Vitamin A is essential throughout life, beginning in the earliest stages. It is required for cell differentiation, the process that determines the function of new cells. This makes vitamin A especially important during fetal development and childhood growth, periods with the highest rates of cell differentiation. Throughout life, vitamin A remains necessary for growth, vision and fighting disease.

The body makes vitamin A from precursors in the diet – either retinyl esters from animal products (and fortified foods) or carotenoids from plants. These precursors can be changed to three forms of vitamin A: retinol (the most common), retinal and retinoic acid. As a group, retinyl esters, retinol, retinal and retinoic acid are called retinoids.

The body is more efficient at transforming retinyl esters into vitamin A than carotenoids (a type of pigment produced by some plants). Therefore, it requires larger amounts of carotenoids than retinyl esters to make the same amount of vitamin A. The best sources of carotenoids are yellow and orange fruits and vegetables, including carrots, mangos and sweet potatoes.

Supplements may be recommended by physicians for people who do not consume enough vitamin A through the diet or as part of the treatment for various conditions. They are available as preformed retinol (ready to be used by the body), beta carotene (must be converted to be used) or a combination of both. However, vitamin A supplementation does significantly increase the risk of overdose. Getting too much vitamin A can lead to birth defects, osteoporosis, liver damage, central nervous system disorders and other problems.

Vitamin A is a group of fat soluble nutrients essential for growth, vision and immune response. Unlike most water soluble vitamins, the body does not immediately remove excess vitamin A. Instead, it is stored in the liver, where excess over time can eventually damage a patient’s health.

Certain internal proteins are necessary for vitamin A to move through the body. Chylomicrons, a type of protein, must be present to move vitamin A from the intestines (where absorption takes place) to the liver for storage. Retinol-binding protein is required to move the vitamin from the liver to cells for use.

Vitamin A itself is not present in food. It is produced in the body once certain precursors in foods are ingested. As foods or supplements are digested, the precursors are converted into a usable form of the vitamin. Once it is in a useable form, vitamin A is active in the entire body, including:

• All body cells. Vitamin A is crucial for cell differentiation – the process that determines each new cell’s function. This occurs throughout life, but is significantly more active during fetal development and childhood growth. During these periods, vitamin A deficiency is known to cause severe debilitation.
  
  
• Epithelial cells. Most of the body’s vitamin A function is performed in these cells that line the skin and internal surfaces (mucous membranes), such as the respiratory, urinary and gastrointestinal tracts. Vitamin A helps synthesize proteins necessary for the function and production of mucus. Mucus creates a barrier that helps prevent invasive organisms and harmful substances from damaging tissues.
  
  
• Eyes. Vitamin A is needed by the retina (in the back of the eye) to convert light energy into nerve signals. It is also necessary to maintain the health of the cornea, the membrane that covers the outside of the eye.
  
  
• Immune system. Vitamin A is needed to create white blood cells, which fight infection.
  
  
• Skeletal system. Throughout life, and especially during growth, the body must take apart sections of bone and then rebuild them. As part of this process, vitamin A is necessary for dissolving the old bone.

To help consumers and medical professionals to determine if they are getting enough vitamin A, the U.S. National Academy of Sciences established dietary reference intakes (DRIs) for the nutrient. DRIs provide the average daily intake of vitamin A necessary to support a healthy body – based on age and gender (including whether women are pregnant or breastfeeding). Tolerable upper intake levels (UL) were also determined. Patients who consume more than the UL for vitamin A increase potential health risks. These recommendations are measured in micrograms per day. One microgram is equivalent to ¼ gram.

Vitamin A DRIs

The term vitamin A is often used to refer to a number of substances aside from its three usable forms (retinol, retinal, retinoic acid). To understand these forms, it is important to know how they are created in the body. Vitamin A itself is not found in foods. Instead, the body converts chemical precursors found in dietary sources once they are metabolized in the body.

Retinyl esters (found in meat and other animal products) are the most efficient vitamin A precursor for bioavailability. The body can also convert some carotenoids (plant pigments) into vitamin A, including beta carotene, alpha carotene and beta cryptoxanthin.

Once in the body, vitamin A precursors can be converted into three forms of vitamin A. Each form plays a specific role in the body and which form is created depends on what the body needs at that time. Retinyl esters, together with the three usable forms of vitamin A, belong to a group called retinoids. The forms of vitamin A are:

• Retinol. The primary form of vitamin A in the body. Both retinyl esters and carotenoids can be changed into retinol in the intestines. Retinol is then either transported to the liver to be stored or to body cells to be used or converted into retinal. Once in the liver, retinol-binding protein must be present for retinol to be transported out of the liver into body cells. Once converted, retinol functions the same way, no matter its original source.
  
  
• Retinal. Retinyl esters must first be converted to retinol. Carotenoids can be converted directly into retinal, if needed. Whatever its original source, retinol can be converted into retinal in body cells when needed. Excess amounts can be further converted into retinoic acid or be changed back into retinol for storage.
  
