Occult Vitamin D Deficiency in Postmenopausal US Women With Acute Hip Fracture FREE

Vitamin D is required for efficient absorption of dietary calcium and for normal mineralization of bone. Reduction in vitamin D levels is associated with impaired calcium absorption and a compensatory increase in the level of parathyroid hormone (PTH) which, in turn, stimulates bone resorption and bone loss. According to the recent National Health and Nutrition Examination Survey Study, an estimated 26 million to 38 million US adults have osteoporosis or are at risk for osteoporosis in the hip.¹ Risk of hip fractures increases exponentially with age. Hip fractures are increasing worldwide, and are the most devastating and costly of the osteoporotic fractures.² Advancing age is associated with reductions in sun exposure, intake and skin-activation of vitamin D, and in vitamin D absorption, all of which may contribute to low vitamin D levels.³

It is striking that in recent studies from England, Scotland, and South Africa, 13% to 33% of patients with hip fractures had histological signs of osteomalacia that may have been caused by prolonged vitamin D deficiency,^4- 7 although not all European studies concur.⁸ In the United States in 1978, Sokoloff⁹ reported occult osteomalacia in 8 (25%) of 31 patients with hip fracture, and another study¹⁰ from an elderly residential facility and hospital found 4 (12.9%) of 31 patients with osteomalacia at the time of hip fracture. Limited data are available on vitamin D status in US patients admitted for acute osteoporotic fractures. We compared such subjects with patients scheduled for elective joint replacementsurgery to test the hypothesis that patients with acute hip fractures have lower vitamin D levels and higher PTH levels than do subjects without fracture with normal or even low bone mineral density.

A total of 805 postmenopausal women scheduled for joint replacement were identified by daily monitoring of orthopedic admissions between January 1995 and June 1998, including 262 women admitted to Brigham and Women's Hospital with acute hip fracture and 543 women scheduled for elective joint replacement either at the Brigham and Women's Hospital or the New England Baptist Hospital in Boston, Mass. Postmenopausal women who experienced either natural or surgical menopause with amenorrhea for 12 months were recruited for this study. Women receiving estrogen replacement therapy were included (46% of patients). Women were excluded if they were taking any other medications or had any disorders or abnormal admission test results that might affect bone, or had any underlying hip disease other than osteoarthritis (Figure 1 and Table 1). Of the 68 women admitted for elective joint replacement, 17 were determined to be osteoporotic (T score <−2.5) according to the World Health Organization bone density criteria¹¹ and were analyzed separately (elective osteoporotic group) from the 51 elective patients admitted who were not osteoporotic (elective control group). There were 30 osteoporotic women with hip fractures (fracture group). Research protocols were approved by the institutional review boards of the Brigham and Women's Hospital and the New England Baptist Hospital. All patients or, if necessary, a designated family member or guardian, gave informed consent.

Using a modification of the Nurses' Health Study questionnaire¹² and dietary and detailed physical activity¹³ questionnaires, all participants provided information regarding lifestyle, reproductive factors, dietary calcium consumption, and physical activity (Table 2).

Bone mineral density of the spine (L1-L4), proximal femur, and total body were measured with the dual x-ray absorptiometry technique (QDR2000, Hologic Inc, Waltham, Mass). In vivo precisions (coefficient of variation percentage) in postmenopausal women for the spine, femoral neck, and trochanteric bone density measurements on different days were 1.21%, 1.74%, and 1.24%, respectively.¹⁴ Women with vertebrae with moderately severe osteoarthritic changes, disk space narrowing, or a fracture were excluded from the analyses as these anatomic findings may elevate the spinal bone mineral density. Results were expressed as SDs and compared with bone mineral density values for age-matched healthy individuals (z score). Body composition (lean and fat mass) was determined by dual x-ray absorptiometry technique. The ratio of fat-to-lean tissue was calculated. Reproducibility of mean (SEM) for fat and lean tissue determinations in our laboratory were 1.09% (0.15%) and 0.89% (0.28%), respectively.

