Prevalence and correlates of vitamin D deficiency in US adults
Introduction
Vitamin D has been traditionally considered as important in skeletal health. However, during the past decade, numerous research findings have revealed that vitamin D produces beneficial effects on extraskeletal tissues as well [1], [2], [3]. Some evidence suggests that vitamin D helps to regulate cell growth and prevent cancer progression [4], [5], [6]. Epidemiological studies have reported that higher vitamin D levels were associated with reduced cancer incidence and decreased cancer-related mortality [7], [8], [9]. Vitamin D was found to be involved in controlling the production of renin, one of the most important hormones for regulating blood pressure [10]. Thus, Vitamin D deficiency might contribute to development and progression of hypertension and cardiovascular disease [11], [12], [13], [14]. Furthermore, vitamin D deficiency has been linked to the development of type 1 diabetes [1], [2], [3], [15], multiple sclerosis [16], [17], rheumatoid arthritis [18], and other autoimmune conditions [1], [2], [3], [19], [20].
Vitamin D can be synthesized by the skin through exposure to ultraviolet light of wavelength 290 to 315 nm that stimulates the conversion of 7-dehydrocholesterol to previtamin D [21]. The other source of vitamin D is from the diet. Vitamin D from food undergoes hydroxylation in the liver to 25-hydroxyvitamin D—the major circulating form—and then in the kidney to 1,25-dihydroxyvitamin D, which optimizes calcium and phosphate absorption from the intestine, as well as having direct effects on bone cells [1], [3]. The recommended adequate intake of vitamin D used to be 200 IU/d for all children and adults 50 years or younger, 400 IU/d for people aged 51 to 70 years, and 600 IU/d for those older than 70 years [22]. However, due to its beneficial effects, the amounts of vitamin D needed for optimal health are probably higher than previously thought [2], [23]. On November 30, 2010, the Institute of Medicine released updated recommendations regarding vitamin D intake: 600 IU/d for people aged 1 to 70 years and 800 IU for people aged 71 and older [24].
Although vitamin D deficiency is commonly defined as a 25-hydroxyvitamin D level ≤20 ng/mL (50 nmol/L) [20], [25], published studies have used different definitions for vitamin D deficiency. By different cutoff points, vitamin D deficiency was found to be common in certain subpopulations, including elder adults (41% using 25-hydroxyvitamin D level ≤20 ng/mL) [20], African Americans (61% using 25-hydroxyvitamin D level ≤15 ng/mL) [26], and women with osteoporosis (64% using 25-hydroxyvitamin D level ≤30 ng/mL [27]. Given that low vitamin D levels are linked to all major health problems in populations, such as cardiovascular disease, cancers, and diabetes, it is of importance to identify how prevalent this condition is and what factors are associated with this condition in the US population. This study analyzed the data from the 2005 to 2006 National Health and Nutrition Examination Survey (NHANES) to describe the epidemiology of vitamin D deficiency in US adults, including prevalence patterns of vitamin D deficiency and its correlates. Based on the findings from the literature, it was hypothesized that vitamin D deficiency was common in the US population, especially certain minority groups. The objective of this study was to examine and compare the prevalence rate of vitamin D deficiency by age, race, other demographic factors, as well as by certain health conditions, using serum 25-hydroxyvitamin D level, the best indicator of vitamin D status.
Section snippets
Study population
The NHANES is an ongoing program conducted by the National Center for Health Statistics to assess the health and nutritional status in the noninstitutionalized US population and track changes over time [28]. The survey combines interviews and physical examinations. The interview includes demographic, socioeconomic, dietary, and health-related questions. The examination component consists of medical and physiologic measurements, as well as laboratory tests.
The NHANES uses a stratified multistage
Results
A total of 4495 individuals aged 20 years or older were included in this study. About 17% of the study population were 65 years or older. Table 1 showed the characteristics of the study population. Fifty-two percent of the study participants were women. The race distribution was 72.5%, 11.1%, 11.3%, and 5.1% for white, black, Hispanic, and other races, respectively. More than 57% of study participants received at least some postsecondary education. Most individuals (83.8%) reported to have a
Discussion
From the results of the current study, the hypothesis was accepted that vitamin D deficiency was common in US adults, especially among minority groups. Although different cutoff points have been used to define vitamin D deficiency, several studies have reported a high prevalence rate of vitamin D deficiency in non-Hispanic blacks [30], [31], [32], [33]. Using the definition of serum 25-hydroxyvitamin D concentrations ≤20 ng/mL, we found that over 80% of black adults, both men and women, would
Acknowledgment
This research was supported by the College of Health, Environment and Science at Slippery Rock University of Pennsylvania.
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