Vitamin A

The Basics

Vitamin A refers to a family of related fat-soluble compounds that are required for numerous functions throughout the body. Although vitamin A was officially recognized as a vitamin until the early 1900s, vitamin A-rich foods have long been valued by traditional cultures as medicinal foods. As far back as 3,500 years ago, ancient Egyptians used liver for various types of blindness, and similar practices are mentioned in ancient Assyrian texts and Chinese medical writings.1 Today we know vitamin A is responsible for numerous important functions throughout the body.

Vitamin A
  • Is involved in embryonic brain, limb, heart, eye and ear development.
  • Is essential for several different aspects of vision. One of the first ocular symptoms of vitamin A deficiency is night blindness and, if left unchecked, can progress to complete blindness.
  • Was once called the “anti-infective” vitamin because it plays a critical role in immune function, including maintaining adequate levels of natural killer cells, increasing the activity of macrophages, producing infection-fighting cytokines, maintaining mucosal immune function and supporting healthy secretory IgA production (the body’s main mucosal defense).2 3 4 Vitamin A also appears to play a role in maintaining healthy immune homeostasis during infection and/or autoimmune diseases.5
  • Is required for epithelial cell integrity throughout the body. Epithelial cells comprise the tissues that form the outer layer of the skin and some internal organs as well as the inner lining of the blood vessels, the respiratory, digestive, and urinary tracts, and the reproductive system.
  • Regulates the expression of over 500 different genes and plays a major role in cell proliferation and differentiation.2
  • Protects against vitamin D toxicity.6 7
  • Helps to set the circadian rhythm as a part of melanopsin, the protein that translates to blue light entering the eye into daylight for the brain. In this role, it may support healthy sleep.8
  • Works synergistically with the other fat-soluble vitamins D, E and K, enhancing and balancing each other.

Vitamin A deficiency is a major concern worldwide and in countries at risk of deficiency, 33% of preschool aged children and 15% of pregnant women are deficient.9 In the US, and most other developed countries, outright vitamin A deficiency is much less common, but considering 57% of teenagers and 51% of adults don’t actually consume the Estimated Average Requirement (EAR) of vitamin A, more subtle, subclinical deficiencies are likely to exist.10 Inadequate intake of vitamin A is associated with decreased resistance to infection, night blindness, follicular hyperkeratosis, and in more extreme deficiency, birth defects.2 11

Provitamin A Carotenoids vs Preformed Vitamin A

Although preformed vitamin A and provitamin A carotenoids are often lumped together (including on food nutrition labels), they are not the same. Carotenoids are yellow, orange and red pigments found in a variety of common foods, especially brightly colored fruits, and vegetables. They are largely believed to act as antioxidants in the body. Of the hundreds of carotenoids found in nature, only about 10% of them can be converted by the body into vitamin A.12 The carotenoids most readily converted into vitamin A include beta-, alpha- and gamma-carotene. The conversion of provitamin A carotenoids into vitamin A is highly variable from person to person.

Factors that impede conversion of carotenoids into vitamin A include
  • Inherited genetic tendencies
  • Digestive problems, including bacterial imbalances
  • Excessive exposure to certain toxic chemicals
  • Excessive alcohol use
  • A low-fat and/or a low-protein diet
  • Inadequate zinc levels
  • Certain over-the-counter and prescription medications
  • Poor thyroid function
  • Diabetes
  • High body fat percentage14

Considering how critical vitamin A is to optimal function, it is probably wise to get at least some preformed vitamin A in your diet or supplement routine, instead of hoping to get enough solely through the conversion of provitamin A carotenoids.


