Vitamin A is a lipid-soluble vitamin that is not synthesized by ourselves but must be taken into the body through food. However, it is used in the preservation of vision, in the differentiation of epithelial cells, the stimulation of the immune system in the metabolism of the railway.
In developing countries, vitamin A deficiency is a common cause of blindness in preschool children.
Vitamin A is a group of compounds that have qualitatively the same biological activity as retinol and provitamin A carotenoids, which are precursors of retinol in food. All of these compounds have in common a molecule form that contains a monocyclic base with five carbon-carbon double bonds and a functional group at the end of the acyclic part of the molecule. Due to the similar structure to retinol, such compounds can also be called retinoids. Retinol can be oxidized to retinal and retinoic acid or esterified with an organic acid.
Most vitamin A is stored in the liver, namely as much as 50-80% of the total amount of vitamin A, and a smaller proportion is also in adipose tissue and other tissues.
The biological activity of compounds having vitamin A activity is expressed in retinol equivalents (RE), where 1 µg of RE is equal to 1 µg of retinol. Due to poor absorption of carotenoids and their conversion to retinol, the European Food Safety Authority (EFSA) has determined that 1 µg of RE is 1 µg of retinol, 6 µg of B-carotene, and 12 µg of another provitamin A carotenoids.
The European Food Safety Authority (EFSA) has approved the following health claims for vitamin A that can be used in foods that are a good source of this vitamin:
- contributes to maintaining healthy mucous membranes
- contributes to maintaining healthy skin
- plays a role in maintaining vision
- plays a role in the functioning of the immune system
- plays a role in cell specialization
- plays a role in iron metabolism
EFSA set a target retinol concentration of 20 µg per gram of liver, based on average requirements. Several factors are taken into account: the ratio of total retinol in the body to retinol in the liver is 1.25; liver weight is 2.4% of body weight; the catabolic rate of retinol is 0.7% per day; storage efficiency of ingested retinol is 50%; the reference body weight in the EU for men is 68.1 kg and for women 58.5 kg.
Up to the age of 14, the average requirement of vitamin A is 480 µg /day for girls and boys, from 15 to 17 years the average need for vitamin A and adult women is 490 µg /day, for boys from 15 to 17 years 580 µg /day and for adult men 570 µg /day. Because retinol accumulates in the uterus and fetus in pregnant and lactating women and is later excreted in breast milk, the increased need for vitamin A is 540 µg/day for pregnant women and 1020 µg/day for breastfeeding mothers.
Vitamin A is present in nature in various forms. Plants that can be green, yellow, and orange synthesize the polyisoprenoid pigments carotenoids, which are metabolized to retinol and have vitamin A activity – called provitamin A carotenoids. These include α-carotene, β-carotene, γ-carotene, and β-cryptoxanthin. Provitamin A carotenoids are also present in products of animal origin because some animals feed on plants that synthesize these pigments. Thus, vitamin A is found in milk, meat, eggs, but in the form of retinol ester with long-chain fatty acids. Retinol is found exclusively in foods of animal origin, it is also produced by some bacteria.
|
Food |
Vitamin A content per 100g of food (mg) |
Recommended dietary allowances (RDA) in % |
|
Carrots |
835 |
104 |
|
Sweet potato |
710 |
89 |
|
Spinach |
469 |
59 |
|
Pumpkin |
430 |
54 |
|
Cabbage |
241 |
30 |
|
Red peppers |
157 |
20 |
|
Apricot |
96 |
12 |
|
Tomato |
42 |
5 |
|
Iceberg lettuce |
25 |
3 |
|
Nectarine |
17 |
2 |
|
Orange |
11 |
1 |
Here are some suggestions for Hungry Pumpkin lunches that were rich in vitamin A:
Vitamin A deficiency
Tissue stores of vitamin A maintain the need for a deficiency of it. However, the deficiency begins to appear when the plasma retinol concentration is less than 20 µg/dl, or when the concentration of retinol in liver tissue is less than 20 µg/g.
Vitamin A deficiency can be caused by malnutrition, especially protein. These are important in the synthesis of retinol transport proteins. Protein deficiency means not only poorer absorption of vitamin A, but also impaired transport and function of target organs.
However, the primary deficiency can also be caused by eating foods that contain very few yellow and green vegetables and fruits. Secondary deficiency, however, may be due to a chronic deterioration in fat absorption, due to various diseases that interfere with the secretion of pancreatic enzymes or bile acids, but may also be due to a low-fat diet. Deficiency can also lead to chronic exposure to cigarette smoke, which contains the oxidant benzopyrene, which can deplete vitamin A sources. Zinc, which affects the absorption, transport, and metabolism of vitamin A, is also important.
The signs of vitamin deficiency are different. Common signs are loss of appetite, stunted growth, dry and keratinizing membranes, and infections. Dry mucous membranes, ulcers, xerosis of the cornea and sclera, night blindness appear near the eye. The skin becomes rough and scaly, the hair can also be rough and the muscles weaken. The mucus in the digestive tract is reduced, the epithelium changes and diarrhea occurs, as well as a decrease in mucus secretion and epithelial changes, as well as a decrease in tissue elasticity in the respiratory tract.
Hypervitaminosis A
This can occur due to prolonged exposure to higher doses of the vitamin. The first changes are first visible on the skin and mucous membranes. Dry mouth and nasal cavity appear, followed by dry eyes and conjunctivitis. The skin becomes dry, itchy, and flaky. Dermatitis, alopecia, and brittle nails can also occur, as well as headache and vomiting. However, we need to know that carotenoids are generally not toxic even at higher doses. Excessive intake of these causes accumulation and yellowing of the skin (carotenoderma), which is not harmful and disappears when the carotenes are normalized.