  
• Retinoic acid. An irreversible form of vitamin A that can be made from retinal in body cells when needed. However, excess cannot be converted back into retinal or retinol for storage. Therefore any retinoic acid produced in body cells must be used. Neither retinyl esters nor carotenoids can be directly converted into retinoic acid.

Much greater amounts of carotenoids than retinyl esters are needed to produce the same amount of vitamin A. In addition, certain carotenoids are more efficient when taken in supplement form than when ingested through diet. To better compare the efficiency of different sources of vitamin A available from various foods and supplements, the U.S. National Academy of Sciences developed retinol activity equivalents (RAEs). For example, to produce 1 RAE, the following amounts of vitamin A sources would be needed:

• 1 microgram retinol
• 2 microgram beta carotene (from supplements)
• 12 microgram beta carotene (from food)
• 24 microgram alpha carotene or beta cryptoxanthin

In supplements, vitamin A can be present in two forms, either separately or together:

• Preformed retinol. A synthetic form of retinol.
  
  
• Beta carotene. A plant-based precursor for vitamin A. Beta carotene in supplement form is used more efficiently by the body than when ingested through food.

Vitamin A precursors are available from a variety of dietary sources. Therefore, a balanced diet usually provides sufficient vitamin A to satisfy the body’s needs. The retinoids come from animal sources and fortified foods while the carotenoids are available from vegetables and fruits (most of which are dark green, yellow or orange).

The carotenoids found in plants are not as well-absorbed as the retinoids found in animal products. However, most experts urge people to get vitamin A from both sources.

Whichever dietary source is used, freezing is known to reduce the amount of vitamin A available. In addition, consuming small amounts of fats helps increase the bioavailability of the vitamin.

Sources of Vitamin A

Vitamin A is essential for the body to function. Without it, growth, vision and fighting infection are impossible. It is made from precursors found in food and is also available in supplements as retinol (ready to be used by the body), beta carotene (must be converted before use), or a combination of both. Vitamin A supplements may be recommended by physicians as part of the established treatments for a variety of conditions, including:

• Night blindness (the inability to see in limited light or recover vision after a sudden bright flash). This is a symptom of the genetic disorder retinitis pigmentosa and vitamin A deficiency.
  
  
• Acne. Topical and oral prescription medications containing specific forms of vitamin A are used to treat serious cases of acne after other treatments have failed. Women who may become pregnant should not use these medications due to the potential risk of severe birth defects.
  
  
• Acute promyelocytic leukemia. An acute form of leukemia characterized by the accumulation of immature white blood cells. Survival increases among patients treated with vitamin A in conjunction with chemotherapy.
  
  
• Erythropoietic protoporphyria. This inherited genetic disorder causes sensitivity to light, gallstones and liver dysfunction. Beta carotene supplements have been used to increase some patient’s ability to withstand exposure to sunlight.

Research continues into other potential health benefits associated with vitamin A. For example, one recent study found that applying topical vitamin A to the skin may improve the appearance of wrinkles associated with aging as well as promote the production of skin-building compounds. Another study found that children who take vitamin A supplements may receive a boost to their immune systems that is helpful prior to receiving vaccines such as tetanus.

Further research is necessary to confirm these findings.

Vitamin A supplements are associated with potential health risks that must be considered along with possible benefits. Improper use of retinol in particular has been linked to certain health problems.

In contrast, beta-carotene supplements have not been linked to health problems affecting the bones or other parts of the body. Experts believe these supplements to be safe, according to the National Institutes of Health (NIH). However, the Institute of Medicine has concluded that beta-carotene supplements are not advisable for the general population. However, they may be appropriate as a provitamin A source for the prevention of vitamin A deficiency in specific populations.

Known health risks associated with vitamin A supplements include:

• Birth defects. Vitamin A is a known teratogen when consumed excessively by pregnant women, especially early in the pregnancy. Unless a deficiency is known, vitamin A is usually not part of recommended prenatal vitamin supplements during the first trimester.
  
  
• Osteoporosis and hip fractures. High doses of vitamin A supplements have been linked to an increased risk of hip fractures and osteoporosis in elderly populations. While it is necessary to remodel bone for growth, excess vitamin A in older patients causes bone cells to break down and weaken. Consuming too much vitamin A also may interfere with the work of vitamin D, which is essential to preserving bone.
  
   
• Delayed growth. In children and teens, growth delays have been associated with excess vitamin A.

Because of these potential risks, some experts urge people not to take vitamin A supplements unless a physician directs them to do so. In particular, men over age 30 and women over age 50 sometimes test for increased levels of retinol, according to the NIH.  

The ability to absorb and use vitamin A can be impacted by other medical conditions. Those that impact the body’s relationship with vitamin A include:

• Liver damage. Patients with damaged livers, as a result of disease or alcohol abuse, are more susceptible to toxicity from excess vitamin A.
  
  
• Inability to absorb fats. Because vitamin A is a fat soluble vitamin, a patient with decreased ability to absorb fats will also have limited absorption of vitamin A precursors. Reduced fat absorption may result from medications to help treat obesity, injury to the alimentary canal, genetic conditions, or certain medical conditions (e.g., pancreatic, liver or gallbladder diseases).
  
