31 May 2014

Sources of vitamin D

Vitamin D is a fat soluble essential  vitamin (sunshine vitamin). Activeform of vitamin D is vitamin D3. It is also called calcitriol, whose main function is promotion of calcium absorption. Liver and kidney plays important role in formation of D3.

What are the Sources of vitamin D
Sunlight and vitamin D
Skin exposure to sunlight is the main source of vitamin D. Active form of vitamin D  that is vitamin D3 is formed in the skin by the action of ultraviolet B exposure (UV-B wavelength 290–315 nm) on 7-dehydrocholesterol, which is a cholesterol metabolite present in skin. The intensity of UV light decreases as we move from equator, so at a latitude more than  50° (including northern Europe ), in winter seasons vitamin D is not synthesized, and there is seasonal variation even above 30° .These ultra violet B rays (wavelength) are present only if the ultra violet index is more than 3. Number of days with ultra violet index more than 3 decreases as we move from equator. 

Apart from that, during winter, people living at high altitude wear thick clothes, spend nearly all the day indoors and pollution of sky with smoke all decreases the ultra violet exposure. So the dietary intake is crucial during these season. They have to consume the body store  accumulated during summer. People living in tropics can get adequate amount of vitamin D, if they expose their unprotected skin to sunlight 15 minutes session twice a week. Though direct sunlight is a great source of vitamin D people should not overdo it . In United states vitamin D deficiency  is considered to be  an epidemic.

Factors that decrease synthesis of vitamin D by skin
  • Living away from equator
  • Cloud cover
  • Smog
  • Sunscreens
  • Wearing clothes 
  • Living indoor
  • People with dark skin pigmentation
  • Image of sun exposure
If the body cannot synthesis adequate amount of vitamin D  due to insufficient sunlight you need dietary supplementation. Daily requirement is about 1000 units in the absence of sun exposure. Deficiency risk increased because of change in life style  such as mainly indoor jobs and change in dressing style.Many countries tried to overcome this by fortifying food with vitamin D, but even that is not enough to meet the complete demand of body. Except for  fish and fortified foods availability of vitamin D from other food is  limited.

Food sources of vitamin D are the following

  • Fish liver oils.
  • Fatty fish (sardines, herring, mackerel, tuna, salmon, pilchards).
  • Margarine (fortified).
  • Infant milk formulas (fortified).
  • Eggs.
  • Liver.
  • Salmon (447 IUs per serving), tuna (154 IUs per serving) mackerel (388 IUs per serving), and  fish liver oils (one tablespoon -1,360 IUs)  are good source of vitamin D .

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