Not all vitamin A is created equal. There are actually two forms of vitamin A in the human diet, which are retinol (aka preformed vitamin A) and provitamin A carotenoids, such as beta-carotene. Even though popular nutrition literature often treats the different forms of vitamin A as equivalent, there are actually significant differences between the two.
Retinol, which is also referred to by some as “true vitamin A,” has a higher level of bioavailability than carotenoids because while both retinol and carotenoids must be metabolized into retinal (the active form of vitamin A), carotenoids must first undergo an enzymatic conversion reaction to produce retinol. The efficiency of this conversion reaction differs from person to person depending on factors like how much fat is in the meal, how healthy one’s intestinal flora is, bile production, whether or not one is taking certain medications, etc. Despite all these variables, the conversion efficiency rate of carotenoids to retinol is considered to be highly inefficient, and is estimated to be between 9-22%.
Since vitamin A is a fat-soluble vitamin, this means that it must be consumed with fat in order to be properly absorbed by the intestinal tract. Once in the blood stream, vitamin A makes its way to the liver, where it can be stored for later use. This critical vitamin is documented to play a role in immune system function, vision, reproduction, and cellular communication, growth, and differentiation. In addition, it plays a pivotal role in the proper functioning of almost every organ, including the heart, lungs, kidneys, epithelium, and gonads.
Obtaining the necessary amounts of vitamin A to attain optimal health is nearly impossible when consuming a diet that omits animal sources of vitamin A due to the poor conversion of carotenoids to retinol. Assuming perfectly efficient conversion ability, one would need to eat three times the amount of carotenoid-containing foods as compared to retinol-containing foods to receive equivalent amounts of vitamin A. The best sources of retinol including butter, liver, egg yolks, organ meats, cream, seafood, and fish liver oils. Beta-carotene and other carotenoids are abundant in red, orange, yellow, and dark green fruits and vegetables, such as sweet potatoes, spinach, kale, broccoli, mangoes, papayas, and apricots.
It’s important to remember that vitamins work in concert with other nutrients present in food that increase their absorption, thus making it best to try and obtain nutrients from food before supplements. Obtaining vitamin A in food form also prevents the development of vitamin A overdose, as other vitamins like vitamin D and vitamin K present in the food (but often missing from supplements) prevent this toxicity. Ultimately, including sources of vitamin A from both plants and animals is a good idea, but it’s important to be aware that optimal vitamin A levels are likely to only be reached through the inclusion of animal products.
Fallon, S., & Enig, M. G. (2000, January 01). Vitamin Primer – Weston A Price. Retrieved November 01, 2016, from http://www.westonaprice.org/health-topics/abcs-of-nutrition/vitamin-primer/
Higdon, J. (2016, August). Carotenoids. Retrieved November 01, 2016, from http://lpi.oregonstate.edu/mic/dietary-factors/phytochemicals/carotenoids
Vitamin A — Health Professional Fact Sheet. (2016, August 31). Retrieved November 01, 2016, from https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
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