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Vitamin A to Fight Cancer

Posted by Ir. Adhi Hartono on 14 February 2010

My wife died in 2003, at the age of 28, due to colon cancer. I also have seen several of my family and friends died due to many kind of cancer. So,  it is my mission to learn more about cancer and to share with you all the beneficial and updated information regarding cancer. Below are one part of the information.

Vitamin A

Vitamin A is a generic term for a large number of related compounds. Retinol (an alcohol) and retinal (an aldehyde) are often referred to as preformed vitamin A. Retinal can be converted by the body to retinoic acid, the form of vitamin A known to affect gene transcription. Retinol, retinal, retinoic acid, and related compounds are known as retinoids. Beta-carotene and other carotenoids  that can be converted by the body into retinol are referred to as provitamin A carotenoids. Hundreds of different carotenoids are synthesized by plants, but only about 10% of them are provitamin A carotenoids. The following discussion will focus mainly on preformed vitamin A and retinoic acid.

Function of Vitamin A

Vision

The retina  is located at the back of the eye. When light passes through the lens, it is sensed by the retina and converted to a nerve impulse for interpretation by the brain. Retinol is transported to the retina via the circulation and accumulates in retinal pigment epithelial cells. Here, retinol is esterified to form a retinyl ester, which can be stored. When needed, retinyl esters are broken apart (hydrolyzed) and isomerized  to form 11-cis-retinol, which can be oxidized  to form 11-cis-retinal. 11-cis-retinal can be shuttled across the interphotoreceptor matrix to the rod cell where it binds to a protein called opsin to form the visual pigment, rhodopsin (also known as visual purple). Rod cells with rhodopsin can detect very small amounts of light, making them important for night vision. Absorption of a photon of light catalyze the isomerization of 11-cis-retinal to all-trans-retinal and results in its release. This isomerization triggers a cascade of events, leading to the generation of an electrical signal to the optic nerve. The nerve impulse generated by the optic nerve is conveyed to the brain where it can be interpreted as vision. Once released, all-trans retinal is converted to all-trans-retinol, which can be transported across the interphotoreceptor matrix to the retinal epithelial cell, thereby completing the visual cycle. Inadequate retinol available to the retina results in impaired dark adaptation, known as “night blindness.”

Immunity

Vitamin A is commonly known as the anti-infective vitamin, because it is required for normal functioning of the immune system . The skin and mucosal cells (cells that line the airways, digestive tract, and urinary tract) function as a barrier and form the body’s first line of defense against infection. Retinol and its metabolites are required to maintain the integrity and function of these cells. Vitamin A and retinoic acid (RA) play a central role in the development and differentiation  of white blood cells, such as lymphocytes, which play critical roles in the immune response. Activation of T-lymphocytes, the major regulatory cells of the immune system, appears to require all-trans-RA binding of RAR.

Growth and development

Both vitamin A excess and deficiency are known to cause birth defects. Retinol and retinoic acid (RA) are essential for embryonic development. During fetal development, RA functions in limb development and formation of the heart, eyes, and ears. Additionally, RA has been found to regulate expression  of the gene for growth hormone.

Red blood cell production

Red blood cells, like all blood cells, are derived from precursor cells called stem cells. Stem cells are dependent on retinoids for normal differentiation into red blood cells. Additionally, vitamin A appears to facilitate the mobilization of iron from storage sites to the developing red blood cell for incorporation into hemoglobin, the oxygen carrier in red blood cells.

Disease Prevention

Cancer

Studies in cell culture and animal models have documented the capacity for natural and synthetic retinoids to reduce carcinogenesis significantly in skin, breast, liver, colon, prostate, and other sites. However, the results of human studies examining the relationship between the consumption of preformed vitamin A and cancer are less clear.

Lung cancer

At least ten prospective studies have compared blood retinol levels at baseline among people who subsequently developed lung cancer and those who did not. Only one of those studies found a statistically significant inverse association between serum retinol and lung cancer risk. The results of the Beta-Carotene And Retinol Efficacy Trial (CARET) suggest that high-dose supplementation of vitamin A and beta-carotene should be avoided in people at high risk of lung cancer. About 9,000 people (smokers and people with asbestos exposure) were assigned a daily regimen of 25,000 IU of retinol and 30 milligrams of beta-carotene, while a similar number of people were assigned a placebo. After four years of follow-up, the incidence of lung cancer was 28% higher in the supplemented group compared to the placebo group. A possible explanation for such a finding is that the oxidative environment of the lung, created by smoke or asbestos exposure, gives rise to unusual carotenoid cleavage products, which are involved in carcinogenesis. Presently, it seems unlikely that increased retinol intake decreases the risk of lung cancer, although the effects of retinol may be different for nonsmokers than for smokers.

