Vitamin A Quizlet

1

Vitamin A is an essential nutrient, meaning that we need it in order to stay alive. It helps maintain healthy vision, cell growth and differentiation, and immune function.

The best sources of vitamin A are liver, fish oils, egg yolks and orange/yellow vegetables like carrots, sweet potatoes and squash.

Vitamin A Quizlet

Terms in this set (22)
Vitamin A Forms
Preformed Vitamin A
Provitamin A
Preformed Vitamin A Forms
Retinol – alcohol form – reproduction and growth
Retinal – aldehyde form – vision
Retinoic acid – acid form – cell differentiation
Retinyl esters – ester form – storage form

ProVitamin A Forms
About 600 known carotenoids, approximately 50 have proVitamin function
Beta carotene
Alpha carotene
Beta cryptoxathin
Conversion of Vitamin A compounds
An enzyme splits beta-carotene into 2 retinals which are further converted to retinols
6 beta carotene = 1 retinol
12 of other ProVitamin A = 1 retinol
Retinyl esters hydrolysed to retinol, retinol oxidised to retinal, retinal oxidised to retinoic acid (irreversible reaction)
Food sources
Retinyl esters: liver, fish oils, eggs, dairy products, fortified margarines
Provitamin A: brightly coloured vegetables, e.g. dark green/orange vegetables – carrots, pumpkin, broccoli, silverbeet
Digestion
Released vitamin A and carotenoids aggregate with lipids and pass through into the small intestine
Pancreatic lipases and esterases hydrolyse lipids, retinyl esters, and any esters in carotenoids
Combine with bile salts to form micelles
Absorption
Preformed vitamin A (~80% absorbed)
Provitamin A (~20-50% absorbed)
Rentinol lipid micelles are taken up
Cellular binding protein (CRBPII) in the muscosal cells carries the retinol to the enzyme LRAT
LRAT esterfies retinol and delivers it to chylomicrons
Retinyl esters and unconverted carotenoids are transported by chylomicrons to the liver where they are stored in stallate cells
Transport from liver to target cells
Retinyl esters are hydrolysed to retinol
Retinol is released to retinol binding protein (RBP) for plasma transport to sruface target cells
Carotenoids that have not been converted to retinoids are transported via LDL to where needed
Storage of Vitamin A
50-80% vitamin A stored in liver
B-carotene also stored in adipose tissue, kidneys and lungs
Excretion
Mainly via faces
Liver excreted vitamin A conjugates into bile
Bile excretion into gut increases with excess vitamin A
Very small amount lost in urine

in the body vitamin a helps

Vitamin A is an essential vitamin that the body needs for growth and development. It is also vital for cell recognition, vision, immune function, and reproduction. It also helps the heart, lungs, kidneys, and other organs function correctly. Vitamin A deficiency is rare in the United States.

Vitamin A is a fat-soluble nutrient that plays a vital role in your body.

It exists naturally in foods and can also be consumed through supplements.

This article discusses vitamin A, including its benefits, food sources of the vitamin, and the effects of deficiency and toxicity.

hands peeling carrots on wood block
Tim Macpherson/Getty Images

What is vitamin A?

Though vitamin A is often considered a singular nutrient, it’s really a group of fat-soluble compounds, including retinol, retinal, and retinyl esters (1Trusted Source, 2Trusted Source).

Two forms of vitamin A are found in food.

Preformed vitamin A — retinol and retinyl esters — occurs exclusively in animal products such as dairy, liver, and fish, while provitamin A carotenoids are abundant in plant foods like fruits, vegetables, and oils (3Trusted Source).

To use both of these forms of vitamin A, your body must convert them to retinal and retinoic acid, the active forms of the vitamin.

Because vitamin A is fat-soluble, it’s stored in body tissue for later use.

Most of the vitamin A in your body is kept in your liver in the form of retinyl esters (4Trusted Source).

These esters are then broken down into all-trans-retinol, which binds to retinol-binding protein. It then enters your bloodstream, at which point your body can use it (5Trusted Source).

SUMMARY

“Vitamin A” is the generic term for a group of fat-soluble compounds found in both animal and plant foods.

Functions in your body

Vitamin A is essential for your health. It supports cell growth, immune function, fetal development, and vision.

