Other Nutrients

What relationship does vitamin A have to other nutrients?

Retinoid Forms

The transport and utilization of vitamin A is dependent upon several vitamin A binding proteins. Because a sufficient dietary intake of protein is required for the manufacture of these binding proteins, inadequate protein intake may result in vitamin A deficiency. In addition, adequate intake of dietary fat and zinc is necessary for the absorption and utilization of vitamin A.

The relationship between retinoid forms of vitamin A and vitamin D status has become an area of special research interest. Recent studies suggest that effects of vitamin D deficiency are worsened by high supplemental intake of vitamin A (in retinoid form). Preliminary studies have shown that when blood levels of vitamin D fall below 50 nM/L (nanomoles per liter), higher supplemental intake of retinol (defined as intake above 2,000 micrograms per day) can worsen problems related to vitamin D deficiency (like bone health). After being consumed in retinol form, vitamin A can be converted by the body into retinoic acid. This retinoid acid, in turn, can stimulate formation and activity of osteoclast cells that then work to remove minerals from bone. Similarly, high doses of retinoic acid also seem to be able to suppress the activity of osteoblast cells, which help deposit minerals into the bone. We'd like highlight the fact that all of these studies on vitamin A and vitamin D involve dose levels of retinoid-form vitamin A not available from food but only from supplements. But if you're among the 34% of all U.S. adults who take supplements containing both vitamin A (in retinoid form) and vitamin D, you may want to consult with a healthcare practitioner about the best ratio of vitamin A and vitamin D for you.

Alongside of this research trend showing exacerbation of vitamin D deficiency following higher intake of supplemental vitamin A is a second research trend showing the helpfulness of vitamin A in support of vitamin D metabolism. This second research trend has made it clear that receptors on our cell membranes for vitamin A (called retinoid X receptors, or RXR) and receptors on our cell membranes for vitamin D (called VDR receptors) actually combine in our cells to produce an VDR/RXR form. (In more technical terms, vitamin A is said to "recruit coactivators" that are needed for expression of vitamin D receptors, and the form of vitamin A needed to assist with vitamin D metabolism in this situation is 9-cis-retinoic acid.) We'll need many future research studies to eventually clarify the relationships between vitamin A and vitamin D, and the exact ramifications for our food choices.

Like its potential interference with vitamin D metabolism, excess retinoid-form vitamin A may also interfere with the metabolism of vitamin K, a fat-soluble vitamin necessary for blood clotting. Like the vitamin A-vitamin D relationship, however, "excess" in this case does not apply to the amount of retinoid-form vitamin A provided by everyday amounts of animal food.

Carotenoid Forms

Recent studies have shown the ability of beta-carotene to improve the availability of two minerals—iron and zinc—from grains. In one lab study, the addition of 2.5 grams of cooked carrot containing 200 micrograms of beta-carotene to a 10-gram portion of cooked rice resulted in a 50% increase in the availability of iron from the rice. In everyday practice, you would need to consume one medium-sized cooked carrot (about 50 grams) along with each cup of cooked rice (about 195 grams) in order to achieve this same nutrient ratio. Similarly, this same addition of beta-carotene was able to increase the availability of zinc in the cooked rice by about 35-40%. The authors of this study speculated that beta-carotene may have been able to form a complex with the minerals to help maintain their solubility and also to help prevent their getting bound together with phytates in rice that would otherwise be able to lower their absorption. While this study was lab-based and not conducted on real people eating real food, we look forward to future studies that may show the ability of beta-carotene content in our plant foods to improve the availability of minerals in those foods like iron and zinc.

Risk of Dietary Toxicity

What food practices might lead to a dietary toxicity of vitamin A?

Retinoid Forms

It is almost impossible for ordinary intake of animal foods to result in vitamin A toxicity. Foods simply do not contain enough preformed vitamin A to expose us to toxicity-producing amounts.

Here are some numbers to provide you with a concrete example. When chronic vitamin A toxicity occurs, it typically involves many months of daily intake of vitamin A in retinoid form in amounts exceeding 14,000 IU (4,200 mcg RE) in children and 25,000 IU (7,500 mcg RE) in adults. Let's compare that amount to the largest amounts found in food. At 135 mcg RE of retinol per cup, cow's milk is the animal food on our WHFoods list that ranks highest in retinol content. As you can see, an adult would have to consume over 55 times this amount every day over a period of several months in order to reach the toxicity level described above.

