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Department of Medicine, Section of Endocrinology (M.F.H.), Boston University Medical Center, Boston, Massachusetts 02118; Department of Medicine, Division of Endocrinology (E.S.S.), Columbia University, New York, New York 10032; Osteoporosis Clinical Research Program (N.B.), University of Wisconsin, Madison, Wisconsin 53705; Clinical Practice (M.K.B.), Utah Osteoporosis and Menopause Center, Salt Lake City, Utah 84111; Division of Endocrinology and Geriatrics, Department of Medicine (A.K.), McMaster University, Oakville, Ontario, Canada L6J 1X8; and Clinical Development (J.T.K., A.E.d.P.), Medical Communications (R.A.P.), and Biostatistics (E.C.), Merck & Co., Inc., West Point, Pennsylvania 19486
Address all correspondence and requests for reprints to: Michael F. Holick, M.D., Ph.D., Boston University School of Medicine, 715 Albany Street, M1013, Boston, Massachusetts 02118. E-mail: mfholick{at}bu.edu.
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Abstract |
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Methods: Serum 25(OH)D and PTH were obtained in 1536 community-dwelling women between November 2003 and March 2004. Multivariate logistic regression was used to assess risk factors for suboptimal (<30 ng/ml) 25(OH)D.
Results: Ninety-two percent of study subjects were Caucasian, with a mean age of 71 yr. Thirty-five percent resided at or above latitude 42° north, and 24% resided less than 35° north. Mean (SD) serum 25(OH)D was 30.4 (13.2) ng/ml: serum 25(OH)D was less than 20 ng/ml in 18%; less than 25 ng/ml in 36%; and less than 30 ng/ml in 52%. Prevalence of suboptimal 25(OH)D was significantly higher in subjects who took less than 400 vs. 400 IU/d or more vitamin D. There was a significant negative correlation between serum PTH concentrations and 25(OH)D. Risk factors related to vitamin D inadequacy included age, race, body mass index, medications known to affect vitamin D metabolism, vitamin D supplementation, exercise, education, and physician counseling regarding vitamin D.
Conclusions: More than half of North American women receiving therapy to treat or prevent osteoporosis have vitamin D inadequacy, underscoring the need for improved physician and public education regarding optimization of vitamin D status in this population.
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Adequate vitamin D and calcium intake is considered an essential component of osteoporosis management (http://consensus.nih.gov/cons/111/111_statement.htm; http://www.nof.org//physguide/risk_assessment.htm; http://www.osteofound.org/osteoporosis/index.html). Currently, in the United States, the recommended daily vitamin D intake is 400 IU (10 µg) for individuals aged 51–70 yr and 600 IU (15 µg) for those aged 70 yr and older (7). In Europe, 400 IU is recommended for people aged 65 yr or older. Although vitamin D is produced in the skin with exposure to sunlight, there is an age-related decline in cutaneous synthesis making older individuals more dependent on dietary intake (8, 9).
Numerous epidemiological studies have assessed the prevalence of low serum 25(OH)D concentrations (10, 11, 12) and have indicated that vitamin D inadequacy is a common problem worldwide (13). Differences in the prevalence of vitamin D inadequacy have been related to a variety of factors, including physiological changes with age, race, body mass index (BMI), sun exposure, latitude, and dietary vitamin D intake. Opinions regarding the optimal concentration of serum 25(OH)D vary widely. Several studies have shown that serum concentrations of at least 20–30 ng/ml are necessary to maximize intestinal calcium absorption and minimize perturbations in PTH, calcium, and phosphorus homeostasis (5, 12, 14, 15). The majority of epidemiological studies that have assessed vitamin D status of postmenopausal women have been performed outside the United States. In addition, although past studies have included both osteoporotic and nonosteoporotic women, none have specifically evaluated those receiving osteoporosis therapies. Therefore, we conducted an epidemiological study in North America to determine the vitamin D status of postmenopausal women currently receiving antiresorptive or anabolic therapies to treat or prevent osteoporosis. Furthermore, we sought to identify key risk factors associated with suboptimal 25(OH)D concentrations within this population.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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From November 2003 to March 2004, a cohort of women over the age of 55 yr and postmenopausal for a minimum of 2 yr who were receiving osteoporosis medications was recruited from 61 study sites that were evenly distributed across geographic regions by latitude (42° north or more, Boston, MA; between 35 and 42° north, and less than 35° north, Memphis, TN). Subjects could be taking vitamin D supplementation and were required to be taking therapy to treat or prevent osteoporosis, including: alendronate, calcitonin, etidronate, raloxifene, risedronate, or teriparatide for a minimum of 3 months to qualify. Subjects were excluded if they had participated in a clinical trial for osteoporosis within the 3 yr before study start or if they had increased and/or decreased use of over-the-counter supplements or prescription forms of vitamin D within 3 months. The study sites received institutional review board approval and signed informed consent from each subject before enrollment.
