Physician's Notebooks 2 - http://physiciansnotebook.blogspot.com - See Homepage
Chapter 11 Continue
2.11e: Vitamin A - Update 01 Dec.,
Chapter 11 Continue
2.11e: Vitamin A - Update 01 Dec.,
2021
Vitamin A is found in egg yolk and liver as retinol; in orange, carrot, tomato as beta-carotene; and in leaf and stem as carotenoid. The basic unit is 1 microgram retinal activity (micro-g RE). In microgram RE equivalencies, the ratio of retinol to beta-carotene to carotenoid is 1 to 6 to 12. The international unit, or IU is used. One IU is 0.300 micro-g RE.Vitamin A is unstable to heat and light. As pill, it is combined with chemical preservative and should be kept in tightly stoppered, dark brown bottle in cool, dry place, with expiration date on bottle. It also comes as a black pill in open plastic pack.Vitamin A is normally eaten attached to food protein, which is digested away in stomach, releasing the free vitamin or beta-carotene and absorbed in upper intestine. It is a fat-soluble vitamin (Others are D, E & K) that requires being eaten together with a little fat and mixing with liver bile and lipase digestive enzyme for good intestinal absorption. So someone who continued on strict fat-free diet could develop Vitamin A deficiency. Diseases of liver and a chronic diarrhea can lead to deficiency due to malabsorption.Once in blood, it is stored in liver, whose storage capacity allows one to go without it for at least a year.
Function: Vision, cellular differentiation in skin, and fetal development. In the eyes, it is needed for converting low light that strikes the retina rods into visual signal for brain. Early symptom of deficiency is bad vision at twilight, night and with sunglasses.
Cell differentiation is when stem cell matures to carry out its function and loses ability to divide. In vitamin A deficiency, the cells of respiratory tract cannot differentiate and it causes excessive drying of mouth, eye and throat. When differentiation is blocked, cancer is more likely. Vitamin A as mega dose 25,000 IU per day in trials lowered squamous cell skin cancer in high-risk groups. But, warning! Two large randomized trials showed current smokers on the mega dose beta-carotene (type of vitamin A) pills developed lung cancers at rate 25% greater than placebo control group and had higher mortality!
Effect of excess vitamin A on fetus: Single large dose, or exposure for 1 week to high daily dose (30-90 mg RE) may induce abortion. And if abortion does not occur, such dose (or daily >4.5 mg RE several month intake) may lead to malformed baby or damaged brain in newborn. In fetal development, retinoic acid, which is produced in the body from vitamin A, and also is used as a medication for skin disease like acne, is very important in directing early development of the brain and spinal cord in human fetuses. Too much of it in early pregnancy can cause neural malformations. So women who are or may be pregnant should be very careful to limit vitamin A to nutritional replacement dose and to rely on foods rather than pill or capsule for its source. I do not advise any pregnant to take vitamin A.Vitamin A strengthens immune response to infection, particularly the activity of the T-helper lymphocyte, important against HIV and other virus. But this is what everyone gets from normal eating and does not require a pill.
Toxicity from Excess Vitamin A: Single or short-term daily mega dose of 150 mg (150,000 microgram or 500,000 IU) of retinol (but not beta-carotene or carotenoid) causes increased pressure in brain (headache, dizziness, double vision with blurring of optic disc on ophthalmic exam; the condition called pseudotumor cerebri.) and, continued, can lead to brain damage and death. Habitual daily intake of >10 mg (10,000 micrograms or 33,300 IU) has caused hair loss, staggering gait, bone and muscle pains, irritating skin rash, fissures at edges of mouth, red-eye, headache like from brain tumor, toxic hepatitis and bad high LDL cholesterol.Excess eating of carrot or tangerine may cause yellowing of palm of hands and sole of feet due to high beta-carotene in blood, which stains the thick layer of skin, seen much in Tokyo among young women who love tangerines. It is not a serious side effect; merely something one may note looking at the hands. A worse similar toxicity can come from habitual ingestion of large amounts of canthaxanthin, a carotenoid that gives the yellow-orange pigment of the chanterelle mushroom. It is marketed as “nutritional product” under registered trade name Carophyll Red. Canthaxanthin is used by tanning parlors to improve tanning effect of UV light treatment. Habitual ingestion of large amounts of canthaxanthin is recently reported to induce retinopathy, which can lead to blindness.
Who is at Risk for Vitamin A Deficiency? Newborn, especially if premature because of low liver reserve; also person with intestinal malabsorption..Medicinal Use: Vitamin A is used in inflammatory skin disease, particularly acne. Increased dietary intake of carotenoid (carrot, tomato, mango, and green leaf) is protective against cancer. But do not use vitamin pills with idea it will prevent or slow a cancer.
Vitamin A Blood Testing: The blood test specimen should be drawn at earliest 8 hours after last eating. Testing should be not long after drawing blood because vitamin A is quickly destroyed in standing blood, resulting in false low value. Taking vitamin A pill within 72 hrs before blood sampling will cause high vitamin A test. A blood test vitamin A <0.35 micromole/L (<10 microgram/dL) indicates deficiency and 0.35 – 0.70 micromole/L (10 – 20 microgram/dL) sub-optimal for good health.
My Approach to Vitamin A: Healthy, adequate intake of natural vitamin A, beta-carotene, or carotenoid in food - yellow/orange fruits and vegetables - is important in preventing cancer, protecting against infection, and fostering healthy skin and mucous membrane. But with vitamin A, even a little too much can turn rotten. Vitamin A supplement should be used only under knowledgeable physician supervision
M
No comments:
Post a Comment