Vitamin A

Vitamin A testing is used to help diagnose vitamin A deficiency in people with symptoms, such as night blindness, or in people with diseases that impair intestinal absorption of nutrients and who are at risk of vitamin A deficiency. Testing for this purpose is not common because vitamin A deficiencies are rare in the United States. Sometimes testing may be used to detect toxic levels caused by ingestion of large amounts of vitamin A.

Vitamin A is an essential nutrient required for healthy vision, skin growth and integrity, bone formation, immune function, and embryonic development. It is required to produce photoreceptors in the eyes and to maintain the lining of the surface of the eyes and other mucous membranes.

The body cannot make vitamin A and must rely on dietary sources of vitamin A. Meat sources provide vitamin A (as retinol), while vegetable and fruit sources provide carotene (a substance that can be converted into vitamin A by the liver). Deficiencies in vitamin A can impair night vision, cause eye damage, and in severe cases, lead to blindness. Acute or chronic excesses of vitamin A can be toxic, cause a range of symptoms, and sometimes lead to birth defects.

A vitamin A test may be ordered when a person has signs and symptoms that suggest a vitamin A deficiency or has general malnutrition. These signs and symptoms include:

• Night blindness
• Dry eyes, skin, and hair
• Ulcers and damage to the cornea, the outermost layer of the front of the eye that covers the iris and pupil
• Skin thickening and lesions
• Grayish spots on eye lining (Bitot spots)
• Repeated infections
• Anemia

Testing may be done periodically when a person has a disease that is associated with malabsorption of nutrients to monitor vitamin A status and ensure that the person is getting sufficient vitamin A. Some diseases that are associated with inadequate vitamin A absorption include:

• Celiac disease
• Irritable bowel syndrome
• Crohns disease, a type of inflammatory bowel disease

Testing may be performed when a person has signs and symptoms that suggest vitamin A toxicity and the person's medical history is consistent with consumption of foods or vitamin supplements containing vitamin A. Some of the signs and symptoms associated with vitamin A toxicity are:

• Headache
• Nausea and vomiting
• Double or blurred vision
• Fatigue
• Weakness
• Dizziness
• Seizures
• Irritability
• Muscle pain
• Bone and joint pain
• Weight loss
• Hair loss
• Mucous membrane dryness
• Itching
• Liver dysfunction
• Cracks (fissures) at the corners of the mouth
• Inflammation of the tongue (glossitis)

A normal vitamin A blood level indicates that a person currently has sufficient vitamin A but does not indicate how much is stored in reserve. The body will maintain vitamin A in the blood at a relatively stable level until stores are depleted.

A low vitamin A blood test result indicates that all reserves have been depleted and the person is deficient.

A high vitamin A blood level typically indicates that the capacity to store vitamin A has been exceeded and excess vitamin A is now circulating in the blood and may be deposited in other tissue, leading to toxicity.

People with a minimal amount of vitamin A stored may have marginal but adequate vitamin A until they have an illness or other condition, such as pregnancy, that puts increased demands on their body. This is one of the reasons that night blindness is prevalent in many parts of the world during pregnancy.

Eating excessive amounts of foods with beta carotene, such as carrots, can cause a person's skin to turn yellow-orange but it does not cause vitamin A toxicity. The body does not convert beta carotene to vitamin A when there is sufficient vitamin A present.

Sources of preformed vitamin A include meat (especially liver), eggs, dairy products, and fortified foods. Sources of carotene (mainly beta carotene) include brightly colored fruits and vegetables such as carrots, spinach, apricots, sweet potatoes, and cantaloupe.