THE IMPORTANCE OF VITAMIN D FOR MUSCULOSKELETAL HEALTH
What is vitamin D? Vitamin D is a fat-soluble vitamin that is naturally found in a few foods and also produced by the body when ultraviolet (UV) rays from sunlight start the formation of vitamin D in the skin. [39] In foods and dietary supplements, vitamin D has two main forms, D2 (ergocalciferol) and D3 (cholecalciferol) , that differ only in their chemical structures. However, the majority of evidence indicates that vitamin D3 raises blood levels to a greater extent and maintains these higher levels longer than vitamin D2, even though both forms are well absorbed in the gut. [40] Most people are also able to get some of their vitamin D needs through exposure to sunlight. Type B UV (UVB) radiation in sunlight penetrates uncovered skin and converts 7-dehydrocholesterol in the skin to previtamin D3, which then becomes vitamin D3. UVB radiation does not penetrate glass, however, so exposure to sunshine indoors through a window does not produce vitamin D. Older people and people with dark skin are less able to produce vitamin D from sunlight. [41] What are the functions of vitamin D? Vitamin D supports calcium absorption in the gut and maintains adequate blood levels of calcium and phosphate to allow the normal bone development process of deposition of minerals on the bone matrix. It is also needed for bone growth and bone remodeling, following injury. In addition, Vitamin D is involved in the reduction of inflammation as well as regulation of neuromuscular and immune function, and glucose metabolism. [41] Finally, Vitamin D is needed for the normal development and growth of muscle fibers and low vitamin D levels can negatively affect muscle strength and lead to muscle weakness, fatigue, and pain. [41, 42]
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