Study conduct
Subjects attended a single study visit during which past medical history, height, and weight were recorded. The subject’s most recent dual x-ray absorptiometry, if available, was provided. Concurrent medication use and use of any bone-related therapies (including hormone therapies ever taken) were collected. A specific request was made to record medications that could potentially affect vitamin D metabolism, e.g. glucocorticoids, rifampin, and anticonvulsants (i.e. phenytoin, phenobarbital, carbamazepine). All calcium, vitamin D, and multivitamin supplements taken within 3 months before the study visit were recorded.
A single blood sample was collected and sent to a central laboratory (Quest Laboratories, Van Nuys, CA) to assess 25(OH)D, calcium, intact PTH, phosphorus, albumin, creatinine, magnesium, alkaline phosphatase, and total bilirubin concentrations; fasting was not required.
A self-administered questionnaire was completed by all study subjects and reviewed by study coordinators for completeness and legibility. If changes or additions were required, these modifications were made at the time of the visit by the subject only.
Laboratory studies
Calcium, phosphorus, albumin, creatinine, magnesium, alkaline phosphatase, and total bilirubin were measured by automated standard laboratory methods. Serum 25(OH)D samples collected at the study sites were measured using the Nichols Advantage system (Nichols Institute Diagnostics, San Clemente, CA). The Nichols Advantage system is a commercially available chemiluminescent assay, in which 25(OH)D is released from vitamin D-binding proteins by a releasing agent and the competitive assay is completed automatically by the Nichols Advantage system. This assay has a normal range in adults of 10–68 ng/ml, with an intraassay coefficient of variation of 3.0–4.5% and an interassay coefficient of variation of 7.1–10.0%.
Serum biointact PTH concentrations were measured using the Nichols Advantage chemiluminescence intact immunoassay; the normal range in adults is 6–40 pg/ml, with an interassay coefficient of variation of 7.0–9.2%.
Liquid chromatography-mass spectrometry (LC-MS/MS) was performed on a random subset of 296 subject samples to determine the contribution of 25(OH)D2 and 25(OH)D3 to serum 25(OH)D measured in the study population. 25(OH)D2 and 25(OH)D3 calibration control solutions (eight per set, spanning 1–200 ng/ml) were generated from standards provided by Calbiochem (San Diego, CA). 25(OH)D samples from study subjects were prepared and analyzed through a turbulent flow LC system (Cohesive Technologies, Franklin, MA) followed by traditional laminar flow chromatography. The study samples were then analyzed relative to the control solutions for detection and quantification of the 25(OH)D2 and 25(OH)D3 component of each sample. The analysis was performed using a TSQ Quantum Ultra triple mass-spectrometer (Thermo Finnigan Corp., San Jose, CA). The intraassay coefficient of variation was 6.0%.
Outcome measures
Vitamin D inadequacy and secondary hyperparathyroidism. Various cut points of serum 25(OH)D were used to define vitamin D inadequacy (<9 ng/ml, <15 ng/ml, <20 ng/ml, <25 ng/ml, or <30 ng/ml). Serum 25(OH)D concentrations less than 9 ng/ml have previously been used to define overt vitamin D deficiency (1). A range of higher values was chosen to define vitamin D insufficiency in the absence of a consensus value (5, 6). The percent of subjects with biochemical evidence of secondary hyperparathyroidism (as defined by normal serum calcium with a PTH value above the upper limit of the manufacturer’s normal reference range) was determined.
Predictor variables. Risk factors evaluated for association with 25(OH)D included demographic and baseline variables collected from patient history and physical examination and health-related variables obtained from the subject questionnaire.
