Vitamin A

Vitamin A is more chemical compounds that have biological functions in all animals. They are partially absorbed directly with the food or formed from carotene (provitamin A ), to which not all animals are able (eg, house cats).

In humans, one counts retinal, retinol, retinoic acid and retinyl palmitate as vitamin A, and 3- dehydroretinol including the aldehyde. They can be made ​​by enzymatically catalyzed reactions with each other, with the only exception that retinoic acid can not be recycled. Chemically, it is to retinoids. There are not enough of them in the body, the result is a hypovitaminosis.

• 4.1 Nervous System
• 4.2 blood cells
• 4.3 protein metabolism
• 4.4 Skin and mucous membranes
• 4.5 skeleton
• 4.6 embryonic growth
• 4.7 reproduction
• 4.8 immune system

• 5.1 hypovitaminosis
• 5.2 hypervitaminosis

History

Already used around 1500 BC, the Chinese liver and honey to cure night blindness. In the 16th century AD Guilleaume described this cure.

Occurrence

In animal foods vitamin A is mainly as retinyl palmitate available in plant as carotenes.

First, the vitamin A content of selected foods (especially animal origin ), measured in / converted to micrograms Retinoläquivalent/100 g:

The body, excess vitamin A little break down, so it is easy to accumulate in the body, especially in the liver. From frequent consumption of liver is therefore not recommended. Pork liver contains, for example, up to 42 mg of vitamin A ( 140,000 IU) per 100 g In this context it should be noted that the liver of the polar bear (Ursus maritimus ) is toxic due to their high vitamin A content and therefore not eaten by the Inuit will. This is true to a lesser extent for other arctic animals, especially various seals, too.

Due to the conversion to retinol equivalents (RAE ), the carotene content of plant foods can now be compared with animal foods. For this purpose, a table of selected plant foods, measured in / converted to micrograms Retinoläquivalent/100 g:

Physiology

Provitamin A ( carotenes ), retinyl palmitate and retinol be taken with food. Retinyl palmitate is hydrolyzed by pancreatic lipase to retinol. Although the carotenes can be implemented in most types of tissue to vitamin A, of the majority of this conversion occurs in the intestinal cells. The resulting retinal and retinol are added in the cytosol of cytosolic retinol binding proteins ( CRBP I to III), in turn converted to retinyl palmitate and transported to the liver by means of chylomicrons. From there they are transported in plasma using the plasma RBP. Receiving at the target cell makes the RBP - receptor. In tissue retinol / retinal can be cached as retinyl palmitate; This is the storage form in which most of the vitamin A is present in the liver.

Digestion and absorption of vitamin A

Retinyl palmitate and retinol be taken with animal food. Retinal and retinoic acid, however, play no role in the diet. In any case it is lipophilic compounds which accumulate together with other lipids in the gut. Retinol binds directly to the cell membrane of enterocytes, retinyl palmitate is first split using the enzyme pancreatic lipase in retinol and palmitate:

H2O H

Since retinol at The spa located inside the cell retinol -binding protein CRBP II binds more strongly than to the membrane, retinol does not remain long in the membrane and moves into the cytosol.

Conversion of carotenes

In addition to β -carotene take animals and humans vegetable α -carotene and β -cryptoxanthin in the diet. The enzyme β -carotene -15, 15' - monooxygenase ( BMO ) is able to convert these carotenoids to retinal, wherein two molecules of retinal entirely occurs only in β -carotene, this reaction, while the other substances are split asymmetrically and are each only a retinal molecule is formed.

O2 2

β -carotene is cleaved into two molecules of the all-trans retinal. BMO is expressed in many tissue types, but the most activity, there is, due to the availability of substrate in the intestine.

Also the retinal from this reaction binds rapidly to CRBP II in the cell inside the enterocytes. Is the vitamin A requirement satisfied, the BCMO1 gene expression is returned. Excess β -carotene located in lipophilic areas of the body, so also in the skin, in extreme cases, be perceived as a harmless yellowing ( Aurantiasis cutis, Karotinämie ).

