Vitamin A is a fat soluble vitamin with a generic name of retinoids. Retinoids include retinol (reproductive system), retinal (vision), retinol esters (storage), retinoic acid (growth and differentiation) and 3,4-Didehydroretinol. Additionally, some carotenoids (α-Carotene, β-Carotene) are turned into retinol in the organism by enzymes. Carotenoids do not have the harmful effects of the other forms of vitamin A.
Which foods contain the vitamin?
- Fish liver oil
- Beef liver
- Pork liver
- Chicken liver
- Sweet potato
Biological functions of the vitamin
Retinol, retinal and retinoic acid have several biological functions.
The most important of them is that retinal is a molecule which receives light in the process of vision. Vitamin A also participates in the functioning of epithelium, especially in the development of the eye mucus, and the immune system. Eye dryness due to lack of vitamin A may cause inflammations which may cause scars and jeopardise vision even after treatment.
Vitamin A also participates in the development of epithelium and skin cells. It also plays an important role in the development of cartilage and bone tissue (also teeth) and the regulation of growth. Vitamin A is also essential for the normal development of embryo and the cells of a young organism, placenta and sperm and regulation of gastric acid secretion. It also has antioxidant and anticarcinogenic propertie
Absorption, storage and excretion
Vitamin A is absorbed in the upper part of the small intestine. It must be kept in mind that absorption is disturbed by hard physical labour, iron, excessive alcohol, coffee, mineral oils, nitrates, phenobarbital, prednisolone and cortisone but also cholestyramine and other medications binding bile acids.
Vitamin A is stored in liver. Liver can store up to 300 µg/g retinol and the supplies are for 1–3 years. The use of the stored vitamin is promoted by zinc.
An interesting fact is that from plant carotenoids, vitamin A is produced from β-Carotene and from approximately 30–60% of the carotenoids contained in food.
Vitamin A is excreted in the composition of bile.
Problems resulting from deficiency
- Hemeralopia (poor night vision)
- Slime formation disorders (dry eye syndrome and increased risk of infection)
- Skin dryness
- Aged look of skin
- Growth disorders (especially for children who grow fast)
- Placenta development disorders
- Gametes formation disorders
- Sudden vitamin C deficiency
Risk groups for developing the deficiency
- Pre-school children
- Pregnant women
- People with hyperthyroidism
- Patients with liver cirrhosis
- Patients with the closure of bile ducts
- People with chronic kidney inflammation
- People with pneumonia
- People with respiratory infection
- People with chronic nephritis
It has been observed that vitamin A functions similarly to steroid hormones, which is why overconsumption of the vitamin (over 1.5 mg/a day) poses several threats.
Continuous overconsumption may result in nausea, vomiting, double vision, headache, bone pain, hypercalcemia and neutropenia.
It is also known that continuous overconsumption during pregnancy may result in fetal developmental disorders. The most risky periods are the second and fifth week of pregnancy, but pregnant women should be careful until the eighth week of pregnancy.
Continuous overconsumption is also toxic to liver. It may result in increase in liver, formation of connective tissues and thus the impairment of liver functions. The liver toxic functions of vitamin A are promoted by hypertriglyceridaemia, chronic alcohol abuse and an already existing liver damage. The consumption of quantities which damage liver also interferes with the ability of the liver to store retinol and thus retinol gets into blood in an unbound form. In this way, retinol has surface-active properties which may damage cell membranes and lysosomes.
It must be kept in mind that vitamin A is contraindicated for people with liver diseases, smokers and the consumers of excessive alcohol.
Use of vitamin preparations
In preparations, vitamin A is mostly in the form of retinyl acetate, retinyl palmitate or retinol.
Keep in mind that the effect is more effective, if vitamin A is administered together with calcium and zinc which promote the absorption of vitamin A. Vitamin E prevents inactivation and thus also promotes the absorption of vitamin A. It is important to administer vitamin A together with B vitamins which promote the accumulation, and also with vitamin C which inhibits the toxicity of vitamin A.
As an additional treatment component, vitamin A is also used for treating alcoholism, gastritis, acne, muscular dystrophy, tuberculosis, asthma, allergy, hepatitis, jaundice, arthritis, angina pectoris, diabetes, glaucoma, meningitis, haemophilia and osteomalacia
Figure 1. Vitamin A: retinol
Figure 2. Vitamin A: alpha-Carotene
Vitamin A content in food
Quantities of food products which should be consumed daily to obtain the recommended daily amount of vitamin A.
|fish liver oil||1 g||2 g||3 g||3 g||3 g||3 g||4 g|
|pate||4 g||5 g||7 g||7 g||7 g||9 g||11 g|
|butter||54 g||81 g||108 g||108 g||108 g||135 g||162 g|
|egg yolk||78 g||118 g||157 g||157 g||157 g||196 g||235 g|
|fish liver oil||3 g||3 g||3 g|
|pate||9 g||9 g||9 g|
|butter||135 g||135 g||135 g|
|egg yolk||196 g||196 g||196 g|