The demographic and baseline variables were age, race, BMI (kilograms per square meter), latitude of primary residence, use of daily vitamin D supplementation, history of a medical condition within the past 5 yr known to affect vitamin D metabolism (gastric surgery, chronic liver disease, chronic renal disease, intestinal malabsorption, morbid obesity, hyperparathyroidism, chronic granulomatous disease, malnutrition), and recent use (within the past 4 wk) of concomitant medications known to affect vitamin D metabolism.
Subject questionnaire. The 28-item questionnaire evaluated factors potentially influencing serum 25(OH)D concentrations including general health, limitation of daily activities, amount of exercise per week, discussions with physician of the importance of vitamin D to bone health, highest level of education obtained, annual household income, fall or fracture history, weekly consumption of vitamin D-rich foods, and weekly sun exposure.
Subjects were asked to provide the number of servings on a daily, weekly, or monthly basis for vitamin D containing foods, such as milk, fortified orange juice and cereals, fish, eggs, cod liver oil, and nutritional supplements.
Subjects were asked to record the number of hours per week spent outside without sun protection and the number of body parts exposed. A sun exposure index was calculated using the number of hours outside without sun protection multiplied by the percentage of the body exposed to sunlight (9% for the face, 1% for each hand, 9% for each arm, and 18% for each leg). Exposure of the chest, back, and abdomen were not included.
Statistical analysis
A sample size of 1500 subjects was estimated based on the assumption that the prevalence of low serum 25(OH)D (<15 ng/ml) was 12% and the half-width of the 95% confidence interval (CI) for the true prevalence was 1.6% (16).
The analysis population included all subjects with valid results of serum 25(OH)D. The distribution (mean, SD, median, range, etc.) of serum 25(OH)D and the percent of patients less than 9, 15, 20, 25, and 30 ng/ml and associated 95% CIs were calculated.
Univariate logistic regression models were used to assess the relative risk for vitamin D inadequacy (<30 ng/ml) for each potential risk factor (such as age, race, BMI). P 
0.05 was considered statistically significant. The unadjusted odds ratio (OR), 95% CI, and P value were estimated for each risk factor. Only those with P 0.10 were included in a multivariate stepwise logistic regression analysis. The final logistic regression model included only risk factors with P 0.05. Because a 25(OH)D concentration less than 30 ng/ml was not a rare event, the OR directly estimated from the logistic regression would overestimate the relative risk (RR) if OR was more than 1 or underestimate the RR if OR was less than 1 (17). Therefore, the RR for the risk factors was calculated based on the OR and predicted prevalence of low vitamin D in the reference groups. A risk cohort was a subgroup of patients who had the same number of significant risk factors identified from the multivariate logistic regression analysis. The prevalence of vitamin D inadequacy [25(OH)D < 30 ng/ml] was evaluated within each risk cohort.
Pearson’s correlation coefficient was calculated to assess the relationship between serum 25(OH)D and serum PTH. The means ± SE of PTH was plotted by 10 serum 25(OH)D subgroups (0–9, 10–14, 15–19, 20–24, 25–29, 30–34, 35–39, 40–49, 50–59, or 
60 ng/ml). An exponential model [PTH = ß0 + 
* expß*25(OH)D] was previously used to describe the relationship between serum 25(OH)D and serum PTH by Chapuy et al. (15). We used a second, similar model (the quadratic fit with plateau model) to describe the relationship between serum 25(OH)D and serum PTH (18). That is, for 25(OH)D less than the certain concentration (X0), the equation relating 25(OH)D and PTH was quadratic, and for 25(OH)D greater than X0, the equation was constant. SAS PROC NLIN (18) can fit such a model when X0 is unknown.
All analyses were performed using SAS statistical software (SAS Institute, Inc., Cary, NC).
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Results |
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One thousand five hundred fifty-four subjects met inclusion criteria. A total of 1536 (98.8%) subjects provided a blood sample for determination of serum 25(OH)D concentration and completed the questionnaire.