Retinal is reduced to retinol then, probably an enzyme, which is localized in the membrane of the ER ( facing the cytosol ) of the retinal reductase RalR1.

Esterification and transport to the liver

Not needed retinol is esterified in many tissue types to retinyl palmitate, catalyzing enzyme lecithin: retinol acyltransferase:

All-trans- retinol, and retinyl palmitate are reacted to give dipalmitoyl lecithin and 2- Palmitoyllecithin. The lecithins are removed from the ER membrane, in the vicinity of the reaction takes place.

For installation in chylomicrons the microsomal triglyceride transfer protein is now necessary. It is not known how many molecules retinyl palmitate have to find their way in and then leads us through the lymph into the blood plasma ultimately in a chylomicron. Since no transfer proteins are known for retinyl palmitate, this remains in chylomicrons to the liver. Their dortiger reduction in the endothelium of liver parenchymal cells by lipoprotein lipase and the immediate hydrolysis there by a Retinylesterhydrolase ( REH ) leads to the uptake of retinol into the cytosol of liver cells.

Function

Vitamin A is essential for the growth, function and structure of the skin and mucous membranes, blood cells, metabolism as well as for vision. The utilization of this vitamin in the body can be affected by liver damage and taking estrogen preparations. Recent studies have shown that, contrary to the presumption to even the smallest amounts of fat in foods vitamin A can be absorbed and used by the body.

Nervous system

Retinol gets healthy nerve cells in the peripheral nerve pathways in the brain and spinal cord.

Blood corpuscle

Retinol promotes the formation of new red blood cells and decisively facilitates the installation of iron.

Protein metabolism

It is involved in protein synthesis and in the fat metabolism in the liver, so that a high-protein diet can lead to a vitamin A deficiency. Even with increased stress increases the vitamin A requirement because stress increases the need for protein. Thus, the retinol - demand rises even with serious diseases like arthritis, AIDS or cancer.

Skin and mucous membranes

Vitamin A plays a central role in the structure and health of these tissues, as it is a normal cell growth ensures not only the skin but also the walls of the respiratory, digestive and urinary tract. Furthermore, it prevents DNA damage in skin cells from, contributes to their repair in and normalizes skin functions, such as healthy cell division of keratinocytes ( → epidermis).

Skeleton

Vitamin A is also in the establishment and growth of bones and the healing after fractures of importance. Thus, a sufficient supply of vitamin A is particularly important for children.

Embryonic growth

Vitamin A acid ( all-trans- retinoic acid ) or its salt, the retinate is an important growth factor for nerve cells during embryonic development. It is secreted by cells of the primitive node and is involved in the formation of the longitudinal axis (front-rear orientation) of the embryo. Nerve cells migrate along the concentration gradient of retinoic acid.

Reproduction

Retinol is involved in the synthesis of testosterone and estrogen, as well as on the spermatozoa and oogenesis, as well as retinal, a component of rhodopsin, which is responsible for the perception of light in the photoreceptors of the eye protein. Furthermore, the amount and form of the sperm of an optimal supply of vitamin A -dependent. On the effects on the human mucous membranes vitamin A ( both lined with mucosa) for maintaining the structure and function of sperm and fallopian tubes important. In women, infertility and miscarriages are associated with Retinolmangel.

Immune system

Firstly retinol increases resistance to infection, since, as already mentioned, vitamin A maintains healthy skin and mucous membranes, thereby supporting effective barrier against bacteria, viruses and parasites. Further increase retinol and beta carotene activity and the number of white blood cells, and further facilitate the production of antibodies. Even a slight deficiency increases the risk of developing pneumonia or diarrhea to get to the two-to three-fold.

Demand

The actual daily requirement is dependent on age, gender and life circumstances. Adults should, on average, 0.8 to 1.0 mg ( = 2600-3300 IU) ingest daily, with men towards women have a slightly increased need. Longer cooking, oxygen and light damage vitamin A. So get foods that contain vitamin A, or unpeeled always packed and dark - preferably in the fridge - store. The cooking losses are between 10 and 30 percent.

