Retinol
The Dietary Reference Intake (DRI) Recommended Daily Amount (RDA) for vitamin A for a 25-year-old male is 900 micrograms/day, or 3000 IU. NHS daily recommended values are slightly lower at 700 micrograms for men and 600 micrograms for women.
Estimates have changed over time of the rate at which β-carotene is converted to vitamin A in the human body. An early estimate of 6:1 was revised to 12:1 and from recent studies and experimental trials carried out in developing nations it was revised again to 21:1. The implication of the reduced estimate is that larger quantities of β-carotene are needed to yield the necessary dietary requirement of vitamin A. This means that more continents are affected by the deficiency of vitamin A than was previously thought. Changing dietary choices in Africa, Asia, and South America will not be sufficient, and agricultural practices on those continents will need to change.
The Food Standards Agency states that an average adult should not consume more than 1500 micrograms (5000 IU) per day, because this increases the chance of osteoporosis.
During the absorption process in the intestines, retinol is incorporated into chylomicrons as the ester form, and it is these particles that mediate transport to the liver. Liver cells (hepatocytes) store vitamin A as the ester, and when retinol is needed in other tissues, it is de-esterifed and released into the blood as the alcohol. Retinol then attaches to a serum carrier, retinol binding protein, for transport to target tissues. A binding protein inside cells, cellular retinoic acid binding protein, serves to store and move retinoic acid intracellularly. Carotenoid bioavailability is 1⁄10 to 1⁄5 that of retinol. Carotenoids are better absorbed when ingested as part of a fatty meal. Also, the carotenoids in vegetables, especially those with tough cell walls (e.g. carrots), are better absorbed when these cell walls are broken up by cooking or mincing. Read more about Retinol on Wikipedia:
Retinol.
Beta-Carotene
Since 2001, the US Institute of Medicine uses retinol activity equivalents (RAE) for their Dietary Reference Intakes, defined as follows:
Retinol activity equivalents (RAEs)
- 1 µg RE = 1 µg retinol
- 1 µg RAE = 2 µg all-trans-β-carotene from supplements
- 1 µg RAE = 12 µg of all-trans-β-carotene from food
- 1 µg RAE = 24 µg α-carotene or β-cryptoxanthin from food
RAE takes into account carotenoids' variable absorption and conversion to vitamin A by humans better than and replaces the older retinol equivalent (RE) (1 µg RE = 1 µg retinol, 6 µg β-carotene, or 12 µg α-carotene or β-cryptoxanthin). RE was developed 1967 by the United Nations/World Health Organization Food and Agriculture Organization (FAO/WHO).
Another older unit of vitamin A activity is the international unit (IU). Like retinol equivalent, the international unit does not take into account carotenoids' variable absorption and conversion to vitamin A by humans, as well as the more modern retinol activity equivalent. Unfortunately, food and supplement labels still generally use IU, but IU can be converted to the more useful retinol activity equivalent as follows:
International Units
- 1 µg RAE = 3.33 IU retinol
- 1 IU retinol = 0.3 μg RAE
- 1 IU β-carotene from supplements = 0.15 μg RAE
- 1 IU β-carotene from food = 0.05 μg RAE
- 1 IU α-carotene or β-cryptoxanthin from food = 0.025 μg RAE1
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Beta-CaroteneVitamin A Palmitate
Retinyl acetate is a synthetic alternate for retinyl palmitate in vitamin A supplements, and is available in oily or dry forms. It is a common vitamin supplement, available in both oral and injectable forms for treatment of vitamin A deficiency, under the brand names Aquasol A, Palmitate A and many others. It is a constituent of intra ocular treatment for dry eyes at a concentration of 138 μg/g (VitA-Pos) by Ursapharm. It is a pre-formed version of vitamin A; therefore, the intake should not exceed the Recommended Dietary Allowance (RDA). Overdosing preformed Vitamin A forms such as retinyl palmitate leads to adverse physiological reactions (hypervitaminosis A).
Retinyl palmitate is used as an antioxidant and a source of vitamin A added to low fat milk and other dairy products to replace the vitamin content lost through the removal of milk fat. Palmitate is attached to the alcohol form of vitamin A, retinol, in order to make vitamin A stable in milk.
Retinyl palmitate is also a constituent of some topically applied skin care products. After its absorption into the skin, retinyl palmitate is converted to retinol, and ultimately to retinoic acid (the active form of vitamin A present in Retin-A), though neither its skin absorption nor its conversion is very effective. Read more about Wikipedia on Wikipedia:
Retinyl_palmitate