Subject characteristics
Subject characteristics are summarized in Table 1
. All subjects were female, 91.5% were Caucasian, and 57% were 70 yr of age, or older. The majority (76%) lived at or above 35° latitude, and 35% lived at or above 42° north (Fig. 1). Fifty-nine percent of subjects reported use of at least 400 IU of vitamin D supplementation daily; 4.2% of subjects had a medical condition associated with vitamin D inadequacy; and 13.4% reported prior and/or concomitant medication use known to affect vitamin D metabolism.
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A total of 84.4% of subjects reported being in good to excellent health (Table 2), 18.7% reported a fracture related to a fall within the past 5 yr, 22% reported a fracture of the hip, spine, or wrist since the age of 45 yr, and the majority (66%) reported that they had previously discussed the importance of vitamin D to bone health with their physician. Subjects had a broad range of educational and financial backgrounds; 51% reported at least some college education. Dietary consumption of vitamin D-rich foods was low (including intake of fatty fish; salmon, mackerel, herring, and sardines); the mean and median milk consumption was one 8-oz glass per day.
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The mean (SD) serum 25(OH)D was 30.4 ng/ml (13.2) and the distribution was nearly symmetric. The median value was 29.0 ng/ml. The 25th percentile value was 22 ng/ml and the 75th percentile value was 37 ng/ml. The discrepancy between the mean and median was due in large part to a single subject with a 25(OH)D level of 212.0 ng/ml who was consuming approximately 18,000 IU of supplemental vitamin D daily and had a high serum calcium of 10.4 mg/dl and a low-normal biointact PTH of 14 pg/ml. The next highest serum 25(OH)D concentration was 100 ng/ml in a subject with a normal serum calcium and PTH (9.5 mg/dl and 28 pg/ml, respectively).
Prevalence of low serum 25(OH)D concentration
Several cut points were used to describe the prevalence of vitamin D inadequacy in the study population (Fig. 2). Fifty-two percent (95% CI 50, 55%) of subjects had a serum 25(OH)D less than 30 ng/ml, and 18% (95% CI 16, 20%) had values less than 20 ng/ml. Regardless of the level used to define inadequacy, the prevalence was significantly greater in the group that reported daily vitamin D supplementation of less than 400 IU, compared with the group that used 400 IU or more (Fig. 3). For example, at a cut point of 30 ng/ml, the prevalence of vitamin D inadequacy in those using less than 400 IU vitamin D supplementation daily was 63%, compared with 45% among those using at least 400 IU/d.
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50,000 IU weekly). Associations with serum 25(OH)D concentration
Factors associated with the prevalence of serum 25(OH)D concentrations less than 30 ng/ml were evaluated using univariate logistic regression analyses (Table 3). Factors found to have a significant relationship (P < 0.05) to vitamin D inadequacy were older age (>80 yr), higher BMI (>25 kg/m2), use of concomitant medications known to affect vitamin D metabolism, vitamin D supplementation (<400 IU daily), poor general health, limitation in daily activities, exercise level (less than six times per week), absence of previous discussion with a physician regarding the importance of vitamin D to bone health, lower education level and annual income, and lower number of weekly servings of vitamin D-containing foods. All of these factors remained significantly associated with vitamin D inadequacy when D inadequacy was defined as a 25(OH)D less than 20 ng/ml (data not shown). When age and BMI were analyzed as categorical variables (with more than two categories), the tests for trend were significant, P < 0.05 (data not shown). With advancing age, there was an increased risk of vitamin D inadequacy; however, even among the youngest age groups (51–60 yr), the prevalence of vitamin D inadequacy was relatively high (51%).
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Prevalence of secondary hyperparathyroidism
Among the overall study population, 1246 subjects had been on chronic (1 yr) therapy for osteoporosis. Of these, 206 (16.5%) had biochemical evidence of secondary hyperparathyroidism. There was a significant inverse correlation between serum PTH and 25(OH)D; r = –0.283, P < 0.001. This correlation was also performed for the overall study population and identified a significant and consistent correlation between serum PTH and 25(OH)D (r = –0.290, P < 0.001). The strength of the relation between PTH and 25(OH)D may be underestimated by this r value because of their nonlinear association (Fig. 4A). When the relationship between serum PTH and 25(OH)D was analyzed using a quadratic model, it was found that PTH values began to increase with 25(OH)D concentrations less than 29.8 ng/ml. At 25(OH)D concentrations above 29.8 ng/ml, PTH values appeared to reach a plateau level of 27.2 pg/ml (data not shown). Consequently, the prevalence of secondary hyperparathyroidism was significantly higher among subjects with lower serum 25(OH)D concentrations (P < 0.001 for trend test) (Fig. 4B).
