By Dr. Mercola
Vitamin A is an important vitamin for healthy vision, immune system function, and cell growth. It works synergistically with a number of other vitamins and minerals, including vitamins D, K2, zinc, and magnesium, without which it cannot perform its functions.
- Retinoids (aka retinol), the bioavailable forms of vitamin A found in animal foods
- Carotenoids, previtamin A found in plant foods
The only type of vitamin A your body can readily use is retinol, found in animal foods like liver and eggs. When you get carotenoids (pre-vitamin A) from plant sources, your body must convert the carotenoids into bioavailable retinol. If you're in perfect health, this should not pose a major problem.
However, a number of factors can inhibit your body's ability to absorb carotenoids and convert them into retinol (Vitamin A).
This includes genetics, digestive problems, alcohol use, certain medicines, toxic exposures, and medical conditions that interfere with the digestion of fat (including Crohn's disease, cystic fibrosis, pancreatic enzyme deficiency, and gallbladder and liver disease).
Most People Cannot Convert Carotenoids into the Active Form of Vitamin A
In a majority of people, the carotene-to-retinol conversion is severely compromised, and in some it may be quite negligible. This is particularly true for infants, diabetics, and those with compromised bile production.
Your body's ability to convert carotenoids into bioavailable vitamin A also depends on your diet in general. If you're on a low-fat diet, your conversion rate is virtually guaranteed to be inadequate.
While carotenoids are water-soluble, you still need healthy fats to promote efficient conversion of carotenoids to retinol. As explained in one 2004 study:3
"[P]rovitamin A carotenoids are converted to retinal by beta-carotene-15,15'-dioxygenase. The enzyme activity is expressed specifically in intestinal epithelium and in liver.
The intestinal enzyme not only plays an important role in providing animals with vitamin A, but also determines whether provitamin A carotenoids are converted to vitamin A or circulated in the body as intact carotenoids.
We have found that a high fat diet enhanced the beta-carotene dioxygenase activity together with the cellular retinol binding protein type II level in rat intestines...
Thus, the bioavailability of dietary provitamin A carotenoids might be modulated by the other food components ingested." [Emphasis mine]
The Different Types of Vitamin A
Many associate vitamin A with beta-carotene alone, and believe as long as they eat plenty of sweet potatoes and carrots, they're getting enough vitamin A.
But if your body cannot properly convert carotenoids into retinol, you might still end up with a deficiency if you shun all animal foods.
Retinoids and carotenoids — which are both part of the umbrella term "vitamin A" — are chemically different, and therefore provide different types of health benefits; some of which are better known than others.
The following list illustrates the relationship between the different vitamin As, along with some of their health benefits.
1. Retinoids (fat-soluble, biologically active vitamin A found in animal foods)
- Retinol: Bioactive form of vitamin A, which is converted into retinal, retinoic acid, and retinyl esters
- Retinal: Vision health and healthy growth
- Retinoic acid: Skin health, tooth remineralization, bone growth
- Retinyl esters:4 Biologically inactive storage form
2. Carotenoids (water-soluble pro-vitamins found in plant foods)
• Alpha-carotene: Antioxidant with potential anti-cancer activity; stimulates intercellular communication5
• Beta-carotene:6 Most efficiently converted into bioactive retinol. (Beta-carotene should be avoided in supplement form though, as studies7 have linked it to increased cancer risk. Beta carotene from whole food is safe, as your body will only convert what it needs into retinol)
• Astaxanthin: High-potency antioxidant with anti-inflammatory properties, shown to benefit rheumatoid arthritis; athletic performance; heart- and brain health; age-related macular degeneration. Also protects cells from UV radiation
• Beta-cryptoxanthin: Antioxidant with anti-cancer activity. Studies8 show it may reduce risk of lung- and colon cancer by 30 percent, and rheumatoid arthritis by 41 percent
• Canthaxanthin: Sometimes used in artificial tanning products, canthaxanthin may help reduce photosensitivity associated with erythropoietic protoporphyria, a genetic disorder9
• Fucoxanthin: A brown seaweed pigment that appears to stimulate fat burning and promote healthy glucose metabolism10
• Lutein: Important for vision health: Lutein, found in your macular pigment, helps protect your central vision, and aids in blue light absorption
• Zeaxanthin: Important for vision health: Zeaxanthin is found in high concentrations in your macula lutea, the small central part of your retina responsible for detailed central vision
Are You at Risk for Vitamin A Deficiency?
While vitamin A deficiency tends to be minimal in the US, it is quite common in developing countries. One of the earliest signs of vitamin A deficiency is night blindness, which can lead to permanent blindness if left unaddressed. Vitamin A deficiency also lowers your immune function, thereby raising your risk of complications from infectious diseases. It also contributes to:
- Hormonal imbalances
- Mood disorders
- Skin problems such as eczema and acne
- Thyroid dysfunction
Strict vegans who avoid all animal-based foods and alcoholics are two groups that tend to be more prone to vitamin A deficiency than the general population. According to Dr. Andrew Weil:11
"Alcoholics... should consequently include rich food sources of vitamin A in their diets (while concurrently sharply curtailing or eliminating alcohol consumption). Supplements may not be wise for alcoholics, however, because vitamin A is stored in the liver, and existing liver damage could make them more susceptible to vitamin A toxicity. In such cases, a doctor's supervision is critical."
