Hello everyone on the internet! It’s vitamin A day! Hooray! We are going to be chatting about all things vitamin A.
This post is to help kick off a new thing I’m calling “The Micronutrient Series.” In case you didn’t know, micronutrients are components of the diet that can’t be made by the human body and are therefore required in the diet, usually in small amounts; they include vitamins and minerals. I plan to chronicle all the micronutrients from vitamin A-zinc in this series.
Each vitamin or mineral will have a post like this one, where I explain what the micronutrient does, how it is absorbed and used in the body, food sources of the micronutrient, and what too much of the vitamin may do to you.
Please take note that this series will be rolling out slowly, as there are many micronutrients! But I hope you’ll stick around for the ride. If you have questions about RDA and DRIs, please see this post. Otherwise, let’s chat vitamin A!
‘Vitamin A’ is the name of not one, but rather, a group of fat-soluble retinoids. It includes retinol, retinal, and retinyl esters.
There are two forms available in the human diet: preformed vitamin A and provitamin A (often called carotenoids). Both must be metabolized into retinal and retinoic acid for use in the human body.
Preformed vitamin A is found primarily in animal food products, such as meat, fish, and diary.
Provitamin A is found mostly in plant foods. Beta-carotene is the most important provitamin A in the human diet, and is prevalent in foods such as carrots and sweet potatoes.
What Vitamin A Does in Your Body:
Vitamin A does a lot of things in your body. Often associated with good vision, vitamin A is indeed critical to make rhodopsin, a protein that absorbs light in retinal receptors in your eye, thereby ensuring proper vision.
But vitamin A does more than help you see. It also plays an important role in cell growth and differentiation, and is critical for normal formation and maintenance of the lungs and kidneys, among other organs. Vitamin A also plays a role in gene expression and a large body of research also supports the role of vitamin A in regulation of genes involved in immunity.
Vitamin A also plays an important role in reproduction and human development. In males, vitamin A participates in sperm development, and in women, vitamin A promotes healthy fetal growth and development. Vitamin A is transferred from the mother to fetus to ensure proper development of the nervous system, lungs, kidneys, heart, skeleton, eyes, and ears.
Some human epi studies have shown that adequate intake of vitamin A from food is associated with lower risks of certain cancers; however, studies have also shown that overconsumption of vitamin A from supplement specifically can be harmful (more on this in the toxicity section).
Vitamin A Metabolism:
Vitamin A is a fat-soluble vitamin. It is often bound to other food compounds, and requires some digestion before it can be absorbed.
For example, retinol is often bound to fatty acid esters. Retinyl esters and carotenoids in foods (other forms of vitamin A) are often pound to protein. Stomach acid helps break the vitamins from their food components, and they often bind to fat globules and travel to the intestines as for further enzymatic digestion. Next, micelles (small aggregates) form with the vitamin A and are absorbed into the intestine.
Around 70-90% of dietary vitamin A is absorbed as long as the meal contains some fat (at least 10 gram). Carotenoid (provitamin A) absorption from uncooked plant foods (like raw carrots) can be as low as 5%, and can range up to much higher values (20-50%) depending on the plant, if the plant food is cooked, and what it is consumed with.
Up to (approximately) a year’s worth of vitamin A can be stored in the body, primarily in the liver.
Food Sources of Vitamin A:
As mentioned above, there are two kinds of vitamin A relevant to the human diet.
Liver, chicken, and fish (including tuna and herring) are rich sources of preformed vitamin A. Eggs, milk, and milk products such as cheese, also contain preformed vitamin A.
Because vitamin A is fat-soluble, it is lost from milk products when they are skimmed. However, most fat-free milk products are fortified with vitamin A to make up for the loss. Check labels if you are unsure of your products.
Provitamin A is found in plant foods, including orange and yellow vegetables (sweet potatoes, carrots, peppers, pumpkins, etc), tomato products, cantaloupe, mangos, apricots, and some other fruits, as well as broccoli, spinach, black-eyed peas, and some other vegetables. Fortified ready-to-eat-cereals and pistachios also contain provitamin A.
How Much Vitamin A Do I Need?
Vitamin A is a little less straightforward than other micronutrients, as it is carotenoids are converted into vitamin A. Attempts have been made to determine how much provitamin A in the diet is converted to retinol in the diet, which is expressed in IU units. This is why you often see vitamin A recommendations expressed in mcg (ug) or IU.