Hypovitaminosis

In vitamin A deficiency leads to increased susceptibility to infections, dryness of the skin, hair, nails and eyes, hair loss, night blindness, reduced visual acuity, increased sensitivity to light, iron deficiency, increased risk of atherosclerotic heart disease, increased risk of cancer in organs with mucous membranes, increased risk for kidney stones due to increased calcium excretion, reproductive disorders, impaired sense of smell, touch and appetite, fatigue and growth disorders such as disorders of bone growth in children.

Causes of hypovitaminosis

• Inflammation, surgery, but also stress
• Smoking and routine inhalation poor air
• Environmental toxins such as cadmium
• Strong sunlight (eg on the beach or in the snow ), especially in fair-skinned people
• Disorders of fat absorption, mostly due to problems with the liver, gallbladder or pancreas
• One in five Europeans will not record enough retinol from the diet. Especially in children quickly develops a deficiency because they have lower storage options, but have a high demand.
• Alcohol impairs the uptake, storage and mobilization
• Diabetics and people with hyperthyroidism can only poorly convert the plant carotenoids to vitamin A.
• Cholesterol-lowering agents and laxatives worsen the recording
• Certain sleeping pills consume memory stores in the liver

The insufficient supply of vitamin A ( vitamin A deficiency or VAD) is a widespread problem in developing countries. Approximately 250 million preschool children suffer from VAD and every year about one million children die from it. Between 250,000 and 500,000 children go blind also by VAD and half died the following year. Also vitamin A deficiency leads to a greatly increased rate of complications in infectious diseases such as measles.

Combat

To combat vitamin A deficiency as it occurs mainly as poverty phenomenon in developing countries, there are several strategies:

• Distributing vitamin supplements: This typically every 6 months Retinoltabletten be administered. Retinol is stored in the liver and released from there over a period of four to six months. This strategy is more cost effective, but there may be difficulties in reaching large proportions of needy children.
• Fortification of food: this food to be enriched during the production or packaging of micronutrients. In Latin America, for example, the accumulation of sugars could contribute significantly to combat VAD. However, the enrichment is only an option if the need to consume industrially processed products. Africa, this is, for example, is often not the case.
• Diversification of the diet: actions such as awareness among those concerned to help that more vitamin A-rich foods are consumed, such as from your own garden. The disadvantage is that the availability of vitamin A- rich foods often varies seasonally strong.
• Biofortification: Here is increased using plant breeding of micronutrient content of crops. Thus, the content of provitamin A, zinc or iron in staple foods such as cassava, maize, rice and sweet potato was increased, using genetic engineering (Golden rice). It is estimated that the cost-effectiveness of biofortification in developing countries is relatively high.

Hypervitaminosis

The oversupply of carotenes not in humans leads to an over- supply of vitamin A ( hypervitaminosis A), because the body down regulates the conversion of carotene to vitamin A accordingly. An excess of carotenes makes itself visually noticeable yellowing of the skin ( Carotinämie, " Karottenikterus "), but is not in need of treatment because no hypervitaminosis present.

A (longer ) oversupply of about 15 mg ( = 50,000 IU) of vitamin A can, in contrast, cause vomiting, diarrhea, headache, increased intracranial pressure ( pseudotumor cerebri ), decrease the Knochengewebsdichte (osteoporosis), enlargement of liver and spleen, reduction in thyroid activity lead and painful growths of the periosteum. In general, higher single doses can be considered safe, while repeatedly rather take higher doses the risk of intoxication in itself. For daily doses of more than 7.5 mg ( = 25,000 IU ) a number of hepatotoxic effects after several years of supply have been described, some of them fatal.

A single study showed the teratogenic properties in receiving 30,000 IU of vitamin A, could not be confirmed. The security of a dose of 10,000 IU is shown more than once. The recommendation of a daily dose of 2,500 IU ( 0.75 mg) seems justified.

Diagnostics

The serum level for the diagnosis of hypervitaminosis is inappropriate. A sure indicator, however, is the relation of vitamin A to RBP (retinol - binding protein). If the serum binding capacity by RBPs, then there is free vitamin A, which unfolds toxic effect.