Vitamin A (Retinol)

Did you know that vitamin A is commonly mistaken with its precursor, beta-carotene? That’s right, vitamin A is NOT the same thing! Vitamin A is actually a name for a group of fat-soluble retinoids: retinal, retinol, and retinyl esters (“retinoid” really just refers to any compound that has vitamin A functions in the body). Vitamin A is absolutely essential for several physiological functions: vision, reproduction, immunity, and cellular communication.  

On the other hand, carotenoids like beta-carotene are phytonutrients with antioxidant and anticancer properties.  But, in order to become biologically active vitamin A, they must be converted into retinoids by our bodies, a process that is extremely inefficient. See also The Amazing World of Plant Phytochemicals: Why a diet rich in veggies is so important!

Vitamin A is probably best known for its function in vision: it is a component of the protein rhodopsin, which absorbs light as it hits the retina of the eye (hence the chemical name retinal/retinol). Plus, it is an essential nutrient that supports the proper functioning of certain parts of the eye (conjunctival membranes and cornea, specifically). While vitamin A’s reputation might be as a major contributor to visual health, it doesn’t stop there.

Normal levels of vitamin A are required in order to reproduce: female reproductive cycles depend on adequate amounts of serum vitamin A, and male sperm production depends on vitamin A to be present as well. Additionally, epithelial cells (those that line the inside and outside of the body – think skin cells, gut cells, etc.) require enough vitamin A for proper differentiation (knowing what type of cell to become as they mature). When there isn’t enough vitamin A, epithelial cells lose their integrity; for example, skin will become scaly and hard, and mucus secretion will be suppressed throughout the gastrointestinal tract. Vitamin A is also necessary for the proper function of cells in our bones, called osteoblasts and osteoclasts – without it, we can’t build bones and/or use the calcium stored in them properly. And, of equal importance, vitamin A is essential for proper immune system function. In fact, we know that vitamin A deficiency is directly linked susceptibility to infectious disease! Likewise, it appears that vitamin A plays a key role in autoimmune disease – that is, deficiency has been associated with increased risk for autoimmunity. Other conditions associated with retinoid deficiency include blindness and diseases of the eyes and thyroid dysfunction.

Yep, vitamin A is incredibly important for our health.

So, how much do we need to consume? The recommended dietary allowance (RDA) for adult men is 900 micrograms/day and for women is 700 micrograms/day.

An essential note is that, while carotenoids (vitamin A precursors) found in plant foods are often touted as great sources of vitamin A, the ONLY source of preformed vitamin A is in animal foods. Carotenoids must be converted to vitamin A in our bodies, which is highly variable and depends upon many factors such as food preparation, individual differences in digestion and absorption, and genetics. Typically, 3% or less of ingested carotenoids are actually absorbed and the conversion rate can be very low in general–a conversion factor of 28:1 for beta-carotene to retinal has been found for some plant foods, including leafy greens. So, if you’ve ever heard that eating lots of carrots is good for your vision, you’d be much better off eating sources of retinoids rather than carotenoids!

The foods richest in retinoids are liver, eggs, quality (grass-fed) high-fat dairy products, and seafood (especially shrimp, salmon, sardines, and tuna). Most of these food groups are welcome in a balanced, nutrient-dense Paleo diet, with dairy and eggs being more “gray area” foods that require individual tolerance assessments (discussed in detail in my new book, Paleo Principles).

Importantly, excess intake of vitamin A during pregnancy has been associated with birth defects (mostly seen in the context of supplements and/or vitamin D deficiency) so make sure to work with a healthcare practitioner if you’re interested in becoming pregnant in the next year!

Vitamin A Toxicity

Vitamin A toxicity is rarely seen in the context of whole foods. According to the Merck Manual:

Acute vitamin A toxicity in children may result from taking large doses (> 300,000 IU [> 100,000 RAE]), usually accidentally. In adults, acute toxicity has occurred when arctic explorers ingested polar bear or seal livers, which contain several million units of vitamin A.

A 4-ounce serving of raw beef liver contains 18,928 IU of vitamin A. Even consuming double this amount daily is well below the toxicity limits for both acute and chronic vitamin A toxicity. However, if you’re taking vitamin A in supplement form, it’s worth talking with your healthcare provider about whether or not to limit organ meat consumption.

Citations

 

Weber D, Grune T. The contribution of beta-carotene to vitamin A supply of humans. Mol Nutr Food Res. 2012;56(2):251-258.

Groff JL. Advanced Nutrition and Human Metabolism. 2nd ed. St. Paul: West Publishing; 1995.

Ross AC. Vitamin A. In: Ross A, Caballero B, Cousins R, Tucker K, Ziegler T, eds. Modern Nutrition in Health and Disease. 11th ed: Lippincott Williams & Wilkins; 2014:260-277.

Food and Nutrition Board, Institute of Medicine. Vitamin A. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, D.C.: National Academy Press; 2001:65-126.