Perhaps one of the best-known functions of vitamin A is its role in vision and eye health.

Retinal, the active form of vitamin A, combines with the protein opsin to form rhodopsin, a molecule necessary for color vision and low light vision (6).

It also helps protect and maintain the cornea, which is the outermost layer of your eye, and the conjunctiva, a thin membrane that covers the surface of your eye and the inside of your eyelids (7Trusted Source).

Additionally, vitamin A helps maintain surface tissues such as your skin, intestines, lungs, bladder, and inner ear.

It supports immune function by supporting the growth and distribution of T cells, a type of white blood cells that protect your body from infection (8).

What’s more, vitamin A supports skin cell health, male and female reproductive health, and fetal development.

SUMMARY

Vitamin A is needed for eye health, vision, immune function, cell growth, reproduction, and fetal development.

Health benefits

Vitamin A is an important nutrient that benefits health in many ways.

Potent antioxidant

Provitamin A carotenoids such as beta carotene, alpha carotene, and beta cryptoxanthin are precursors of vitamin A and have antioxidant properties.

Carotenoids protect your body from free radicals — highly reactive molecules that can harm your body by creating oxidative stress (10Trusted Source).

Oxidative stress has been linked to chronic conditions such as diabetes, cancer, heart disease, and cognitive decline (11Trusted Source).

Diets high in carotenoids are associated with a lower risk of many of these conditions, such as heart disease, lung cancer, and diabetes.

Essential for eye health and preventing macular degeneration

As mentioned above, vitamin A is essential to vision and eye health.

Adequate dietary intake of vitamin A helps protect against certain eye diseases, such as age-related macular degeneration (AMD).

Studies show that higher blood levels of beta carotene, alpha carotene, and beta cryptoxanthin may reduce your risk of AMD by up to 25% (15Trusted Source).

This risk reduction is linked to carotenoid nutrients’ protection of macular tissue by lowering levels of oxidative stress.

May protect against certain cancers

Due to their antioxidant properties, carotenoid-rich fruits and vegetables may protect against certain types of cancer.

For example, a study in more than 10,000 adults found that smokers with the highest blood levels of alpha carotene and beta cryptoxanthin had a 46% and 61% lower risk of dying from lung cancer, respectively, than nonsmokers with the lowest intake of these nutrients (16Trusted Source).

What’s more, test-tube studies demonstrate that retinoids may inhibit the growth of certain cancer cells, such as bladder, breast, and ovarian cancer cells (17Trusted Source).

Vital for fertility and fetal development

Vitamin A is essential for both male and female reproduction because it plays a role in sperm and egg development.

It’s also critical for placental health, fetal tissue development and maintenance, and fetal growth (9Trusted Source).

Therefore, vitamin A is integral to the health of pregnant people and their developing babies, as well as people who are trying to become pregnant.

Boosts your immune system

Vitamin A impacts immune health by stimulating responses that protect your body from illnesses and infections.

Vitamin A is involved in the creation of certain cells, including B cells and T cells, which play central roles in immune responses that guard against disease.

A deficiency in this nutrient leads to increased levels of pro-inflammatory molecules that diminish immune system response and function (18Trusted Source).

SUMMARY

Vitamin A positively affects health by keeping oxidative stress in check, boosting your immune system, and protecting against certain diseases.

Deficiency

Though vitamin A deficiency is rare in developed countries such as the United States, it’s common in developing countries, where populations may have limited access to food sources of preformed vitamin A and provitamin A carotenoids.

Vitamin A deficiency can lead to severe health complications.

According to the World Health Organization, vitamin A deficiency is the leading cause of preventable blindness in children worldwide (19Trusted Source).

Vitamin A deficiency also increases the severity and risk of dying from infections like measles and diarrhea (20, 21Trusted Source).

Additionally, research has found that vitamin A deficiency raises the risk of anemia and death in pregnant women and negatively impacts the fetus by slowing growth and development (22Trusted Source).

Less severe symptoms of vitamin A deficiency include skin issues such as hyperkeratosis and acne (23, 24Trusted Source).

Certain groups — such as premature infants, people with cystic fibrosis, and pregnant or breastfeeding people in developing countries — are more at risk of vitamin A deficiency (25).