Currently, vitamin A is listed on foods and dietary supplements in IU, even though nutrition scientists don’t often use IU in research. This can make interpreting scientific data confusing for the everyday non-nutrition-scientist. For reference:
- 1 IU retinol = 0.3 ug RAE
- 1 IUbeta-carotene from dietary supplements = 0.15 ug RAE
- 1 IU beta-carotene from food = 0.05 ug RAE
- 1 IU alpha-carotene or beta-cryptoxanthin = 0.025 ug RAE
The Recommended Dietary Allowance (RDA)* for vitamin A for healthy adult males is 900 ug (~3,000 IU) RAE and 700 ug (~2,333 IU) RAE for adult females.
Pregnant and lactating women should consume 770ug/day and 1300 ug/day, respectively. Children 1-3 require 300 ug/day, 4-8 year olds require 400 ug/day, and 9-13 year olds require 600 ug/day. 14-18 year old females require 700 ug/day, and 15-18 year old males require 900 ug.day.
For reference, a sweet potato has about 1,403 ug RAE vitamin A (or 28,058 IU).
The UL (aka the maximum amount most can consume before experience adverse effects) is 3,000 ug RAE or 10,000 IU. As detailed below, these UL are mostly for the preformed form of vitamin A found in animal foods.
*Curious about what a RDA is? Read this post!
Vitamin A Deficiency:
Once a person’s body is depleted of any storage of vitamin A, they become at risk for deficiency. In vitamin A deficiency, cell differentiation and maturation become impaired and some epithelial cells (cells on the surface of skin and mucous membranes) flatten and produce a tough, inflexible protein called keratin. In the eye, this leads to drying and hardening of the cornea, which eventually progresses to blindness.
Vitamin A deficiency is the leading causes of preventable blindness in children. It also increases the risk of disease and death from infections, dry mouth may cause xerophthalmia (night blindness), and increased risk of disease and death from severe infections. Evidence also suggests vitamin A reduces the risk of measles infections and diarrhea. Pregnant women who are vitamin A deficient are at increased risk of maternal mortality.
Vitamin A deficiency is rare in the United States and many other developed countries. However, vitamin A deficiency is common in many developing countries, especially African and South-East Asia, due to low access to vitamin A-rich foods. Poor access is often linked to poverty.
Vitamin A Toxicity:
Because vitamin A is fat soluble, excess can be stored in the human body. It is most often stored in the liver, where it can accumulate.
Toxicity from provitamin A and beta-carotene is not associated with adverse effects. However, large amounts of preformed vitamin A can cause toxicity and severe adverse effects, including hypervitaminosis A.
Hypervitaminosis A can cause increases in intracranial pressure, dizziness, nausea, headaches, skin irritation, joint pain, coma, and death. It has been documented in humans in a few instances, usually with Arctic explorers who ate large amounts of polar bear liver.
Excess preformed vitamin A intake is also associated with reduced bone mineral density and increased risk of fracture. It is also considered a teratogenic, and may lead to serious congenital birth defects, including malformation of the heart, eyes, skull and lungs in a developing fetus.
While toxicity from food sources of provitamin A and beta-carotene are rare, consumption of provitamin A and beta-carotene in the form of supplementation is associated with increased risk of certain types of cancer and cardiovascular disease. In fact, some studies (the famous CARET study) have been ended early as they found high-dose vitamin supplementation was associated with earlier deaths from cancer and cardiovascular disease in the test group.
But how can a vitamin that’s an antioxidant lead to cancer? Well, at certain levels, antioxidants become pro-oxidants, and can fuel cancer.
Random Fun Facts About Vitamin A:
Remember that one super intense acne medicine, Accutane? Yeah, that was actually a large dose of a derivative of vitamin A. All of the warnings about Accutane and birth defects reflect the mentioned risks of excess vitamin A consumption and adverse impacts on the fetus
Vitamin A is a fat soluble vitamin important for healthy vision, immunity, and for proper human reproductive function and fetal growth development. It is found in two forms: preformed vitamin A (found in animal foods) and provitamin A (found in plant foods such as carrots and sweet potatoes).
It is rare to experience adverse effects of over-consuming provitamin A from foods, but large provitamin vitamin A supplements carry significant risks. It is also possible to experience toxicity from consuming preformed vitamin A in the human diet. Vitamin A deficiency is rare in developed countries, but is a significant problem in many developing nations.