Vitamin A Requires Zinc to Benefit Your Vision
Vitamin A is very important for good vision. Lutein and zeaxanthin are particularly crucial for preventing age-related macular degeneration, the most common cause of blindness among seniors. Vitamin A affects vision primarily by regulating gene expression, but in order for this to occur, it must be activated in a two-step process, converting from retinol to retinal, and finally to retinoic acid. As previously explained by Christopher Masterjohn in his article on fat-soluble vitamins:
"Vitamin A supports vision, however, in its semi-activated form as retinal. Retinal binds to a protein known as opsin, forming a vitamin-protein complex known as rhodopsin. Each photon of light that enters our eye and collides with rhodopsin causes the retinal to change shape and release itself from the complex. This event then translates into an electrical impulse that our optic nerve transmits to our brain.
The brain synthesizes myriad such electrical impulses at every moment and interprets them as vision. While the function of opsin is to help generate visual images by binding and releasing vitamin A, opsin can only maintain its proper shape and function when it is bound to zinc. In addition, zinc supports the conversion of retinol to retinal, the form of vitamin A that binds to opsin.
We could predict, then, that vitamin A would only be able to support vision in the presence of adequate zinc. This can be studied by determining dark adaptation thresholds, which determine the dimmest spots of light we are able to see after having spent a period of time in the dark to maximize our visual sensitivity. When vitamin A is insufficient, we lose the ability to see the dimmer spots of light."
Researchers at Tufts University showed the importance of zinc in a 2000 study,12 which included 10 vitamin A deficient patients who had failed the dark-adaptation test. After supplementing with 10,000 IUs of vitamin A for two to four weeks, eight of them achieved normal dark-adaptation thresholds. Two of them, however, also had deficient blood levels of zinc. Vitamin A supplementation alone did not work for them, but when 220 milligrams of zinc per day was added to their regimen for two weeks, their vision was also brought back to normal. These results show that vitamin A requires zinc to support healthy vision.
Vitamin A Supplementation Can Be Risky, so Use Caution
When it comes to vitamin A, supplementation carries risks for most people, not just alcoholics, so your best bet is to make sure you're getting your vitamin A from real food — both animal- and plant-based. Some of the most vitamin A-rich foods include the following:
Sources for bioavailable vitamin A (retinoids)13 Pro-vitamin A carotenoid-rich foods Pasture-raised beef or duck liver Carrots Eggs from organic pastured chickens Sweet potatoes Raw organic butter and cheese from grass-fed cows Kale Whole raw milk and heavy cream from organic grass-fed cows Spinach Shrimp Butternut squash Fatty fish like wild-caught salmon (and to a lesser degree sardines) Mustard greens and collard greens
A number of studies have raised warnings about vitamin A supplementation; showing high doses may lead to toxicity, and may raise your risk of heart disease, cancer, and all-cause mortality. Be particularly cautious with retinol or retinoic acid supplements, as the risk of toxicity is higher with these fat-soluble forms. Synthetic versions should also be strictly avoided. Signs of vitamin A toxicity include:
- Hair loss
- Bone loss
- Liver damage
Plant-derived vitamin A precursors such as beta-carotene, or supplements containing "mixed carotenoids" are far better and carry a much lower risk of toxicity, as your body will not convert more than it needs. Of the carotenoids, beta-carotene is the most efficient converter. Compared to alpha-carotene or beta-cryptoxanthin, half as much beta-carotene is required to convert into the same amount of retinol. If you need to supplement, another option is to take desiccated liver pills.
Vitamin A Works in Tandem with Many Other Nutrients
In addition to zinc, vitamin A also works synergistically with vitamins D and K2, magnesium, and dietary fat. Vitamins A, D, and K2 interact synergistically to support immune health, provide for adequate growth, support strong bones and teeth, and protect soft tissues from calcification. Magnesium is required for the production of all proteins, including those that interact with vitamins A and D. Many of the proteins involved in vitamin A metabolism and the receptors for both vitamins A and D only function correctly in the presence of zinc.
Vitamins A and D also cooperate together to regulate the production of certain vitamin K-dependent proteins. Once vitamin K activates these proteins, they help mineralize bones and teeth, protect arteries and other soft tissues from abnormal calcification, and protect against cell death.
This kind of complexity is one of the key reasons why I strongly recommend getting most of your nutrients from real, whole food (and in when it comes to vitamin D, from sensible sun exposure). This is particularly true for vitamin A, as this will circumvent any toxicity issues. Consuming a well-rounded, nutrient-dense diet, with plenty of vegetables and healthy fats, will go a long way toward warding off nutritional deficiencies and serious nutrient imbalances. Any time you opt to supplement with any given vitamin or mineral, you run the risk of throwing it out of balance with its synergistic partners.