Frequency of hypovitaminosis D

There have been a large number of (mostly small) studies describing the prevalence of hypovitaminosis D between 1971 and 1990, as summarized in a review by McKenna et al.3

As a result of these early observational studies, the major risk factors for hypovitaminosis D in the general population are now recognized to be season (less exposure to sunshine during winter months), latitude (less penetrating ultraviolet B [UVB] radiation in countries at higher latitudes), nutritional deficiencies (less hypovitaminosis D in countries with fortification of foods with vitamin D or with dietary habits predisposing to more vitamin D intake), age (less skin production of vitamin D in the elderly), and mobility (less mobility predisposing to decreased sun exposure especially in debilitated and institutionalized elderly people).

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Contemporary studies performed after 1990 have largely corroborated these findings,4-29 and added to the above listed risk factors for hypovitaminosis D is race-
ethnicity, with dark skinned individuals displaying a much higher prevalence of low vitamin D levels.4,5,19 The best description of vitamin D levels in the general U.S. population has come from studies using data from the National Health and Nutrition Examination Survey (NHANES), which offers a snapshot assessment of the entire non-institutionalized civilian U.S. population. In one study of 18,875 NHANES III participants, 25OHD levels below 7, 10, and 25 ng/mL were described in less than 1%, 1%-5%, and 25%-57% of subpopulations assessed either in the winter or at a higher latitude.20

The prevalence of hypovitaminosis D in the summer/higher latitude subpopulations was less than 1%-3% for 25OHD levels less than 10 ng/mL and 21%-49% for 25OHD levels below 25 ng/mL. In addition, 25OHD levels were highest in non-Hispanic whites, intermediate in Mexican-Americans, and lowest in non-Hispanic blacks.20 Interesting population trends in 25OHD levels were shown by a study comparing 18,883 participants in NHANES III and 13,369 participants in NHANES 2001-2004.28 Mean 25OHD levels decreased from 30 to 24 ng/mL during the 10 years between these two phases of NHANES.

The prevalence of 25OHD levels below 10 ng/mL increased from 2% to 6% and the prevalence of 25OHD levels above 
30 ng/mL decreased from 45% to 23% during these 10 years. The results of this study indicate a progressive trend of gradually worsening 25OHD levels in the general population, and the reasons remain obscure.

In addition to the above listed geographic and demographic characteristics, CKD has also emerged as a condition associated with a significantly higher prevalence of hypovitaminosis D, possibly because uremia may adversely affect the photoproduction of cholecalciferol in the skin.30

Wolf et al examined 825 incident hemodialysis (HD) patients and found that 78% of patients had 25OHD levels below 
30 ng/ml, with 18% having extremely low levels of <10 ng/ml.27 Levin et al examined 1,814 patients with various stages of CKD (all non-dialysis dependent), describing median serum 25OHD levels of 20-30 ng/mL, and slightly, but not significantly, lower levels in patients with more advanced CKD.31

It is unclear if the lower 25OHD levels seen in patients with CKD and end-stage renal disease (ESRD) are related to a higher frequency of conditions that affect these levels (such as more black patients, more advanced age, or a higher comorbidity burden with lesser mobility and sun exposure) versus uremia-specific mechanisms leading to lower production, absorption and/or higher degradation of vitamin D.