Blood chemistries, complete blood cell counts, and tests for urinary calcium levels were performed in hospital clinical laboratories; all remaining tests were performed in the General Clinical Research Center laboratory unless otherwise specified. Blood samples were obtained preoperatively in 88% of the patients. In some instances, elective or hip fracture patients were rushed to surgery and informed consent and blood samples were obtained postoperatively. Serum intact PTH levels were measured with the sensitive Allegro immunoradiometric assay (Corning Nichols Institute, San Juan Capistrano, Calif). Serum 25-hydroxyvitamin D levels were measured using a radioimmunoassay (RIA) procedure (Incstar Corp, Stillwater, Minn) approved by the Food and Drug Administration. Levels of 1,25-dihydroxyvitamin D were measured by RIA. Urinary N-telopeptide levels, an index of bone resorption, were determined in a 24-hour urine collection by an enzyme-linked immunosorbent assay that measures cross-linked collagen peptides (Osteomark Assay, Ostex International Inc, Seattle, Wash). Bone formation markers including serum levels of osteocalcin by RIA¹⁵ and bone-specific alkaline phosphatase (BSAP) were measured by immunoradiometric assay (Tandem-R Ostase assay, provided by Hybritech, San Diego, Calif) by Dr Gundberg of Yale University School of Medicine, New Haven, Conn. The BSAP levels were measured by immunoassay (Alphase B assay, provided, in part, by Metra Biosystems, San Diego, Calif). The interassay and intra-assay coefficients of variation for all the assays ranged from 2.3% to 12.1%.

Patient characteristics and markers of bone turnover were summarized as means and SDs. Analysis of variance was performed with prespecified contrasts comparing the fracture group with each of the elective groups, and having estrogen replacement therapy as a covariate. Normality tests were rejected for several calciotropic hormones, biochemical values, and baseline characteristics; therefore, medians and 5th and 95th percentiles were presented. The natural log transformed normalized intact PTH, 25-hydroxyvitamin D, N-telopeptides, BSAP (Metra Biosystems), BSAP (Hybritech), and creatinine. Analysis of variance was used for phosphate, 1,25-dihydroxyvitamin D, urinary calcium, transformed intact PTH, 25-hydroxyvitamin D, N-telopeptides, BSAP (Metra Biosystems), BSAP (Hybritech), and creatinine, with age and estrogen replacement therapy as covariates. Cochran Mantel-Haenszel tests were used for albumin, total calcium, osteocalcin, total calcium intake, alcohol, smoking, and physical activity with estrogen replacement therapy and age above or below the median as stratification factors. Percentages and χ² tests were used to compare groups for dichotomized calciotropic hormones. Ninety-one subjects had hip surgery and 7 had knee surgery (6 from the elective control group, 1 from the elective osteoporotic group). Results were nearly identical with and without knee surgery subjects; combined results were presented. Analyses were performed using SAS software (SAS Inc, Cary, NC).

Characteristics of 98 women who were enrolled in the study are summarized in Table 2. The groups with osteoporosis (with fracture or elective) did not differ in the years since menopause, and they had comparable bone density levels (z scores) in the spine and proximal femur sites, but they differed in total body bone density. Body fat, percentage fat, and the fat-to-lean ratio showed monotonic changes among the 3 groups. Compared with the elective control group, the postmenopausal women admitted for hip fracture had reduced exercise hours.

Fifty percent (n=15) of the postmenopausal women admitted with acute hip fractures had deficient 25-hydroxyvitamin D levels (≤30.0 nmol/L [≤12 ng/mL]), 36.7% (n=11) had elevated PTH levels (>6.84 pmol/L [>65 pg/mL]), and 81.8% had urinary calcium levels below the median (<3 mmol/d [120 mg/24 h]) (Figure 2). Among the women with acute fracture, 8 had both deficient 25-hydroxyvitamin D levels (≤30 nmol/L) and elevated PTH levels (>6.84 pmol/L).

Circulating calcium levels were lower in the women with hip fractures than in the elective control and osteoporotic groups (Table 3). Compared with the women in the control group admitted for elective hip replacement, urinary N-telopeptide levels were higher in the osteoporotic elective and fracture groups (with similar z scores for the spine and hip). Levels of N-telopeptide were not different between the osteoporotic elective and fracture groups and were higher than the elective control group. Osteocalcin, BSAP (Hybritech), and BSAP (Metra Biosystems) were lower in women with fractures than in the elective osteoporotic group. The correlation between the BSAP (Hybritech) and the BSAP (Metra Biosystems) samples was 0.82 (Spearman rank correlation, P<.001). Serum thyrotropin levels were comparable among the 3 groups (data not shown). There were no significant differences in calcium, albumin, phosphate, PTH, 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, urinary calcium, osteocalcin, creatinine, or N-telopeptide levels between women whose serum sample was obtained preoperatively and the 12% of women whose serum sample was obtained postoperatively.

