Factors that Affect Function deficiency of vitamin D

What factors might contribute to a deficiency of vitamin D?
Insufficient sun exposure
By far the most important D-deficiency contributing factor faced worldwide is insufficient exposure to sunlight. More specifically, it is ultraviolet B sunlight in the range of 290-300nm that is needed to convert 7-dehydrocholesterol found in our skin cells into cholecalciferol (the preliminary form of vitamin D3).

Although the task of �getting enough sunlight� may seem like a fairly straightforward one, the relationship between our vitamin D status and our time in the sun is not nearly as simple as many people might think. First, there is the fluctuating nature of UVB light. UVB light�the kind needed for skin synthesis of vitamin D�is not always present with the same intensity just because there is visible sunlight. The intensity of UVB light varies dramatically with geographical location (latitude), time of year, time of day, degree of cloud cover, and other factors. In other words, there are times when your eyes might leave you thinking that you are getting good intensity UVB light and synthesizing optimal amounts of vitamin D in your skin cells, but you actually are not. In addition, there is the issue of latitude, angle of sunlight, and time of day. In higher-latitude countries across the globe, UVB light in the range of 290-300nm wavelengths may not be available except for a few hours in the middle of the day. In those countries, being outside on a �bright and sunny day� would not be enough to guarantee adequate skin synthesis of vitamin D. Skin pigmentation also plays an important role in skin synthesis of vitamin D. Darker skin pigmentation means less vitamin D synthesis per minute exposure to UVB light. (In the United States, for example, it is estimated that African American adults are 2-3 times more likely to have vitamin D insufficiency than Caucasian adults.) As all of these examples indicate, even when common sense tells you that you are getting good sun exposure and must be synthesizing plenty of vitamin D, you might not be, for a variety of reasons.
Some lifestyles and occupations provide unusual amounts of sun exposure. Individuals who work outdoors throughout the day in warmer climates have a great chance of getting good exposure to UVB light and synthesizing adequate amounts of vitamin D. Under optimal circumstances, our skin can synthesize between 10,000-20,000 IU of vitamin D (cholecalciferol) in 30 minutes. Yes, all of the practical factors still have to line up correctly (like angle of the sun, time of day, degree of cloud cover, etc.), but when people spend generous amounts of times outdoors in the sun on a regular basis, their chances of adequate vitamin D synthesis are greatly increased. At the other end of the spectrum, if an individual typically wears clothing that blocks a lot of the skin from exposure to sunlight, or typically uses sunblock, or works in a job that keeps him or her indoors throughout the day, chances of adequate vitamin D synthesis are greatly decreased.
Based on all of the factors listed above, we believe that the majority of U.S. adults are unlikely to be getting enough exposure to UVB sunlight in the 290-300nm wavelength required to allow for optimal skin synthesis of vitamin D. In addition, we believe that many people may assume that they get plenty of sunlight and have plenty of vitamin D synthesis in their skin cells when they actually do not. Except in cases where lifestyle and/or occupation makes exposure to sunlight an issue that is free from debate, we encourage individuals to avoid assumptions about the adequacy of their sunlight exposure and to treat their vitamin D needs as a matter for special focus and potential reason for follow-up healthcare consultation.
The importance of sunlight for vitamin D health has been the subject of longstanding research. The term �vitamin D winter� was coined several decades ago to summarize the impact of this lower-sunlight season on potential compromise in vitamin D status. More recently, the idea of a �vitamin D winter� has been revisited to include a look at diseases which are related to vitamin D deficiency. Researchers are discovering that many autoimmune conditions have greater severity in winter versus summer, and they are investigating possible links with vitamin D deficiency under these circumstances. Similarly, there has been a strong research trend for studying the occurrence of vitamin D-related diseases at different latitudes on the earth. Over the past 10 years, researchers have found �South to North� trends for increased occurrence of multiple sclerosis, Crohn�s disease, and type 1 diabetes. In other words, there is lesser incidence of these vitamin D deficiency-related diseases at lower latitudes (closer to the equator) where UVB light exposure is presumably higher, and greater incidence of these vitamin D deficiency-related diseases at higher latitudes (farther from the equator).
Breastfeeding and pregnancy
Since nursing mothers must help supply their infants with vitamin D, breastfeeding can pose a challenge both to mothers and infants in terms of vitamin D status. The American Academy of Pediatrics (AAP) and the Canadian Pediatric Society (CPS) have both recommended vitamin D supplementation for both mothers and infants for this reason. (See the section on �Public Health Recommendations� for more information.)
Insufficient dietary fat or inability to absorb dietary fat
Since vitamin D is a fat-soluble vitamin, a diet that is extremely low in fat and/or the presence of certain medical conditions that cause a reduction in the ability to absorb dietary fat may cause vitamin D deficiency. These medical conditions include pancreatic enzyme deficiency, Crohn's disease, celiac sprue, cystic fibrosis, surgical removal of part or all of the stomach, gall bladder disease, and liver disease. Symptoms of fat malabsorption include diarrhea and greasy stools.
Health conditions that involve the parathyroid gland or kidney
Under certain circumstances, the conversion of inactive forms of vitamin D to calcitriol is impaired. For example, diseases that affect the parathyroid gland, liver, and/or kidney impair the synthesis of the active form of vitamin D.
The production of vitamin D precursors in the skin decreases with age. Additionally, with age the kidneys and many other organ systems and cell types are less able to convert vitamin D to its active hormone form.
Genetic susceptibility
Some individual's genetic inheritance includes genetic polymorphisms that result in the production of vitamin D receptors (VDR) that don't work very well. To help compensate for such VDR defects, these individuals need more vitamin D than would normally be necessary.

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