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Vitamin A Knowledge graphic © naturalhealthzone.org.
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Vitamin A is a fat-soluble vitamin made up of closely related unsaturated hydrocarbons, including retinol, retinal, retinoic acid, and several provitamin A carotenoids.  Vitamin A and its derivates, either natural or synthetic, are collectively and generally referred to as retinoids. 
This vitamin carries the distinction of having being used therapeutically approximately 3000 years ago, in ancient Egypt to treat endemic night blindness,  a condition wherein a person finds it difficult to see in dim light.
The Father of Medicine himself, Hippocrates (460–325 BC) of Cos, Greece, suggested a diet of raw liver (liver cells store around 80% of the total body vitamin A in lipid droplets  ) to cure night blindness.  To date, a myriad of studies have furnished sufficient research data that scientifically support vitamin A’s role in embryogenesis, reproduction, vision, and regulation of inflammation, and growth and differentiation of cells. 
Vitamin A is involved in the cyclic visual process. Borrowing the words of Wald (1935), “Vitamin A is the precursor of visual purple (rhodopsin, or the biological pigment in photoreceptor cells of the retina) as well as the product of its decomposition; the visual processes therefore constitute a cycle.”  Moreover, the retinal form of vitamin A plays a role in transducting light into neural signals essential for vision.  Vitamin A’s dermatological function and importance has also long been investigated, starting perhaps with Wolbach and Howe’s 1925 study on epidermal changes as abnormal keratinization following vitamin A deprivation in albino white rat models. 
In a randomized, double-blind, vehicle-controlled study, vitamin A topically administered was reported to improve the fine wrinkles associated with natural aging among the elderly subjects and to significantly increase the expression of glycosaminoglycan (a well-known and important “water retainer”) and the production of collagen.  Vitamin A, anti-infective in nature, also contributes to the integrity and normal functioning of diverse immune defenses. Because vitamin A protects epithelial barriers such as the skin and the mucosal cells of the airways, digestive tract, and urinary tract from any adverse alterations and regulates immune function, the body’s first line of defense against invading pathogens – and in general infection – is kept intact and functioning optimally and ideally. 
Vitamin A exists in two different forms in the diet: preformed vitamin A and provitamin A, the former being found in animal products such as meat, fish, poultry, and dairy foods, whereas the latter in plant-based foods such as fruits and vegetables.  Preformed vitamin A includes retinol (an alcohol) and retinal (an aldehyde that can be converted by the body to retinoic acid, the isomers of which are known to act as hormones that influence gene expression and thereby numerous physiological processes). Provitamin A on the other hand refers to β-carotene and other carotenoids. 
Although animal products (e.g., eggs, meat, fortified milk, cheese, cream, liver, kidney, cod, and halibut fish oil) are good sources of vitamin A, it comes unfortunately with a price no diabetic or cardiovascular patient would appreciate – these sources, excluding vitamin A–enriched skim milk, pack considerable amounts of saturated fat and cholesterol. Good β-carotene sources include bright yellow and orange fruits such as cantaloupe, pink grapefruit, and apricots and vegetables such as carrots, pumpkin, sweet potatoes, winter squash, and most dark-green leafy vegetables. 
List Of Foods Considered As Top Natural Sources Of Vitamin A:
(sorted by vitamin A content, as adapted from the data of the USDA National Nutrient Database for Standard Reference, Release 25)
1 cup of carrot juice (canned) – 45133 IU
1 cup of carrots (cooked, boiled, drained, without salt) – 26571 IU
1 cup of carrots (frozen, cooked, boiled, drained, without salt) – 24715 IU
1 cup of pumpkin (canned, without salt) – 38129 IU
1 cup of pumpkin (cooked, boiled, drained, without salt) – 14100 IU
One 146 g sweet potato (cooked, baked in skin, without salt) – 28058 IU
One 156 g sweet potato (cooked, boiled, without skin) – 24554 IU
1 cup of spinach (frozen, chopped or leaf, cooked, boiled, drained, without salt) – 22916 IU
1 cup of turkey (whole, giblets, cooked, simmered) – 22326 IU
85 g/3 oz beef (variety meats and by-products, liver, cooked, pan-fried) – 22175 IU
1 cup of collards (frozen, chopped, cooked, boiled, drained, without salt) – 19538 IU
1 cup of kale (frozen, cooked, boiled, drained, without salt) – 19115 IU