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Among the potential risk factors identified, only high BMI, lack of exercise, absence of a previous discussion with a physician regarding the importance of vitamin D to bone health, vitamin D supplementation less than 400 IU daily, and education level below grade 12 were consistently found to be associated with vitamin D inadequacy (when defined as either <30 or <20 ng/ml) in both the univariate and multivariate analyses. The association of vitamin D inadequacy with obesity is well documented (19). Although obesity and lack of exercise are linked to additional health risks, modifications to diet and activity level are difficult to achieve (20, 21). However, two measures that could be readily implemented in clinical practice to reduce the prevalence of vitamin D inadequacy are physician-initiated discussions regarding the importance of vitamin D to bone health and use of vitamin D supplementation. This study suggests that the prevalence of vitamin D inadequacy could be reduced if subjects took at least 400 IU of vitamin D supplementation per day. This is not an excessive recommendation as the results of this study suggest that some women need more vitamin D to obtain optimal serum concentrations. Moreover, the current recommended daily allowance of 400 IU/d is based on the dose of vitamin D necessary to prevent osteomalacia.
Previous epidemiological studies have shown a wide variation in the prevalence of low 25(OH)D concentrations, but comparisons between studies are difficult because of differences in 25(OH)D cutoffs and 25(OH)D assays used. Thomas et al. (22) reported that 57% of hospitalized patients on a general medical service had 25(OH)D less than 15 ng/ml. Even higher rates of vitamin D deficiency have been reported for institutionalized and inactive elderly individuals (23, 24). Low 25(OH)D concentrations are not invariably associated with the sick or elderly because suboptimal 25(OH)D concentrations (20 ng/ml) have been reported in healthy adolescents (25). The prevalence in our study was consistent with the results of a previously reported multinational osteoporosis study (13). Indeed, vitamin D inadequacy is being recognized as an increasingly significant public health problem.
The optimal serum concentration of 25(OH)D remains the subject of much debate. Heaney et al. (14) concluded that serum 25(OH)D concentrations of 32–36 ng/ml (80 to 90 nmol/liter) may be required to attain maximum intestinal calcium absorption, whereas Malabanan et al. (5) concluded that a serum 25(OH)D concentration of 20 ng/ml (50 nmol/liter) was the minimum required to optimize serum PTH levels and prevent secondary hyperparathyroidism. We included a cut point of 30 ng/ml based on the findings of Chapuy et al. (15), who showed that mean serum PTH concentrations began to rise when serum 25(OH)D concentrations fell below 31 ng/ml. Our findings are consistent with Chapuy et al. demonstrating an increase in serum PTH at 25(OH)D concentrations less than 29.8 ng/ml. Recognition of functional hypoparathyroidism in up to one third of patients with hypovitaminosis D suggests that biochemical evidence of secondary hyperparathyroidism may not be the most sensitive indicator of vitamin D inadequacy in an individual patient (26). A recent cross-sectional study of 4100 ambulatory elderly (>60 yr) suggested that serum 25(OH)D concentrations greater than 40 ng/ml were the minimum needed for optimal musculoskeletal function of the lower extremities (27). Given the above, it is clear that the lower limit of normal for current laboratory 25(OH)D ranges is set too low, yet the optimum serum concentration of 25(OH)D remains uncertain.
Vitamin D deficiency has long been recognized to lead to rickets in children and osteomalacia in adults. There is growing evidence that less severe degrees of vitamin D insufficiency also lead to skeletal disease. There is a positive association between serum 25(OH)D concentrations and total hip bone mineral density (28), and lower 25(OH)D concentrations have been reported in hip fracture patients, compared with controls (29). Moreover, a placebo-controlled study by Chapuy et al. (30, 31) demonstrated a significant reduction in hip and nonvertebral fractures in elderly women living in retirement homes after daily supplementation with vitamin D3 and elemental calcium. A reduction in nonvertebral fracture incidence was also observed in a study of elderly community-dwelling men and women by Dawson-Hughes et al. (32) with daily use of vitamin D3 and calcium. Intermittent administration of vitamin D has been shown to be as effective as daily use, in that 100,000 IU of oral vitamin D3 given every 4 months for 5 yr in an elderly British population significantly reduced the incidence of osteoporotic fractures (33).
