Vitamin A: food sources, functions, side effects and daily dosing
Vitamins are organic substances, required in small amounts in the diet for growth, maintenance and normal functions of body tissues. They can be either fat soluble or water soluble. Vitamin A is defined as a group of nutritional unsaturated organic compounds. It includes retinol, retinal, retinoic acid, carotenes (alpha carotenes, beta carotenes and gamma carotenes).
The form of vitamin A absorbed from animal food sources is retinol. Retinol is commercially produced and administered in the form of esters. Carotenes upon intestinal enzymatic digestion convert into retinol.
There are two types of vitamin A: preformed vitamin A and pro-vitamin A.
Pro-vitamin A is found in vegetables and fruits while preformed vitamin A is present in animal products.
In 1947, Vitamin A was first synthesized by two Dutch scientists, David Adriaan van Dorp and Jozef Ferdinand Arens. Its structure was discovered prior to its synthesis in 1931 by Paul Karrer. Vitamin A is absorbed in the upper part of small intestine.
As it is fat soluble vitamin, factors causing mal-absorption of fat can also affect vitamin A absorption. Digestion and absorption mechanisms of vitamin A involve participation of many proteins. Vitamin A is distributed in liver, pancreas and fat tissues overall the human body. Humans cannot synthesize vitamin A, so they are strictly dependant on external sources for fulfilling their daily needs.
As it is fat soluble vitamin, it is not excreted readily and feces are the way of its excretion. Due to its fat solubility, it can store in the body cells.
Excessive accumulation of vitamin A leads to its toxicity. Excessive cooking and improper storage can lead to the loss of vitamin A.
Functions of vitamin A
Vision: Whenever we discuss vision the vitamin which comes instantly in our mind is vitamin A. It has an important role in visual cycle. In the eye 11-cis-retinal is attach with a protein known as opsin. When light strikes our eye, this 11-cis-retinal converts to 11-trans-retinal. This conversion send signal to brain via nerve of vision (optic nerve) for visual adaptation. This Trans form then dissociates from the opsin and after enzymatic reaction it converts back to 11-cis-retinal. In the last step this 11-cis-retinal rebinds to opsin to form rhodopsin. Rhodopsin is a protein necessary for night vision and visual contrast. In case of vitamin A deficiency there will be no reformation of rhodopsin leading to night blindness.
Treatment of skin diseases: Vitamin A maintains normal healthy skin by converting immature epithelial cells into mature cells. It is necessary for the maintenance of epithelial tissues, both in terms of their growth and division. It decreases the size and secretions of sebaceous glands in skin. Due to fewer secretions, fewer nutrients will be available for bacterial growth resulting in decline in their numbers. This mechanism explains its role in the treatment of acne. It reduces wrinkles and protects skin from ultraviolet radiation. It is also helpful in treating sunburns, sores, eczema and psoriasis.
As an antioxidant: Vitamin A acts as an antioxidant. Antioxidant plays a role in protection of cells from free radicals. Free radicals are the reactive oxygen molecules having unpaired electron. These radicals can cause oxidation of membranes and DNA, so these are very toxic molecules and should be eliminated from the body. Vitamin A helps stabilize these molecules.
Vitamin A role in diseases
Cancer: Vitamin A plays significant role in cellular growth. Supplementation with vitamin A decreases the risk of lung, prostate, ovarian, esophageal, colorectal and stomach cancer. Exact mechanism of this role of vitamin A is unknown.
Age related macular degeneration: It is a disease of unknown etiology in which significant vision loss occurs in older people. Due to its role in vision (already discussed) vitamin A supplementation helps in the treatment and prevention of this disease.
Cataractand glaucoma: Vitamin A improves cataract (in which formation of cloudy areas in lens occur) and glaucoma (disease of eye in which there is increased intraocular pressure that can damage optic nerve).
Measles: Measles is one of the major problems of developing countries leading to increased morbidity and mortality. Vitamin A deficiency increases its risk because body needs vitamin A to maintain epithelial and corneal surface. If not supplemented properly, this deficiency of vitamin A can lead to blindness in children suffering from measles.
HIV: Vitamin A decreases the complications of HIV. It also decreases the risk of transmitting HIV to fetus during pregnancy.
Other conditions: It helps in managing following conditions.
- Gastrointestinal ulcers
Role in reproduction
Vitamin A acts as a hormone. Vitamin A plays an essential role in growth, differentiation, reproduction and embryonic development. Vitamin A insufficiency in pregnant women may cause birth defects in the fetus.
During lactation need of vitamin A increases; as it is essential for infant’s growth and development. Women use vitamin A for treating heavy vaginal bleeding and premenstrual symptoms.
Vitamin A also plays a role in spermatogenesis by regulating germ cell differentiation.