Breast cancer

Retinol and its metabplites have been found to reduce the growth of breast cancer cells in vitro, but observational studies of dietary retinol intake in humans have not confirmed this in vivo. The majority of epidemiological studies have failed to find significant associations between retinol intake and breast cancer risk in women, although one large prospective study found that total vitamin A intake was inversely associated with the risk of breast cancer in premenopausal women with a family history of breast cancer. Blood levels of retinol reflect the intake of both preformed vitamin A and provitamin A carotenoids like beta-carotene. Although a case-control study found serum retinol levels and serum antioxidant levels to be inversely related to the risk of breast cancer, two prospective studies did not observe significant associations between blood retinol levels and subsequent risk of developing breast cancer. Presently, there is little evidence in humans that increased intake of preformed vitamin A or retinol reduces breast cancer risk.

Food sources

Free retinol is not generally found in foods. Retinyl palmitate, a precursor and storage form of retinol, is found in foods from animals. Plants contain carotenoids, some of which are precursors for vitamin A (e.g., alpha-carotene, beta-carotene, and beta-cryptoxanthin). Yellow and orange vegetables contain significant quantities of carotenoids. Green vegetables also contain carotenoids, though the pigment is masked by the green pigment of chlorophyll. A number of good food sources of vitamin A are listed in the table below along with their vitamin A content in micrograms of retinol activity equivalents (mcg RAE). In those foods where retinol activity comes mainly from provitamin A carotenoids, the carotenoid content and the retinol activity equivalents are presented. You may use the USDA food composition database to check foods for their content of several different carotenoids, including lycopene, lutein, and zeaxanthin. The vitamin A IU listings in the USDA database, however, do not take into account bioavailability of the various carotenoids. To obtain a more accurate estimate of the number of IUs of vitamin A in carotenoid-containing foods, multiply the RAE by 3.33.

Food Serving Vitamin A, RAE

Vitamin A, IU Retinol, mcg Retinol, IU
Cod liver oil 1 teaspoon 1,350 mcg 4,500 IU 1,350 mcg 4,500 IU
Fortified breakfast cereals 1 serving 150-230 mcg 500-767 IU 150-230 mcg 500-767 IU
Egg 1 large 91 mcg 303 IU 89 mcg 296 IU
Butter 1 tablespoon 97 mcg 323 IU 95 mcg 317 IU
Whole milk 1 cup (8 fl oz.) 68 mcg 227 IU 68 mcg 227 IU
2% fat milk (vitamin A added) 1 cup (8 fl oz) 134 mcg 447 IU 134 mcg 447 IU
Nonfat milk (vitamin A added) 1 cup (8 fl oz.) 149 mcg 497 IU 149 mcg 497 IU
Sweet potato, canned 1/2 cup, mashed 555 mcg 1,848 IU 0 0
Sweet potato, baked 1/2 cup 961 mcg 3,203 IU 0 0
Pumpkin, canned 1/2 cup 953 mcg 3,177 IU 0 0
Carrot (raw) 1/2 cup, chopped 538 mcg 1,793 IU 0 0
Cantaloupe 1/2 medium melon 467 mcg 1,555 IU 0 0
Mango 1 fruit 79 mcg 263 IU 0 0
Spinach 1/2 cup, cooked 472 mcg 1,572 IU 0 0
Broccoli 1/2 cup, cooked 60 mcg 200 IU 0 0
Kale 1/2 cup, cooked 443 mcg 1,475 IU 0 0
Collards 1/2 cup, cooked 386 mcg 1,285 IU 0 0
Squash, butternut 1/2 cup, cooked 572 mcg 1,907 IU 0 0

Supplements

The principal forms of preformed vitamin A (retinol) in supplements are retinyl palmitate and retinyl acetate. Beta-carotene is also a common source of vitamin A in supplements, and many supplements provide a combination of retinol and beta-carotene. If a percentage of the total vitamin A content of a supplement comes from beta-carotene, this information is included in the Supplement Facts label under vitamin A. Most multivitamin supplements available in the U.S. provide 1,500 mcg (5,000 IU) of vitamin A, which is substantially more than the current RDA for vitamin A. This is due to the fact that the Daily Values (DV) used by the FDA for supplement labeling are based on the RDA established in 1968 rather than the most recent RDA, and multivitamin supplements typically provide 100% of the DV for most nutrients. Because retinol intakes of 5,000 IU/day may be associated with an increased risk of osteoporosisis in older adults, some companies have reduced the retinol content in their multivitamin supplements to 750 mcg (2,500 IU).

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7 Responses to “Vitamin A to Fight Cancer”

  1. LCD TV said

    Hi, I cant understand how to add your site in my rss reader. Can you Help me, please 🙂

  2. Adhi said

    Hello LCDTv,

    Sorry I’m late to reply. Thanks for your visit. For RSS reader, go to the top side of the right sidebar and click the RSS-Posts link.

    Good luck!

  3. Could anyone provide me personally with any extra resources on this area?

  4. Hi there, I found your blog via Google while searching for first aid for a heart attack and your post looks very interesting for me.

  5. Adhi said

    Hello folks,

    Thanks a lot for your visit and comments.

  6. thanks !! very helpful post!

  7. You you could change the blog subject title Vitamin A to Fight Cancer PILIHSEHAT.wordpress.com to more specific for your webpage you write. I liked the the writing even sononetheless.

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