The term vitamin A encompasses a family of retinoids, which includes retinol, retinal and retinoic acid. Preformed vitamin A from animal foods comes mainly as retinyl esters (such as retinyl palmitate) and some retinol. Retinol is taken up directly by the intestinal cells, while retinyl esters are first converted to retinol by enzymes in the intestine. The body can then convert retinol to retinal and retinoic acid. A portion of provitamin A carotenoids are converted into retinal in the intestine before being absorbed and from there can also be converted into retinoic acid. In supplements, both preformed vitamin A and provitamin A carotenoids can be labeled under the term “vitamin A”, so be sure to check the Supplement Facts panel to determine what percentage of the vitamin A in a product comes from each form. Generally, preformed vitamin A comes as either retinyl palmitate or retinyl acetate. It is sometimes sourced from cod liver oil and in these instances will say so on the label. Dry and solubilized vitamin A forms are water dispersible and may improve absorption.

Understanding Vitamin A Labeling

Carotenoids are usually grouped together with preformed vitamin A but since carotenes do not convert to vitamin A at equal rates and have different bioactivities, an elaborate system has been developed to account for this difference. Because the body converts all dietary sources of vitamin A into retinol, a measurement of retinol activity equivalents (RAE) is used. In this system, one food-sourced microgram (mcg or μg) of retinol is equal to 12 mcg of beta-carotene and 24 mcg of alpha-carotene or beta-cryptoxanthin. For beta-carotene from dietary supplements, 2 mcg of beta-carotene is equal to 1 mcg of retinol. (The higher bioavailability of carotenoids from supplements is due partly to the fact that they are not associated with proteins in the plant matrix like carotenoids are in whole foods. 2 ) Currently vitamin A is listed in International Units, or IUs, on food and supplement labels, which requires a conversion to mcg RAE by source, but luckily, beginning in 2018, manufacturers will be required to list vitamin A in micrograms (mcg) instead of IUs, to help make this system slightly less confusing.15

Food Sources

Preformed vitamin A is found exclusively in animal foods, with liver being the best source. Egg yolks, butter, milk, fish, fish liver oil and chicken are also good sources, especially if the animal they come from was fed their natural diet and had ample access to carotene-rich foods like grass and algae, which the animal could convert to preformed vitamin A. Plant-based foods with a high RAE per serving include sweet potato, spinach, carrots, and pumpkin.15


Signs of vitamin A toxicity include dry, itchy or peeling skin, loss of appetite, bone and joint pain, and, in extreme cases, may result in liver damage. Toxicity is relatively rare and usually occurs only from extremely high acute doses or long term (months or years) daily supplementation with very high doses (25,000- 30,000IUs/day).2 Although vitamin A deficiency can cause birth defects, toxicity is also associated with an increased risk of birth defects. The Tolerable Upper Limit set for pregnant women is 10,000IUs/day (3,000 mcg RAE), although individual diet and vitamin A status should be considered when supplementing with preformed vitamin A.15 There is no known toxicity associated with provitamin A carotenoids.

Food Sources of Vitamin A and Carotenes and RAE
Food Serving Vitamin A micrograms Carotenes micrograms Retinol Activity Equivalent
Beef liver, cooked 1 slice (68 g) 6,421   6,421
Cod liver oil (be sure to check label as this varies by brand) 1 teaspoon 1,350   1,350
Egg, conventional 1 large 73 5 73
Egg, pastured 1 large 115 39 118
Butter, conventional 1 tablespoon 78 35 81
Butter, pastured 1 tablespoon 105 105 114
Beef, conventional 4 ounces   0 0
Beef, pastured 4 ounces   51 4
Sweet Potato, baked ½ cup   11,552 961
Pumpkin, canned ½ cup   14,376 953
Carrot, raw ½ cup chopped   7,527 534
Cantaloupe ½ medium melon   5,622 466
Mango 1 fruit   2,214 181
Spinach, cooked ½ cup   5,659 472
Kale, cooked ½ cup   5,312 443
Butternut squash, baked ½ cup   9,036 572


*Chart assumes 100% efficient conversion of carotenes to retinol. If this conversion is impeded due to one of the factors listed above RAE numbers would be lower for foods containing carotenes.



References Available Upon Request