This study shows low vitamin D and high PTH levels among elderly, community-dwelling women in Boston admitted for acute hip fracture with no secondary causes of bone loss. Half had vitamin D levels in the deficient range (≤30 nmol/L) with high-range normal PTH levels or secondary hyperparathyroidism. While there is increased awareness about the importance of calcium nutrition for skeletal health, these data suggest that deficient vitamin D levels should be of concern in women admitted with hip fractures.

Circulating calcium and phosphate levels were lower in women with hip fractures compared with those admitted for elective joint replacement, consistent with vitamin D deficiency, impaired calcium and phosphate absorption, and higher PTH levels. The higher PTH levels, in turn, partially raise circulating calcium to the lower limit of the normal range and decrease the serum phosphate concentration. Low albumin levels in women with hip fractures compared with the elective groups may reflect nutritional status and are an independent predictor of vitamin D deficiency in hospitalized subjects.¹⁶ Although we excluded patients from nursing homes and with comorbid conditions affecting bone density, it is possible that the fracture patients were more frail, had poorer nutritional status, and were exposed to less sunlight than the control group.

Data were analyzed adjusting for estrogen replacement, as indicated. Estrogen can affect bone and calciotropic hormones but not 25-hydroxyvitamin D levels. Estrogen potentially can affect PTH levels, but in women who were postmenopausal for less than 20 years (as in the case, on average, for the elective subjects), PTH levels increase with age. In women with more than 20 years since menopause, similar to the women with hip fractures, estrogen may lower PTH levels. Thus, estrogen use cannot account for higher PTH levels in women with acute hip fracture compared with the elective groups. According to Khosla et al,¹⁷ levels of vitamin D predict PTH levels regardless of estrogen therapy or menopausal status. In our study, the increase in PTH levels in postmenopausal women with hip fractures cannot be explained by differences in creatinine level; the groups were similar and no patient had a creatinine level above 159 µmol/L (1.8 mg/dL), a level at which intact PTH levels increase to more than 6.84 pmol/L.

Levels of 1,25-dihydroxyvitamin D did not differ between the women with hip fractures and those admitted for elective joint replacement. The conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D in the kidney may be augmented in patients with hip fractures by the stimulatory effects of higher PTH and lower phosphate levels on renal 1-hydroxylation of vitamin D.¹⁸ Levels of vitamin D metabolites presented herein reflect the vitamin D status at the time of hip fracture. One large study found low levels of 1,25-dihydroxyvitamin D but not 25-hydroxyvitamin D in women with hip fractures, although the vitamin D metabolites were obtained approximately 3.9 years before the fracture.¹⁹ Others did not find low levels of 1,25-dihydroxyvitamin D in women with hip fractures after correction for a decrease in vitamin D-binding protein.

Osteoporosis results from an imbalance between bone resorption and bone formation. The development of markers of bone turnover such as the urinary N-telopeptide level, which is an index of bone resorption, and serum osteocalcin and BSAP, which are bone formation markers, make it possible to estimate bone remodeling noninvasively. In our study, both the osteoporotic women admitted for elective orthopedic surgery and those with hip fractures and comparable axial bone density and years since menopause had higher urinary N-telopeptide levels compared with the elective women without osteoporosis. Levels of N-telopeptide, osteocalcin, and BSAP increase with age and increased bone turnover is a major determinant of osteoporosis in the elderly.²⁰ Osteocalcin and BSAP levels were lower in the women with fractures than women with osteoporosis in the elective group. This low osteocalcin level in the women with hip fractures compared with the osteoporotic elective group, in conjunction with higherN-telopeptide levels, suggests an uncoupling of bone formation, with increased bone resorption that may be a consequence of vitamin D deficiency. Another study showed that urinary excretion of pyridinolines was higher in elderly patients with vitamin D deficiency and elevated PTH levels than in healthy young adults.²¹ Studies by Akesson et al²² showed lower osteocalcin levels and higher urinary pyridinoline levels in women with hip fractures than in age-matched controls.

