Answer to Question 1
Sunlight supplies the needed vitamin D for most of the world's people. When ultraviolet (UV) light rays from the sun reach a cholesterol compound in human skin, the compound is transformed into a vitamin D precursor and is absorbed directly into the blood. Slowly, over the next day and a half, the liver and kidneys finish converting the inactive precursor to the active form of vitamin D. Like natural sunscreen, the pigments of dark skin protect against UV radiation. To synthesize several days' worth of vitamin D, dark-skinned people require up to 3 hours of direct sun (depending on the climate). Therefore, dark-skinned people and those who lack sufficient exposure to sunlight often lack vitamin D.
Answer to Question 2
Beta-carotene is an orange pigment with antioxidant activity. It is a vitamin A precursor made by plants and stored in human fat tissue. Retinol is one of the active forms of vitamin A made from beta-carotene in animal and human bodies. It is an antioxidant nutrient. Foods derived from animals provide the active form of vitamin A, which is readily absorbed and put to use by the body. Foods derived from plants provide beta-carotene, which must be converted to active vitamin A before it can be used as such. The vitamin A precursor, beta-carotene, is naturally present in many vegetable and fruit varieties. For vitamin A precursors, activity is measured in retinol activity equivalents (RAE). It takes about 12 micrograms of beta-carotene from food to supply the equivalent of 1 microgram of retinol to the body.
The seasons in the Northern and Southern Hemispheres.
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