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.
- 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
- 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."