Vitamin A is required for immunity. It helps in treating infectious diseases by increasing regeneration of mucous membranes damaged by infection. Vitamin A defecient children are susceptible to many diseases.
Vitamin A when applied to skin increases wound healing.
Food sources of vitamin A
Vitamin A is present naturally in many foods. Fruit sources of vitamin A:
- Cantaloupe Melon
- Tomato Vegetable and plant sources of vitamin A: sweet potato, canned pumpkin, carrots,collards, kale, spinach, lettuce, red peppers, squash, mustard, greens, beet greens, turnip greens, dandelion greens, pea, broccoli, seaweed
- Animal sources of vitamin A: milk, egg, cheese, yogurt, beef liver, chicken liver, pork liver, fish liver, oyster
Daily tolerable of vitamin A
Like all others vitamins, vitamin A has specific safe upper limits for specific age groups called as daily tolerable upper levels.
Daily tolerable values are listed as follows:
INFANTS: 600 micrograms/day CHILDREN: 1-3 years: 600 micrograms/day 4-8 years: 900 micrograms/day
MALES: 9-13 years: 1700 micrograms/day 14-70 years: 2800-3000 micrograms/day
- General guidelines: 9-13 years: 1700 micrograms/day 14-70 years: 2800-3000 micrograms/day; 13-18 years: 2800 micrograms/day 19-50 years: 3000 micrograms/day
- Pregnant and lactating females: 13-18 years: 2800 micrograms/day 19-50 years: 3000 micrograms/day
Side effects of vitamin A
Toxicity can occur if this limit is crossed. Toxic effects resulting from excess intake of vitamin A are called as “Hypervitaminosis”.
These are listed as follows:
Gastrointestinal disturbance: Excess intake of vitamin A causes nausea, vomiting, abdominal pain, diarrhea, feeling of fullness and anorexia. People with high intake of alcohol and liver diseases are prone to develop liver toxicity. Vitamin A toxicity can also lead to intra-hepatic cholestasis in which flow of bile from the liver into intestine stops.
Neurological problems: Consuming excessive vitamin A over a long period can cause fatigue, lethargy, excessive sweating, irritability, headache, dizziness, drowsiness, delirium, coma and mental problems. It leads to increased intracranial pressure (psuedotumor cerebri) and cerebral edema.
Birth defects: Intake of vitamin A more than upper limit can be a cause of birth defects. These defects include malformation of skull, heart, lungs and eyes. Vitamin A excess particularly affects period of organogenesis in pregnant ladies.
Skin problems: Long term ingestion of large amounts of vitamin A produces cartenodermia. This disease is manifested by yellow orange discoloration of the skin of hands and feet. Distinguishing feature of this condition from jaundice is that sclera remains white. Other skin problems are dryness of mucous membranes, redness followed by peeling of skin, soreness, brittle nails, psoriasis.
Increased bone turnover: Vitamin A increases activity of bone dissolving cells known as osteoclasts and suppresses activity of bone forming cells called as osteoblasts, resulting in increased bone loss and decreased bone formation. This increases the risk of bone fractures, osteoporosis and bone lesions. It leads to skeletal deformation and pain.
- Increased risk of lung and heart disease in smokers
- Hair loss
- Blurry vision
- Increased risk of pneumonia
Interactions of vitamin A
- Vitamin A increases the risk of bleeding when taken with anti-coagulants, anti-platelets, aspirin and NSAIDS.
- Vitamin A may decrease blood pressure. Caution should be taken when combined with blood pressure lowering agents.
- Vitamin A interacts with drugs using cytochrome p450 enzyme system of liver.
- Vitamin A may interact with anti-diarrheals, anti-depressants, anti-cancers, antibiotics, anti fungals and weight reducing agents.
Recommendations of vitamin A (RDA)
Every vitamin has its own RDA value that varies in different age group people. RDA stands for recommended dietary allowance. It is the daily intake level of a nutrient that is sufficient to meet the requirement of a person. The RDA values of vitamin A are:
- For infants: 0-6 months: 400 micrograms/day 7-12 months: 500 micrograms/day
- For children: 1-3 years: 300 micrograms/day 4-8 years: 400 micrograms/day
- For males: 9-13 years: 600 micrograms/day 14-70 years: 900 micrograms/day
- For females: 9-13 years: 600 micrograms/day 14-70 years: 700 micrograms/day
- Pregnant females: 13-18 years: 750 micrograms/day 19-50 years: 770 micrograms/day
- Lactating females: 13-18 years: 1200 micrograms/day 19-50 years: 1300 micrograms/day
|Written by:||Michal Vilímovský (EN)|
|Education:||Medical student, 3rd Faculty of Medicine, Charles University, Prague, Czech Republic|
|Published:||April 1, 2014 12:59 AM|
|Next scheduled update:||April 1, 2016 12:59 AM|