While vitamin A toxicity can be a problem for our health, it comes from improper use of retinoid-containing supplements, not from our diet. Most causes of vitamin A toxicity are due to accidental ingestion of supplemental doses exceeding 660,000 IU (200,000 mcg retinol equivalents) and 330,000 IU (100,000 mcg retinol equivalents) by adults and children, respectively.

In 2000 the National Academy of Sciences (NAS) set Tolerable Upper Intake Levels (ULs) for preformed vitamin A. These recommendations were designed to help prevent excessive amounts of supplemental intake by the general public, not to discourage intake of foods high in retinoid forms of vitamin A. Here were the recommendations of the NAS for maximal intake of retinoid-form vitamin A

• Children 3 years or younger, 600 micrograms (2,000 IU) of preformed vitamin A (retinol) per day
• Children 4-8 years, 900 micrograms (3,000 IU) of preformed vitamin A (retinol) per day
• Children 9-14 years, 1,700 micrograms (5,666 IU) of preformed vitamin A (retinol) per day
• Teenagers 14-18 years, 2,800 mcg (9,333 IU) of preformed vitamin A (retinol) per day
• Adults 19 years and older, 3,000 mcg (10,000 IU) of preformed vitamin A (retinol) per day
• Pregnant or lactating women 18 years or younger, 2,800 mcg (9,333 IU) of preformed vitamin A (retinol) per day
• Pregnant or lactating women 19 years or older, 3,000 mcg (10,000 IU) of preformed vitamin A (retinol) per day

Carotenoid Forms

A telltale sign of excessive consumption of beta-carotene is a yellowish discoloration of the skin, most often occurring in the palms of the hands and soles of the feet. This condition is called carotenodermia and is generally considered to be reversible and harmless. Excessive consumption of the carotenoid lycopene can cause a deep orange discoloration of the skin. Like carotenodermia, lycopenodermia is generally considered reversible and harmless.

High intake of carotenoid-containing foods or supplements is not associated with any specific toxic side effects. As a result, the Institute of Medicine at the National Academy of Sciences did not establish a Tolerable Upper Intake Level (UL) for carotenoids when it reviewed these compounds in 2000.

Disease Checklist

What diseases might have a special connection with vitamin A?

Retinoid forms of vitamin A may play a role in the prevention and/or treatment of the following health conditions:

• Acne
• AIDS
• Alcoholism
• Atopic dermatitis
• Cataracts
• Cervical dysplasia
• Cystic Fibrosis
• Diabetes
• Dry Eyes
• Fibrocycstic breast disease
• Hyperkeratosis
• Impotence (Lack of sperm production)
• Inflammaotory bowel disease
• Kaposi's sarcoma
• Leukoplakia
• Measles
• Osteoarthritis
• Otitis media (ear infection)
• Poor vision
• Premature Delivery
• Psoriasis
• Thyroid disorders
• Ulcers
• Vaginitis
• Varicose veins
• Viral infections

Carotenoid forms of vitamin A may play a role in the prevention and/or treatment of the following health conditions:

• Acquired Immunodeficiency Syndrome (AIDS)
• Age-related macular degeneration
• Angina pectoris
• Asthma
• Cataracts
• Cervical cancer
• Cervical dysplasia
• Chlamydial infection
• Heart disease
• Laryngeal cancer (cancer of the larynx)
• Lung cancer
• Male and female infertility
• Osteoarthritis
• Photosensitivity
• Pneumonia
• Prostate cancer
• Rheumatoid arthritis
• Skin cancer
• Vaginal candidiasis

Public Health Recommendations

What are current public health recommendations for vitamin A?

Public health recommendations for vitamin A can be confusing due to units of measurement. Food and supplement labels present vitamin A content in terms of the Daily Values (DVs). The DVs were established in 1993 by the U.S. Food and Drug Administration (FDA) and are measured in IU (International Units). However, about 20 years earlier (in 1974), the National Academy of Sciences (NAS) had already established its own set of public health recommendations, setting vitamin A requirements in terms of micrograms retinol equivalents (micrograms RE, or mcg RE). In 2001, the NAS changed this measurement standard from mcg RE to mcg RAE (micrograms retinol activity equivalents). Today, you might find public health recommendations using all three units of measurement: RE, RAE, and IU.