1 cup of turnip greens (frozen, cooked, boiled, drained, without salt) – 17655 IU
1 cup of mustard greens (cooked, boiled, drained, without salt) – 17318 IU
1 cup of beet greens (cooked, boiled, drained, without salt) – 11022 IU
1 cup of winter squash (all varieties, cooked, baked, without salt) – 10707 IU
1 cup of chicken (broilers or fryers, giblets, cooked, simmered) – 8510 IU
2 slices of pork Braunschweiger (a liver sausage) – 7967 IU
1 cup of Chinese cabbage (cooked, boiled, drained, without salt) – 7223 IU
1 cup of dandelion greens (cooked, boiled, drained, without salt) – 7179 IU
1 cup of raw cantaloupe melons – 5411 IU
1 head of raw butterhead lettuce (includes Boston and Bibb types) – 5399 IU
1 cup of raw cos or romaine lettuce – 4878 IU
1 cup of raw sweet red peppers – 4665 IU
1 cup of raw green leaf lettuce – 4147 IU
1 cup of apricots (canned, juice pack, with skin, solids and liquids) – 4126 IU
1 cup of tomato products (canned, paste, without salt added) – 3996 IU; 1 cup of raw red tomatoes – 1499 IU
1 cup of peas (green, frozen, cooked, boiled, drained, without salt) – 3360 IU
1 raw papaya – 2888 IU
1 cup of broccoli (cooked, boiled, drained, without salt) – 2415 IU
One raw mango – 2240 IU
1 cup of tangerines or mandarin oranges (canned, light syrup pack) – 2117 IU
One raw medium-sized plantains – 2017 IU
One wedge of raw watermelon – 1627 IU 
Please note that the vitamin A IU listings given from the USDA database do not take into account the bioavailability of various carotenoids. For men, the Recommended Dietary Allowance (RDA) is 900 μg retinol activity equivalents (RAE)/day, while for women, 700 μg RAE/day. The Tolerable Upper Intake Level (UL) for adults is set at 3,000 μg/day of preformed vitamin A. 
 Vitamin A. Retrieved 5 April 2013 from http://en.wikipedia.org/wiki/Vitamin_A
 Roos T. C., Jugert F. K., Merk H. F., & Bickers D. R. (1998). Retinoid metabolism in the skin. Pharmacological Reviews, 50(2): 315–333. Retrieved 5 April 2013 from http://pharmrev.aspetjournals.org/content/50/2/315.full
 Mukherjee S., Date A., Patravale V., Korting H. C., Roeder A., & Weindl G. (2006). Retinoids in the treatment of skin aging: an overview of clinical efficacy and safety. Clinical Interventions in Aging, 1(4): 327–348. Retrieved 5 April 2013 from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2699641/
 Higashi N. et al. (2005). Vitamin A storage in hepatic stellate cells in the regenerating rat liver: with special reference to zonal heterogeneity. Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology, 286(2): 899–907. Retrieved 5 April 2013 from http://www.ncbi.nlm.nih.gov/pubmed/16086432
 Littré, E. (Eds.). (1861). Oeuvres complètes d’Hippocrate. Paris, France: J. B. Balliére.
 Wald G. (1935). Carotenoids and the visual cycle. Journal of General Physiology, 19(2): 351–371. Retrieved 5 April 2013 from http://jgp.rupress.org/content/19/2/351.abstract
 Saari J. C. (1994). Retinoids in photosensitive systems. In Sporn M. B., Roberts A. B., & Goodman D. S. (Eds.). The retinoids; biology, chemistry and medicine (2nd ed.). New York: Raven Press, 351–385.
 Wolbach S. B. & Howe P. R. (1925). Tissue changes following deprivation of fat-soluble a vitamin. Journal of Experimental Medicine, 42(6): 753–777. Retrieved 5 April 2013 from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2131078/
 Kafi R. et al. (2007). Improvement of naturally aged skin with vitamin A (retinol). Archives of Dermatology, 143(5): 606–612. Retrieved 5 April 2013 from http://www.ncbi.nlm.nih.gov/pubmed/17515510
 McCullough F. S., Northrop-Clewes C. A., & Thurnham D. I. (1999).The effect of vitamin A on epithelial integrity. Proceedings of the Nutrition Society, 58(2): 289–293. Retrieved 5 April 2013 from http://www.ncbi.nlm.nih.gov/pubmed/10466169?dopt=Abstract
 Vitamin A. MedlinePlus. Retrieved 5 April 2013 from http://www.nlm.nih.gov/medlineplus/ency/article/002400.htm
 Higdon J. (2003). Vitamin A. Linus Pauling Institute Micronutrient Information Center, Oregon State University. Retrieved 5 April 2013 from http://lpi.oregonstate.edu/infocenter/vitamins/vitaminA/index.html
 Vitamin A content of selected foods per common measure. USDA National Nutrient Database for Standard Reference, Release 25. Retrieved 5 April 2013 from https://www.ars.usda.gov/SP2UserFiles/Place/12354500/Data/SR25/nutrlist/sr25w318.pdf
 Anonymous. (2001). 4 Vitamin A. Dietary reference intakes for vitamin a, vitamin k, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc. Washington, DC: The National Academies Press. Retrieved 5 April 2013 from http://www.nap.edu/openbook.php?record_id=10026&page=82
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