اعداد *** د. رامي مشتهى صيدلي مخازن الادوية الرعاية الاولية

الأربعاء, 16 تشرين2/نوفمبر 2011 00:00

Vitamin A

Vitamin A can be found in two principal forms in foods:

Retinol, the form of vitamin A absorbed when eating animal food sources, the vitamin is found in tissues in a form of retinyl ester. It is also commercially produced and administered as esters such as retinyl acetate or palmitate.
Provitamin A carotenoids from plant sources as yellow/orange fruits and vegetables principally carrots

The absorption of provitamins depends greatly on the amount of lipids ingested with the provitamin; lipids increase the uptake of the provitamin.

90% of retinoids can be absorbed but only 30% of carotenoids are absorbed.

Intestinal cells can convert carotenoids to retinoids

Beta carotene is safe at any dose

Sources

Vitamin A is found naturally in many foods:

liver (beef, pork, chicken, turkey, fish) (6500 μg 722%)
carrot (835 μg 93%)
broccoli leaf (800 μg 89%)
sweet potato (709 μg 79%)
butter (684 μg 76%)
kale (681 μg 76%)
spinach (469 μg 52%)
pumpkin (400 μg 41%)
collard greens (333 μg 37%)
Cheddar cheese (265 μg 29%)
cantaloupe melon (169 μg 19%)
egg (140 μg 16%)
apricot (96 μg 11%)
papaya (55 μg 6%)
mango (38 μg 4%)
pea (38 μg 4%)
broccoli (31 μg 3%)
milk (28 μg 3%)

Note: data taken from USDA database bracketed values are retinol activity equivalences (RAEs) and percentage of the adult male RDA, per 100 grams of the foodstuff (average).

Conversion of carotene to retinol varies from person to person and bioavailability of carotene in food varies.

Recommended daily intake

Vitamin A

Dietary Reference Intake

Life Stage Group	Recommended Dietary Allowances (RDA)Adequate Intakes (AI*) μg/day	Upper Limitμg/day
Infants 0–6 months 7–12 months	400* 500*	600 600
Children 1–3 years 4–8 years	300 400	600 900
Males 9–13 years 14–18 years 19 - >70 years	600 900 900	1700 2800 3000
Females 9–13 years 14–18 years 19 - >70 years	600 700 700	1700 2800 3000
Pregnancy <19 years 19 - >50 years	750 770	2800 3000
Lactation <19 years 19 - >50 years	1200 1300	2800 3000

10000IU or more during pregnancy lead to birth malformation

ADVICE to pregnant women for limit intake of liver and avoid cod-liver oil capsules

Metabolic functions

Vitamin A plays a role in a variety of functions throughout the body, such as:

Vision
Gene transcription
Immune function
Embryonic development and reproduction
Bone metabolism
Haematopoiesis
Skin health
Antioxidant activity
Retinol participates in sperm development

Vitamin A deficiency

Night blindness
Blindness( xerophthalmia)
Slow bone growth
Impaired immune function
Birth defects
Keratinization( less goblet cells, some cells secrete keratin)
Rough, scaly skin

Toxicity

nausea, jaundice, irritability, anorexia (not to be confused with anorexia nervosa, the eating disorder), vomiting, blurry vision, headaches, hair loss, muscle and abdominal pain and weakness, drowsiness and altered mental status

Acute toxicity generally occurs at doses of 25,000 IU/kg of body weight, with chronic toxicity occurring at 4,000 IU/kg of body weight daily for 6–15 months. However, liver toxicities can occur at levels as low as 15,000 IU per day to 1.4 million IU per day, with an average daily toxic dose of 120,000 IU per day. In people with renal failure 4000 IU can cause substantial damage. Additionally, excessive alcohol intake can increase toxicity. Children can reach toxic levels at 1,500 IU/kg of body weight.