The term vitamin A describes the compounds that exhibit the biological activity of retinol. Vitamin A is fat-soluble vitamin and two key ingredients in foods are retinol and carotenoids.
Dosage
In the absence of malabsorption or gastrointestinal disease, should not be exceeded in the following regular daily downloads (from food and supplements). The therapeutic dose should not exceed these limits, except when the patient is under medical supervision. For example, in cystic fibrosis, can be given doses of 1200 - 3300 g (4000 - 10000 Units).
Action
Vitamin A is essential for normal functioning of the retina, and particularly on the adaptation of vision in the dark, to maintain the structural and functional integrity of epithelial tissue and the immune system, cell differentiation and division, to develop bone for the operation of the genital organs, and fetal growth. Also vitamin A can act as a co factor in biochemical reactions.
Metabolism
Vitamin D is readily absorbed from the upper intestine (duodenum and nistida) through a body-moderator. For absorption requires the presence of gastric juice, bile, pancreatic and intestinal lipase, and protein and dietary fat.
The liver contains at least 90% of stored vitamin A in the body (the amount you need an adult for about two years). Small quantities are stored in the kidneys and lungs.
Vitamin A is excreted in bile and urine (as metabolites). Appears in breast milk. The absorption decreases significantly with consumption of food low in fat (<5g) and the presence over-oxidized grease and other oxidants in the food. The lack of protein, vitamin E, zinc and heavy alcohol consumption affects the transport, storage and utilization of vitamin A.
The vitamin A deficiency is prevalent in children of developing countries and generally associated with malnutrition. Symptoms of deficiency include:
Weakness of vision in the dark.
Dry eyes.
Dry skin and in the presence of discontinuity in the skin layers.
Metaplasia and cornification of the cells of the respiratory tract and other organs.
Increased susceptibility to respiratory infections and urinary tract.
Occasionally diarrhea and loss of appetite.
Uses
Most children aged from six months to five years is recommended to take supplements of vitamins A and D, unless it can ensure its adequacy from their diet.
There is no evidence of the value of vitamin A in eye problems and prevent and treat infections not related to vitamin A. Vitamin A supplements to a normal, safe doses have no proven benefits to skin problems (eg acne), but synthetic retinoids may be prescribed for this purpose.
In the literature have occurred during periods of independent studies suggest that vitamin D may be beneficial in premenstrual syndrome, but these results have not been confirmed at random.
There is little evidence for a beneficial effect of supplementation of vitamin D beyond the cases of deficiency. The available data on the role of vitamin D in cancer are more related than carotenoids to vitamin A in general.
Precautions
Pregnancy and breastfeeding
Excessive doses of vitamin D appear to be teratogenic, although the levels at which this happens are not well defined.
Side effects
With a single dose of 300 mg retinol in adults, 60 mg retinol in children, or 30 mg retinol in infants can cause acute toxicity. The signs and symptoms are usually temporary (often occurring approximately 6 hours after ingestion of an acute dose and subside after 36 hours) and include: severe headache, stomatalgia, bleeding gums, dizziness, confusion, vomiting, blurred vision, hepatomegaly, irritability.
When the daily intake is> 15 mg retinol in adults and 6 mg in infants and young children may be signs of chronic toxicity.
Symptoms may include: dry skin, dermatitis, exfoliation, skin rash, hives the blade skin disorders, hair growth, pain in bones and joints, headache, fatigue, irritability, insomnia, anorexia, nausea, vomiting, diarrhea, loss weight, hepatomegaly, hepatotoxicity, increased intracranial pressure, hypercalcemia (due to the increased functionality of alkaline phosphatase).
The emerging signs and symptoms vary widely from person to person. Most disappear within one week after the reduction of intake, but changes in the skin and bones remain visible for several months.
Vitamin D interacts with the metabolism of other nutrients:
Iron: In vitamin A deficiency reduced iron levels in plasma.
Vitamin C: In conditions of hypervitaminosis A, vitamin C levels in the tissues can be reduced while increasing the excretion of vitamin C in urine. Vitamin C may reduce the toxic effects of vitamin A.
Vitamin C: Large doses of vitamin D increase the need for Vitamin E - Vitamin E reduces the oxidative destruction of vitamin A.
Vitamin K: In conditions of hypervitaminosis A can occur ypothromvinaimia, which is corrected by administration of vitamin K.
No comments:
Post a Comment