The lack of wetting and spreading of water and simple saline solutions
Dietary Sources of Vitamin A
animal source
Preformed vitamin A (mainly retinyl palmitate and small amounts of other retinyl esters including stearate, myristate, and oleate) and very small amounts of provitamin A procarotenoids are obtained from animal sources of. Particularly rich sources of preformed vitamin A include liver, whole milk, egg yolks and fish.
plant source
Dietary vitamin A is also available from plants as provitamin A carotenoids. Among the carotenoids, β-carotene is the most abundant. It is absorbed more efficiently and converted to retinol more easily than other carotenoids. Good sources of provitamin carotenoids include carrots, spinach and other dark fruits and vegetables. These foods also contain carotenoids (such as lutein and lycopene) that cannot be converted into vitamin A but are also thought to have health benefits.
Vitamin A Activity of Food
Retinol activity equivalents (RAE) are corrected for the bioavailability of provitamin A carotenoids and are used to compare the vitamin A activity of foods. One RAE is defined as 1 microgram of retinol, 2 micrograms of beta-carotene in oil (ie as a supplement), 12 micrograms of beta-carotene in food, or 24 micrograms of other provitamin A carotenoids in food. One International Unit (IU) is equivalent to 0.3 micrograms of retinol. The recommended daily dietary intake (RDI) for vitamin A is 900 RAE (3000 IU) and 700 (2333 IU) RAE for adult males and nonlactating females, respectively. Carotenoids do not have individual RDIs. The 5000 IU Daily Value (DV) on US food labels is based on earlier recommendations. The DV percentages on food labels provide a percentage of the recommended DV based on a 2,000-calorie diet.
The lack of wetting and spreading of water and simple saline solutions on non-polar surfaces can lead to bubble problems when casting hydrophobic impression materials (such as silicone rubber, 7 § 6) in gypsum slurries or in embedding wax patterns. In the latter case, a surfactant solution (often called a defoamer) is usually applied to the surface of the stamp or pattern and then dried to leave a film wetted by the slurry (Figure 8.5). That is, the wax or impression material itself is no longer "seen" by the stearate water.
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