SUMMARY

Vitamin A deficiency can lead to blindness, increased infection risk, pregnancy complications, and skin issues.

Food sources

There are many dietary sources of both preformed vitamin A and provitamin A carotenoids.

Preformed vitamin A is more readily absorbed and utilized by your body than plant-based sources of provitamin A carotenoids.

Your body’s ability to effectively convert carotenoids, such as beta carotene, into active vitamin A depends on many factors — including genetics, diet, overall health, and medications (26Trusted Source).

For this reason, those who follow plant-based diets — especially vegans — should be vigilant about getting enough carotenoid-rich foods.

Foods highest in preformed vitamin A include:

  • egg yolks
  • beef liver
  • liverwurst
  • butter
  • cod liver oil
  • chicken liver
  • salmon
  • cheddar cheese
  • liver sausage
  • king mackerel
  • trout

Foods high in provitamin A carotenoids like beta carotene include (27):

  • sweet potatoes
  • pumpkin
  • carrots
  • kale
  • spinach
  • dandelion greens
  • collard greens
  • winter squash
  • cantaloupe
  • papaya
  • red peppers

SUMMARY

Preformed vitamin A exists in animal foods like liver, salmon, and egg yolks, while provitamin A carotenoids are found in plant foods, including sweet potatoes, kale, and carrots.

Toxicity and dosage recommendations

Just as vitamin A deficiency can negatively impact health, getting too much can also be dangerous.

The Recommended Dietary Allowance (RDA) for vitamin A is 900 mcg and 700 mcg per day for men and women, respectively. This intake level is easy to reach if you consume plenty of whole foods (28Trusted Source).

However, to prevent toxicity, it’s important not to exceed the Tolerable Upper Intake Level (UL) of 10,000 IU (3,000 mcg) per day for adults (28Trusted Source).

Though it’s possible to consume excessive preformed vitamin A through animal-based sources like liver, toxicity is most commonly linked to excessive supplement intake and treatment with certain medications, such as isotretinoin (29Trusted Source).

Since vitamin A is fat-soluble, it’s stored in your body and can reach unhealthy levels over time.

Taking too much vitamin A can lead to serious side effects and can even be fatal if you ingest extremely high doses.

Acute vitamin A toxicity occurs over a short time period when a single excessively high dose of vitamin A is consumed. Chronic toxicity occurs when doses more than 10 times the RDA are ingested over a longer time span (30Trusted Source).

The most common side effects of chronic vitamin A toxicity — often referred to as hypervitaminosis A — are:

  • vision disturbances
  • joint and bone pain
  • poor appetite
  • nausea and vomiting
  • sunlight sensitivity
  • hair loss
  • headache
  • dry skin
  • liver damage
  • jaundice
  • delayed growth
  • decreased appetite
  • confusion
  • itchy skin

Though less common than chronic vitamin A toxicity, acute vitamin A toxicity is associated with more severe symptoms, including liver damage, increased cranial pressure, and even death (31Trusted Source).

What’s more, vitamin A toxicity can negatively impact the health of pregnant people and their babies and may lead to fetal development irregularities (9Trusted Source).

To avoid toxicity, steer clear of high dose vitamin A supplements.

The UL for vitamin A applies to animal-based food sources of vitamin A and to vitamin A supplements.

A high intake of dietary carotenoids is not associated with toxicity, though studies link beta carotene supplements with an increased risk of lung cancer and heart disease in people who smoke cigarettes (32Trusted Source).

Since too much vitamin A can be harmful, consult a healthcare professional before taking vitamin A supplements.

SUMMARY

Vitamin A toxicity may have negative effects such as liver damage, vision disturbances, nausea, and even death. Do not take high dose vitamin A supplements unless a healthcare professional prescribes them for you.

The bottom line

Vitamin A is a fat-soluble nutrient vital to immune function, eye health, reproduction, and fetal development.

Both deficiency and surplus intake may cause severe side effects. Therefore, while it’s crucial to meet the RDA of 700–900 mcg daily for adults, it’s also essential not to exceed the daily upper limit of 3,000 mcg.

healthy, balanced diet is a great way to provide your body with a safe amount of this essential nutrient.

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