  
• Malnutrition. Any condition that causes a patient to consume insufficient nutrients may lead to vitamin A deficiency. Examples include gluten intolerance, Crohn’s disease, chronic diarrhea, pancreatic disorders and alcoholism.
  
  
• Iron deficiency or zinc deficiency. Iron and zinc are minerals involved in processes throughout the body, including the synthesis of vitamin A from foods. Without either of them, a vitamin A deficiency is likely to occur.

Dietary choices and behaviors can also impact vitamin A intake and the body’s reaction to the nutrient. Some examples include:

• Vegetarian and vegan diet. Vitamin A precursors from animal sources (retinyl esters) are more efficiently converted to forms the body can use than the carotenoids from plants. Additional servings of fruits and vegetables or beta carotene supplements are necessary for patients who consume only plant materials.
  
  
• Smoking. Vitamin A supplements containing beta carotene are not recommended for smokers. Studies have shown their use is associated with a higher mortality rate than smokers in general. Researchers are investigating the potential reasons.
  
  
• Alcohol abuse. Alcohol impacts the body’s relationship to vitamin A in multiple ways. It depletes vitamin stores, increasing the possibility of a deficiency and damages the liver, causing the body to become more susceptible to a possible overdose.

At the proper levels, vitamin A is necessary for the human body to function. Too little or too much stresses cells and tissues and may lead to serious, life-threatening disease. A variety of symptoms occurs and the entire body is impacted.

Symptoms of Vitamin A Overdose or Deficiency

Hypervitaminosis A is the term used for the body’s response to an overdose (hypervitaminosis) of vitamin A. It results when all the binding proteins are occupied by retinoids. The condition is rare and the majority of cases results from consuming excessive animal products (especially liver), fortified foods or supplements. If the overdose is the consequence of a single excessive dose, it may be classified as acute. Chronic hypervitaminosis A results from consuming smaller excesses over a long period. Children and the elderly are most vulnerable to both types.

As hypervitaminosis A advances, it can cause eye and liver damage, osteoporosis, and disorders of the central nervous system. Certain populations show significant impact due to vitamin A overdoses. They include:

• Pregnant women. Birth defects may result if vitamin A intake is over the recommended amount during the first seven weeks of pregnancy.
  
  
• Children. Slower growth than other children in their age group is a common affect of hypervitaminosis A. The growth rate returns to normal when vitamin A intake is reduced.
  
  
• Infants. A bulging soft spot in the head may develop in infants. Additional signs and symptoms may include swelling around the optic nerve and double vision.

When additional vitamin A is necessary, physicians may recommend that patients consume more beta carotene from fruits and vegetables rather than supplements. Dietary sources are preferred because the body usually does not convert the beta carotene available in foods to retinol if sufficient vitamin A is present, avoiding the risk of an overdose. While not physically harmful, excess beta carotene is stored in fat under the skin and, in extreme cases, may cause the skin to turn yellow or orange.

Beta carotene from supplements poses health hazards that are not associated with carotenoids from food. In the body, overdoses of supplemental beta carotene may cause destruction of vitamin A and increase cell division. Most cases are associated with alcohol and cigarettes use.

In the United States, vitamin A deficiency is rare. For a healthy adult, more than a year of no intake is required for deficiency to occur. Children use the vitamin faster and have smaller stores, so deficiency can occur much faster. Deficiencies are most commonly associated with strict dietary restrictions, chronic diarrhea or alcohol abuse. They may also be related to zinc deficiency or iron deficiency, which impact the body’s ability to use vitamin A. Beginning with dry eyes and night blindness, deficiency may eventually result in total blindness or death.

Vitamin A deficiency is treated with supplements containing preformed retinol (a synthetic form) to replenish retinol stores within the body. Patients are initially given high doses of the vitamin, which are reduced over time. After several weeks, enough retinol should once again be stored in the body and supplementation may no longer be needed.

Preparing questions in advance can help patients to have more meaningful discussions with health professionals regarding their conditions. Patients may wish to ask their doctor or registered dietitian the following questions related to vitamin A:

1. Am I at risk for vitamin A deficiency or toxicity?
   
   
2. How can I learn more about sources of vitamins and how much I need each day?
   
   
3. Should I consult a registered dietitian to reduce my risk of vitamin A deficiency or overdose? Can you recommend one?
   
   
4. What early signs and symptoms of vitamin A deficiency should I be on the lookout for?
   
   
5. If I need treatment for my vitamin deficiency, how long will it last?
   
   
6. Should I eat more animal or plant sources to increase my vitamin A intake?
   
   
7. Is it possible for me to consume too much vitamin A?
   
   
8. What are the risks of consuming excess vitamin A?
   
   
9. As I age, how should I change the amount of vitamin A I consume?
   
   
10. Should I be concerned about how my lifestyle impacts my body’s ability to use vitamin A?