Several lines of evidence indicate that subclinical vitamin D deficiency may contribute to increased fractures. First, there is seasonal variation in vitamin D levels, with a decrease in winter and spring that parallels the increased hip fracture rates during those seasons.²³ Second, the increase in PTH levels due to vitamin D deficiency leads to PTH-mediated bone loss. Third, vitamin D and calcium therapy reduce PTH and lead to a decrease in markers of bone resorption, thereby reducing bone breakdown^24- 25 and fractures.^24,26

Low vitamin D levels and elevated PTH levels have been reported in patients with hip fractures from Europe, Scandinavia, and Australia.^27- 29 However, in the United States, milk is fortified with vitamin D so the intake of vitamin D is greater, and vitamin D levels are higher than in Scandinavia and Europe.³⁰ Peacock et al³¹ determined that a level of 25-hydroxyvitamin D of less than 9.98 nmol/L can cause osteomalacia. Other studies show that a level of 25-hydroxyvitamin D below 30.0 nmol/L is a threshold for decreased bone density.³² Results may not be comparable, however, because vitamin D assays used in earlier studies from Europe and Amsterdam had lower values than the RIA assays now approved by the Food and Drug Administration and often used in the United States.^33- 34 International standards for vitamin D assays and for normality are needed for application to clinical care of patients with metabolic bone disorders.

The decision to treat patients with low 25-hydroxyvitamin D levels depends on the risk of a rise in PTH levels that may have deleterious effects on the skeleton. For example, PTH levels increase well before the 25-hydroxyvitamin D level decreases to 30.0 nmol/L (or 22.5 nmol/L, the lower limit of normal for some assays). A few individuals were identified with 25-hydroxyvitamin D levels between 30.0 and 54.9 nmol/L and with elevated PTH levels. According to different models, several 25-hydroxyvitamin D threshold levels for PTH have been proposed. Ooms et al³⁵ reported that 25-hydroxyvitamin D levels of 25.0 nmol/L are negatively related to PTH and low bone density at a cutoff of 30 nmol/L. In elderly nursing home patients including those in Boston, PTH levels were elevated with 25-hydroxyvitamin D concentrations less than 37.4 nmol/L, thus vitamin D depletion was indicated by a 25-hydroxyvitamin D level below 37.4 nmol/L.^3,36 Peacock et al³¹ defined vitamin D deficiency as 25-hydroxyvitamin D levels between 9.98 and 49.9 nmol/L, when there may be mild hyperparathyroidism and decreased bone density.³¹ Chapuy et al³⁷ studied 1569 French subjects and found that PTH levels were s when the 25-hydroxyvitamin D level was higher than 77.4 nmol/L, using the same RIA vitamin D assay used in our study. Haden et al³⁸ also showed that PTH levels start to increase at 25-hydroxyvitamin D levels of less than 62.4 nmol/L. Thus, levels of 25-hydroxyvitamin D may need to be higher than 49.9 to 73.4 nmol/L to protect against secondary hyperparathyroidism.

Our study was not designed to assess seasonal changes and it did not equally span all seasons in the study interval. The study may have underestimated the frequency of low vitamin D levels in women with hip fractures by excluding 190 subjects with comorbid conditions and fractures who may have had reduced sun exposure and vitamin D intake (Table 1). A recent study showed that 57% of hospitalized patients had low vitamin D levels.¹⁶

In conclusion, this study shows occult vitamin D deficiency in postmenopausal women with hip fractures and no other secondary causes of bone loss in the northeastern United States. Correction of vitamin D deficiency or insufficiency with vitamin D supplementation or sun exposure may lead to a reduction in the exponential rise of fractures that occur with age. The new 1997 Dietary Reference Intake guidelines from the National Institutes of Health recommend 400 IU of vitamin D supplement daily for individuals from age 51 through 70 years and 600 IU daily for those older than 70 years. Supplements of about 800 IU of vitamin D per day and calcium may be necessary to attenuate bone loss in the winter and to reduce fractures.^24,26 These findings may play an important role in the orthopedic management of acute fractures in postmenopausal women. In patients with hip fractures, vitamin D repletion at the time of fracture and resultant suppression of hyperparathyroidism may facilitate hip fracture repair, implant incorporation, and reduction of future fracture risk. Vitamin D deficiency is preventable and heightened awareness is necessary to institute public health programs to ensure adequate vitamin D nutrition in the elderly, particularly in northern latitudes.