Key Concepts

  • Vitamin A plays a crucial role in vision and eye development, is involved in regulating the specialization and growth of nearly all cells in the body, in immune system functioning, and organ formation during fetal development
  • Amount you need in diet can be confusing – eat veggies like spinach, butternut squash, sweet potatoes and you’ll be set!
  • Risk of Toxicity HIGH from animal-based supplement sources (retinoids); low toxicity risk from plant-based sources (carotenoids)
  • Supplementation generally not advised in medical literature for retinoids
  • Vitamin A – $0.30/day

Intro to Vitamin A

Vitamin A does not disappoint when it comes to confusing topics in nutrition. Vitamin A is a generic term referring to fat-soluble compounds found as preformed vitamin A and provitamin A. Preformed vitamin A are the retinoids found in animals and animal products and provitamin A are the carotenoids found in vegetables and fruits. Retinoids include retinyl esters, retinol, retinal, retinoic acid, and related compounds. Carotenoids with vitamin A activity include beta-carotene, alpha-carotene, gamma-carotene, beta-cryptoxanthin, and over 45 more compounds. There are also carotenoids without vitamin A activity which may serve important roles in the body. Examples include lycopene, lutein, and zeaxanthin.

For our discussion, it is important to understand the differences between the two main forms (preformed vitamin A and provitamin A) in terms of foods sources, bioavailability, function in the body, and toxicity.

Food Sources of Preformed Vitamin A – Retinoids – Animals, Animal Products, & Fortified Products

  • Beef liver, pan fried, 3 ounces – 444% Daily Value
  • Butter, 1 Tbsp – 7% Daily Value
  • Eggs – 5%
  • Fortified Milk – 10%
  • Fortified Cereal – 10%

Food Sources of Provitamin A – Carotenoids – Fruits, Veggies, & Plant Products

  • Sweet Potato in Skin, 1 whole – 561% Daily Value
  • Butternut Squash, 1 cup – 297%
  • Spinach, frozen, boiled, ½ cup – 229%
  • Carrots, raw, ½ cup – 184%
  • Peppers, sweet, red, raw, ½ cup – 47%
  • Mangos, raw, 1 whole – 45%
  • Broccoli, boiled, ½ cup – 25%
  • Black-eyed peas (cowpeas), boiled, 1 cup – 26%

For a more exhaustive list, click here

Absorption Differences

Preformed vitamin A is better absorbed than provitamin A. The absorption rates of each are as follows:

  • Retinyl esters – 70-90%
  • Beta-carotene – 20-50%
  • Other carotenes – <5-60%

Cooking and processing foods, such as blending carrots, increase absorption. Fiber in plants tend to bind to carotenoids and decrease absorption. Luckily, nature has worked out beautifully and put a bunch of it in fruits and veggies so that we can absorb what we need for proper functioning!

Recommended Intakes

You may have noticed that I did not include any units (such as mcg or IU) next to the foods I listed. Most foods and supplements list the amount of vitamin A as IUs (international units) but this can be deceiving due to the different rates of absorption. Starting in July of 2018, the FDA will require vitamin A to be listed in micrograms (mcg or (μg)). This is because vitamin A is actually measured in terms of micrograms of rational activity equivalents  (RAEs).

Quantity Consumed Quantity Bioconverted to Retinol RAE Ratio
1 μg of dietary or supplemental vitamin A 1 μg of retinol* 1:1
2 μg of supplemental β-carotene 1 μg of retinol 2:1
12 μg of dietary β-carotene 1 μg of retinol 12:1
24 μg of dietary α-carotene 1 μg of retinol 24:1
24 μg of dietary β-cryptoxanthin 1 μg of retinol 24:1
*1 IU is equivalent to 0.3 microgram (μg) of retinol, and 1 μg of retinol is equivalent to 3.33 IU of retinol.

Source: http://lpi.oregonstate.edu/mic/vitamins/vitamin-A#RAE

With all that being said, the take home message is that unit conversions are a pain in the butt and it’s easier to just rely on the percentages of daily value as they do take into account the differences in the source of vitamin A. Or, go eat some spinach and butternut squash and call it a day!

Toxicity

it is really important to note the differences in toxicity between preformed vitamin A (retinoid) and provtaimin A (carotenoid). Retinoids are far more toxic than carotenoids. Just above 3 times the recommended amount of retinoids have been linked to increased risk of birth defects. And just below two times the recommended amount of retinoids may increase the risk of bone loss. Liver damage, nausea, vomiting, and headache are also risks of hypervitaminosis A (vitamin A toxicity). This typically occurs at exposures of about 8 times more than the recommended intake level for several weeks.

Carotenoid sources do not pose any of these risks, although you may turn orange if you eat enough of them ;-).

Because of these findings, supplementation of retinoids does not appear to be wise because the amount you need for proper function and the amount that causes toxicity are within short range of one another.

Function in the Body

What is really neat about vitamin A is that our body can take either form (preformed vitamin A or provitamin A) and convert it to the form it needs. Retinyl esters are converted to retinol in the body and supports reproduction. Beta-carotene is converted into retinal in the body which participates in vision. Iron is required as a cofactor for the enzyme β-carotene-monooxygenase which converts carotenoids to retinal. The body can convert retinol to retinal and retinal to retinol depending on its needs. Retinal can also be converted to retinoid acid which regulates growth, although this is an irreversible reaction.

A zinc deficiency can decrease stored vitamin A’s ability to be released from the liver and decrease the conversion of retinol to retinal. Chronic alcohol consumption also decreases vitamin A status.

Vitamin A plays a crucial role in vision and eye development, is involved in regulating the specialization and growth of nearly all cells in the body, in immune system functioning, and organ formation during fetal development.

Vitamin A Deficiency 

Deficiencies are primarily associated with inadequate intake of vitamin A and secondarily associated with fat malabsorption. A vitamin A deficiency can cause conjunctival xerosis (bitot’s spots), premature skin aging (excess keritization of the skin), increased susceptibility to infections, night blindness, and delayed growth in children. Vitamin A deficiency is the leading cause of preventable blindness in children. It is a major public health concern affecting more than half of all countries, especially in Africa and south-east Asia.

Supplementation

Vitamin A – $0.30/day

 

References/Resources

  1. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
  2. http://lpi.oregonstate.edu/mic/vitamins/vitamin-A#RAE
  3. http://www.who.int/nutrition/topics/vad/en/

 

Facebook Comments

comments