From 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