If you need to make conversions between these different units of measurement for vitamin A, here are the rules you must follow:

In the RE and RAE measurement systems, 1 microgram of retinol is equivalent to 1 RAE/1 RE while 1 RE is equal to 6 micrograms of beta-carotene or 12 micrograms of alpha-carotene or beta-cryptoxanthin. Additionally, 1 RAE is equal to 12 micrograms of beta-carotene or 24 micrograms of alpha-carotene or beta-cryptoxanthin, and 1 IU is equal to 0.3 micrograms of retinol, 0.6 micrograms of beta-carotene, or 1.2 micrograms of alpha-carotene or beta-cryptoxanthin. (Please note that the word "microgram" is commonly abbreviated as "mcg.")

When converting between these two units of measure, 1 retinol equivalent (or RE) is equal to 3.33 International Units (or IU) of preformed vitamin A.

In 2000, the National Academy of Sciences established the following Adequate Intake (AI) levels for consumption of vitamin A by infants:

• Males and females 0-6 months: 400 mcg RAE (1,333 IU)
• Males and females 7-12 months: 500 mcg RAE (1,666 IU)

In 2000, the National Academy of Sciences established the following Recommended Dietary Allowances (RDAs) for consumption of vitamin A by children and adults. (Please note that the acronym "RAE" used below stands for Retinol Activity Equivalents, and the abbreviation "mcg" stands for micrograms.)

• Males and females, 1-3 years: 300 mcg RAE (1,000 IU)
• Males and females, 4-8 years: 400 mcg RAE (1,333 IU)
• Males and females, 9-13 years: 600 mcg RAE (2,000 IU)
• Males, 14 years and older: 900 mcg RAE (3,000 IU)
• Females, 14 years and older: 700 mcg RAE (2,333 IU)

In 2000, the National Academy of Sciences established the following Estimated Average Requirements (EARs) for consumption of vitamin A by pregnant and lactating women:

• Pregnant women less than 18 years of age: 750 mcg RAE (2,500 IU)
• Pregnant women 19 years and older 770 mcg RAE (2,567 IU)
• Lactating women, 18 years or younger: 1,200 mcg RAE (4,000 IU)
• Lactating women, 19 years or older: 1,300 mcg RAE (4,333 IU)

The Institute of Medicine set Tolerable Upper Intake Levels (ULs) for preformed vitamin A as follows:

• Children 3 years or younger, 600 micrograms (2,000 IU) of preformed vitamin A (retinol) per day
• Children 4-8 years, 900 micrograms (3,000 IU) of preformed vitamin A (retinol) per day
• Children 9-14 years, 1,700 micrograms (5,666 IU) of preformed vitamin A (retinol) per day
• Teenagers 14-18 years, 2,800 mcg (9,333 IU) of preformed vitamin A (retinol) per day
• Adults 19 years and older, 3,000 mcg (10,000 IU) of preformed vitamin A (retinol) per day
• Pregnant or lactating women 18 years or younger, 2,800 mcg (9,333 IU) of preformed vitamin A (retinol) per day
• Pregnant or lactating women 19 years or older, 3,000 mcg (10,000 IU) of preformed vitamin A (retinol) per day

The Daily Value (DV) for vitamin A - as updated in 2008 by the U.S. Food and Drug Administration - is 5,000 IU. This DV standard is the one that you currently see on food labels, and it is based on a 2,000 calorie diet. The 5,000 IU Daily Value also translates into 1,500 mcg RAE of vitamin A.

For our WHFoods standard, we chose the National Academy of Sciences (NAS) most recent DRI recommendation of 900 mcg RAE of vitamin A for men 19 years and older. This amount translates into 3,000 IU of vitamin A. Like the NAS, we chose to adopt a standard for total vitamin A in all of its forms, and we did not adopt separate standards for individual forms of vitamin A, for example, total carotenoids or beta-carotene.

For more details on toxicity of preformed vitamin A, please see the Risk of Dietary Toxicity section above.

For more details on toxicity of preformed vitamin A, please see the Toxicity Symptoms section above.

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