Vitamin D also plays an important role in the maintenance of proximal muscle strength, and reduction of falls is a potential benefit of vitamin D supplementation (34, 35, 36, 37). A 3-month clinical trial of 122 patients in long-term care facilities demonstrated that daily vitamin D3 and calcium provided a 49% reduction in fall rates, compared with calcium alone (38). A recently published metaanalysis demonstrated that vitamin D supplementation reduced the likelihood of falling by more than 20%, compared with calcium or placebo (39). Thus, the above-noted reduction in fracture risk with vitamin D supplementation may reflect skeletal and extraskeletal effects.
Serum 25(OH)D assays are widely available from commercial laboratories throughout the world. The wide interlaboratory variation as well as the even greater intermethod variation in serum 25(OH)D measurement has been the subject of several reports (40, 41). It has recently been reported that most of the commercial assays tested substantially overestimated serum 25(OH)D when compared with a gold standard HPLC method (42, 43). As such, the assay used in the present study (the Nichols Advantage system) may be reporting a higher 25(OH)D concentration than is actually present. Thus, the prevalence of vitamin D inadequacy might be even higher than reported here. This assay was chosen because of its proven precision, ease of use, and widespread availability. Conversely, this specific assay has been reported to underestimate circulating 25(OH)D2 (43, 44). However, because milk is routinely supplemented with vitamin D3, and less than half of the available multivitamin preparations contain vitamin D2, it is unlikely that assay-dependent underestimation of vitamin D2 contributed in a major way to the results of this study. In addition, in this study few subjects reported taking large doses of vitamin D2. Furthermore, LC-MS/MS on a subset of subjects confirmed that very few had 25(OH)D2 as the major component of their serum 25(OH)D concentrations. LC-MS/MS is currently among the most accurate measures of the separate contributions of both 25(OH) D2 and 25(OH)D3 to total 25(OH)D concentrations. Results using LC-MS/MS and the Nichols Advantage assay were almost identical for the prevalence of vitamin D inadequacy in this subgroup of patients.
The major strength of our study is that it provides estimates of the prevalence of vitamin D inadequacy among women receiving therapy to treat or prevent osteoporosis. These women were expected to be more aware of the importance of vitamin D to bone health than the general population. There are several limitations of our study. First, our study was performed during the winter months only, when serum 25(OH)D concentrations are typically near the lowest levels of the year. Very little vitamin D is made in the skin after November at latitudes above 35° north (45). For this reason, we may not have seen a significant variation across latitudes, although an inverse trend was observed as reported by Lips et al. (13). Even with adequate sun exposure during the summer and fall, subjects are at risk of low serum 25(OH)D concentrations because of the relatively short half-life of 25(OH)D (
2 wk). Second, our study population was confined to North American women only. The great majority of subjects in our study were Caucasian; thus, we could not fully assess the prevalence of vitamin D inadequacy in non-Caucasians, although, as reported by others, non-Caucasians are likely to have an even higher prevalence of vitamin D inadequacy (46, 47).
Conclusions
There is an unacceptably high level of vitamin D inadequacy among postmenopausal North American women receiving therapy to treat or prevent osteoporosis. Even in the absence of traditional risk factors for vitamin D deficiency, vitamin D inadequacy was highly prevalent. In our study, supplemental use of vitamin D less than 400 IU/d and lack of physician counseling regarding the importance of vitamin D were two risk factors independently and significantly associated with vitamin D inadequacy that could easily be addressed through physician and patient education. These results underscore the need for better education of the public and physicians regarding the optimization of vitamin D status in the care of postmenopausal women with osteoporosis.
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Acknowledgments |
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Footnotes |
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First Published Online March 29, 2005
Abbreviations: BMI, Body mass index; CI, confidence interval; LC-MS/MS, liquid chromatography-mass spectrometry; 25(OH)D, 25-hydroxyvitamin D; OR, odds ratio; RR, relative risk.
Received December 3, 2004.
Accepted March 21, 2005.
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