Vitamin A (Retinol): Health Benefits and Deficiency
Vitamin A is a fat soluble vitamin needed for the maintenance of healthy cell membranes and for cell reproduction. It is involved in the formation of protein, growth hormones, bone and retina. As a result, vitamin A is necessary for ensuring good vision, healthy skin, hair, teeth, bones, mucous membranes and immune system, and normal growth and development. It is also important for proper functioning of internal organs including the lungs, heart and kidneys. Alongside vitamins C and E, it is one of the major antioxidants helping to fight free radicals in the body.
Vitamin A is present in three forms including retinal, retinol and retinoic acid. Human diet mainly provides retinol, which is found in animal foods, and a precursor to retinol known as carotene (alpha, beta or gamma-carotene). Beta-carotene is the most abundant form of carotene. It commonly occurs in vegetables (especially those that are yellow, orange or red in color) and can be easily converted into retinol by the body. Beta-carotene is thus the major dietary source of vitamin A. The third dietary form of vitamin A, the so-called preformed vitamin A (retinyl ester) can be found in dietary supplements. Vitamin A is stored in the liver.
Deficiency of Vitamin A
Deficiency of vitamin A is rare in the developed world and usually only occurs in people with digestive conditions that prevent proper absorption of nutrients from food (e.g., inflammatory bowel disease, celiac disease or leaky gut syndrome) or those who eat very little fruits, vegetables, fish, eggs and dairy products. Likewise, malabsorption of fats (e.g., due to cystic fibrosis) may contribute to deficiency of fat-soluble vitamins including vitamin A. Vegans and alcoholics also appear to be more susceptible to developing vitamin A deficiency. In addition, early weaning of breast milk may put infants at increased risk of deficiency in vitamin A. However, malnutrition (often combined with zinc deficiency) is still the main cause of vitamin A deficiency in the impoverished third world countries.
Symptoms: the most typical signs of vitamin A deficiency are vision problems (especially impaired night-time vision and extreme eye dryness) and compromised immune system (particularly frequent are respiratory infections occurring in young children as well as ear and urinary tract infections). Other health problems that may result from the lack of vitamin A include dry, scaling skin and skin lesions, broken fingernails, enamel hypoplasia, hyperkeratosis (abnormal thickening of the skin and hair follicles), keratinisation of mucous membranes and airway metaplasia, slow growth in children, poor utilisation of iron, fatigue, insomnia, infertility, miscarriage and possibly also age-related macular degeneration. In addition, vision problems may eventually develop into keratomalacia and lead to total irreversible blindness.
Vitamin A Toxicity (Hypervitaminosis)
Vitamin A overdose usually only occurs in people who overuse dietary supplements. Just like deficiency, too much vitamin A may cause vision problems (e.g., blurred vision) and skin changes. Other health problems associated with vitamin A overdose include joint and muscle pain, headaches, nausea, vomiting, low appetite, liver damage and confusion. Chronic overdose may also lead to increased susceptibility to bone fractures, hair loss, fatigue, anemia, chronic diarrhea and weight loss. In addition, long-term supplementation with beta-carotene has been found to increase the risk of developing lung cancer as well as cardiovascular disease in current and former smokers. Too much beta-carotene may also cause yellowing of the skin known as carotenodermia, a harmless and reversible condition.
Dietary Recommendations for Vitamin A
Since there are several chemically different dietary sources of vitamin A, the recommended dietary allowances and the tolerable upper intake levels are provided in micrograms (mcg) of retinol activity equivalents (RAE). This is because 1 mcg of retinol is equivalent to 12 mcg of beta-carotene and 24 mcg of alpha-carotene from dietary sources. The current recommendations are as follows:
|RDA (mcg of RAE per day)||Upper Limit (mcg of RAE per day)|
|Birth to 6 months||400 (*)||400 (*)||600||600|
|7 – 12 months||500 (*)||500 (*)||600||600|
|1 – 3 years||300||300||600||600|
|4 – 8 years||400||400||900||900|
|9 – 13 years||600||600||1,700||1,700|
|14 -18 years||900||700, pregnant 750, lactating 1,200||2,800||2,800|
|19 – 50 years||900||700, pregnant 770, lactating 1,300||3,000||3,000|
|51 years and older||900||700||3,000||3,000|
* Adequate intake as no RDA has been established
Dietary Sources of Vitamin A
As it was mentioned above, animal foods are the chief source of readily available vitamin A known as retinol. These mainly include liver and fish oil. Other good sources of active vitamin A are milk and dairy products, eggs, chicken, fish and meat. Carotene, a precursor to vitamin A, is found in the pigment of fruits and vegetables. Hence, carrots, sweat potatoes, green leafy vegetables, squash, cantaloupe melon, dried apricots, peppers, tropical fruits and most other yellow and orange-colored fruits and vegetables are all good sources of beta-carotene.