Answer to Question 1
The vitamin E family consists of two subgroupsthe tocopherols and the tocotrienolseach containing four members designated by letters of the Greek alphabet (alpha, beta, gamma, and delta). All consist of a complex ring structure with a long saturated (in tocopherols) or unsaturated (in tocotrienols) side chain. The positions of methyl groups (CH3) on the side chain and their chemical rotations distinguish the four members within each subgroup.
Of all the members of the vitamin E family, only alpha-tocopherol is maintained in the body and can meet the body's needs for the vitamin. The others are not converted to alpha-tocopherol in the body, nor are they recognized by its transport protein. For these reasons, the RDA is based only on alpha-tocopherol.
Most vitamin E research has focused on alpha-tocopherol, but recent studies suggest that the other tocopherols and tocotrienols might also be beneficial. For example, gamma-tocopherol and possibly delta-tocopherol, appear to be most effective in inhibiting inflammation and cancer growth. In addition to preventing cancer, tocotrienols may also protect against osteoporosis, diabetes, heart disease, and neurological disorders.
Answer to Question 2
Only a few foods contain vitamin D naturally. Fortunately, the body can make vitamin D with the help of a little sunshine. In setting dietary recommendations, however, the DRI Committee assumed that no vitamin D was available from skin synthesis. In order to reach sufficient levels of vitamin D in the blood without contributions from the sun, dietary recommendations were recently increased.
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