The Fat-Soluble Vitamins: A, D, E, and K

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Vitamin A plays a key role in maintaining your vision. Without it, you would go blind.

Types

Vitamin A is not a single compound. Rather, it is a group of fat-soluble compounds collectively known as retinoids.

The most common dietary form of vitamin A is retinol. Other forms — retinal and retinoic acid — are found in the body, but absent or rare in foods.

Vitamin A2 (3,4-dehydroretinal) is an alternative, less active form found in freshwater fish (1).

Summary

The main dietary form of vitamin A is known as retinol.

Role and function of vitamin A

Vitamin A supports many critical aspects of your body function, including:

  • Vision maintenance. Vitamin A is essential for maintaining the light-sensing cells in your eyes and for the formation of tear fluid (2).
  • Immune function. Vitamin A deficiency impairs your immune function, increasing susceptibility to infections (3, 4).
  • Body growth. Vitamin A is necessary for cell growth in your body. Deficiency may slow or prevent growth in children (5).
  • Hair growth. It is also vital for hair growth. Deficiency leads to alopecia, or hair loss (6).
  • Reproductive function. Vitamin A maintains fertility and is vital for fetal development (7).

Summary

Vitamin A is best known for its vital role in maintaining vision. It’s also essential for body growth, immune function, and reproductive health.

Dietary sources

Vitamin A is only found in animal-sourced foods. The main natural food sources are:

  • liver
  • fish liver oil
  • butter

The table below shows the amount of vitamin A in 3.5 ounces (100 grams) of some of its richest dietary sources (8):

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Vitamin A can also be derived from certain carotenoid antioxidants found in plants. They are collectively known as provitamin A.

The most efficient of these is beta-carotene, which is abundant in many vegetables, such as carrots, kale, and spinach (9, 10).

Summary

The best dietary sources of vitamin A include liver and fish oil. Sufficient amounts can also be derived from provitamin A carotenoids, like beta-carotene, which are found in vegetables.

Recommended intake

The table below shows the recommended daily allowance (RDA) for vitamin A. The RDA is the estimated amount of vitamin A that the vast majority (about 97.5%) of people need to meet their daily requirements.

This table also shows the tolerable upper intake limit (UL), which is the maximum daily intake considered unlikely to cause adverse health effects (11).

Summary

The RDA for vitamin A is 900 mcg RAE for adult men and 700 mcg RAE for women. For children, it ranges from 300 mcg RAE to 600 mcg RAE.

Vitamin A deficiency

Vitamin A deficiency is rare in developed countries.

However, vegans may be at risk since preformed vitamin A is only found in animal-sourced foods.

Although provitamin A is abundant in many fruits and vegetables, it is not always efficiently converted into retinol, the active form of vitamin A. The efficiency of this conversion depends on your genetics (12, 13).

Deficiency is also widespread in some developing countries where food variety is limited. It is common in populations whose diet is dominated by refined rice, white potatoes, or cassava and lacking in meat, fat, and vegetables.

A common symptom of early deficiency includes night blindness. As it progresses, it may lead to more serious conditions, such as:

  • Dry eyes. Severe deficiency may cause xerophthalmia, a condition characterized by dry eyes caused by reduced tear fluid formation (2).
  • Blindness. Serious vitamin A deficiency may lead to total blindness. In fact, it is among the most common preventable causes of blindness in the world (14).
  • Hair loss. If you are vitamin A deficient, you may start to lose your hair (15).
  • Skin problems. Deficiency leads to a skin condition known as hyperkeratosis, or goose flesh (16).
  • Poor immune function. Poor vitamin A status or deficiency makes you prone to infections (3).

Summary

Severe vitamin A deficiency may lead to blindness. Other symptoms may include hair loss, skin problems, and an increased risk of infections.

Vitamin A toxicity

Overdosing on vitamin A leads to an adverse condition known as hypervitaminosis A. It’s rare but may have serious health effects.

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Its main causes are excessive doses of vitamin A from supplements, liver, or fish liver oil. In contrast, a high intake of provitamin A does not cause hypervitaminosis.

The main symptoms and consequences of toxicity include:

  • fatigue
  • headache
  • irritability
  • stomach pain
  • joint pain
  • lack of appetite
  • vomiting
  • blurred vision
  • skin problems
  • inflammation in the mouth and eyes

It may also lead to:

  • liver damage
  • bone loss
  • hair loss

At extremely high doses, vitamin A can be fatal (17).

Healthcare professional advise against exceeding the upper limit for intake, which is 3,000 mcg of preformed vitamin A per day for adults.

Higher amounts may cause acute hypervitaminosis A in adults. Children can experience harmful effects at much lower amounts (18).

Individual tolerance varies considerably. Children and people with liver diseases like cirrhosis and hepatitis are at an increased risk and need to take extra care.

Pregnant women should also be especially careful, since high doses of vitamin A may harm the fetus (19).

Summary

High doses of vitamin A may lead to hypervitaminosis A, which is associated with various symptoms. Pregnant women should avoid eating high amounts of vitamin A because of the risk of birth defects.

Benefits of vitamin A supplements

While supplements are beneficial for those who have a deficiency, most people get enough vitamin A from their diet and do not need to take supplements.

Yet, controlled studies suggest that vitamin A supplements may benefit certain people even if their diet meets the basic requirements.

For instance, vitamin A supplements may help treat measles in children (20, 21).

They protect against measles-associated pneumonia and reduce the risk of death by 50-80%. Studies suggest that vitamin A acts by suppressing the measles virus (22).

Summary

Supplements mainly benefit those who are low or deficient in vitamin A. One exception is children with measles, as studies show that supplements may help treat the disease.

Summary of vitamin A

Vitamin A, also known as retinol, is a fat-soluble vitamin traditionally associated with vision and eye health.

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The most abundant dietary sources of vitamin A are liver, fish liver oil, and butter.

It can also be derived from provitamin A carotenoids found in red, yellow, and orange vegetables, as well as some leafy, dark green vegetables.

Deficiency is rare in developed countries but is most common among people who follow diets lacking in food variety, especially those dominated by rice, white potatoes, and cassava.

Early symptoms of vitamin A deficiency include night blindness, and severe deficiency may eventually lead to total blindness.

Nevertheless, while getting enough vitamin A is vital, too much may cause harm.

Pregnant women should be extra careful not to eat excessive amounts of vitamin A because of the risk of birth defects.

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