J C E M B r i e f
R e p o r t — E n d o c r i n e
O N L I N E
R e s e a r c h
Serum Sclerostin Levels Vary With Season Bess Dawson-Hughes, Susan S. Harris, Lisa Ceglia, and Nancy J. Palermo Jean Mayer United States Department of Agriculture Human Nutrition Research Center on Aging (B.D.H., S.S.H., L.C., N.J.P.), Tufts University, and Division of Endocrinology, Diabetes, and Metabolism (B.D.H., L.C.), Tufts Medical Center, Boston, Massachusetts 02111
Context: To establish the clinical utility of serum sclerostin levels, it is important to know whether there is seasonal variation in the measurements. Objective: This study was done to determine whether serum sclerostin levels vary by season in healthy older men and women. Methods: Serum sclerostin levels were measured in archived serum of 314 healthy men and women aged 65 years and older and examined for seasonal variation. Several factors known to vary by season and previously reported to be associated with serum sclerostin levels, including serum osteocalcin, physical activity, and serum PTH levels, were also measured in these subjects. Sex did not modify the association of season with sclerostin, so the men and women were analyzed together. Results: Serum sclerostin levels varied significantly by season (P ⬍ .001, after adjustment for sex). Sclerostin levels in the wintertime were 20% higher than the all-year mean, the levels gradually declined through the spring and summer, and by the fall, they were 20% below the all-year mean. Adjustment for serum osteocalcin, physical activity, and serum PTH did not alter the seasonal means. Seasonal differences in serum osteocalcin, physical activity, and serum PTH were not statistically significant. Conclusions: This study documents marked seasonal variation in serum sclerostin levels. It is important to recognize this source of biological variability when considering the potential clinical utility of sclerostin measurements. (J Clin Endocrinol Metab 99: E149 –E152, 2014)
clerostin is a glycoprotein produced by osteocytes. Serum sclerostin levels have been strongly correlated with bone marrow plasma sclerostin in postmenopausal women (1). Sclerostin is best known as an inhibitor of bone formation. Bone formation, as indicated by serum osteocalcin levels, has been shown to vary with season. Season influences several factors that have been associated with serum sclerostin levels; however, it is not known whether serum sclerostin levels vary with time of year. Although not observed consistently (2), in healthy postmenopausal women studied in Boston, Massachusetts, serum osteocalcin levels were 4%– 6% lower in winter/ spring than in summer/fall, whereas 24-hour urinary deoxypyridinoline, a marker of bone resorption, did not
S
vary with season (3). A similar seasonal pattern of osteocalcin was observed in young adults at the higher latitude of Denmark (4). If serum sclerostin tracked inversely with bone formation markers, as might be expected of an inhibitor of bone formation, and then sclerostin levels would be higher in winter than in the summer. Physical activity is known to influence serum sclerostin levels. Sclerostin levels decrease in response to mechanical strain (5, 6) and increase during bed rest (7). Any wintertime decreases in physical activity might therefore be expected to result in higher wintertime sclerostin levels. Serum PTH varies with season, with levels higher in winter than in summer, at least in subjects with vitamin D insufficiency. In a cross-sectional analysis of healthy post-
ISSN Print 0021-972X ISSN Online 1945-7197 Printed in U.S.A. Copyright © 2014 by The Endocrine Society Received August 13, 2013. Accepted November 5, 2013. First Published Online November 18, 2013
Abbreviation: 25OHD, 25-hydroxyvitamin D.
doi: 10.1210/jc.2013-3148
J Clin Endocrinol Metab, January 2014, 99(1):E149 –E152
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menopausal women studied in Boston, latitude 42 degrees, mean serum PTH levels varied from 3.8 pmol/L in late winter to 3.0 pmol/L in late summer (8). PTH is a negative regulator of serum sclerostin in ambient conditions (9) and when exogenous PTH is administered (1). Thus, wintertime increases in PTH would be expected to suppress serum sclerostin levels during the winter season. Currently it is unknown whether serum sclerostin levels in humans fluctuate with season and, if they do, in what pattern. There is one cross-sectional study comparing serum sclerostin levels in American black bears during two seasons (10). Sclerostin levels were higher in bears in the wintertime (during hibernation) than in bears measured in the spring. It is unclear whether the higher levels resulted from reduced mechanical loading, from seasonal influences, or other factors. The objective of this study was to measure serum sclerostin levels in a group of healthy older men and women to determine whether levels varied by season. It is important to understand the scope of seasonal variation in serum sclerostin because such variation needs to be considered in the design and interpretation of clinical studies in which it is measured. Additionally, understanding the cause of seasonal variation could provide some insight into the mechanisms of osteocyte function.
Materials and Methods This study was carried out using existing baseline data and new sclerostin measurements in serum archived at baseline from 314 healthy men and women aged 65 years and older who participated in our STOP/IT (Studies to stop bone loss from the hip) trial (11). Of 389 who completed the study, we excluded 14 nonwhite subjects because they have different PTH homeostasis (12), 21 with diabetes because they have higher sclerostin levels (13, 14) and subjects with no stored serum (n ⫽ 40). Subjects were asked
Table 1.
J Clin Endocrinol Metab, January 2014, 99(1):E149 –E152
not to take any supplemental calcium or vitamin D for 2 months prior to entry into the study. Subjects were enrolled at approximately an even rate over a 13-month period, from February 1992 through February 1993. For purposes of categorizing study visit dates by season, winter was defined as January through March, spring as April through June, summer as July through September, and fall as October through December. The study was approved by the Tufts Medical Center Human Investigation Review Committee, and all participants gave written informed consent. Criteria for exclusion were bone-altering conditions or medications, kidney or liver disease, and current cancer (see Reference 11 for a detailed list). Blood was collected between 7:00 and 9:00 AM after the subjects had fasted for at least 8 hours. Serum osteocalcin was measured during the study by immunoradiometric assay (Nichols Institute) with a coefficient of variation of 5.6 %-7.7%. Serum 25-hydroxyvitamin D (25OHD) was measured by the method of Preece et al (15) with coefficients of variation of 5.6%–7.7%. Serum sclerostin was batch analyzed in 2013 in serum archived at ⫺80°C for 20 years and not previously thawed. The samples were assayed on a MesoScale Discovery, using a proprietary combination of electrochemiluminescence detection and patterned arrays. The reference range for this assay is 18 –156 ng/L, the mean coefficient of variation of this assay is 4%, and the lower level of detection (defined as 2.5 SD above the background) is 1.1 ng/L. Leisure, household, and occupational activity was estimated with use of the Physical Activity Scale for the Elderly questionnaire (16).
Statistical methods Analyses were conducted with SPSS version 21.0 (IBM Corp). Baseline characteristics (Table 1) were compared across seasons with ANOVA (continuous variables) and 2 tests (categorical variables). Adjusted mean sclerostin values (Figure 1) were computed and compared with the Lsmeans option in the General Linear Models command. Values of P ⬍ .05 were considered to indicate statistical significance.
Results Mean serum sclerostin levels did not differ in the men and the women (30.1 and 28.1 ng/L, respectively, P ⫽ .290),
Clinical characteristics of the 314 subjects (Mean ⫾ SD)
N Age, y Females, % BMI, kg/m2 Calcium intake, mg/d eGFR, mL/min per 1.73 m2 PASE score Serum 25OHD, nmol/L Serum osteocalcin, nmol/L Serum PTH, pmol/L
Winter
Spring
Summer
Fall
88 70.9 ⫾ 4.9 55.7 26.3 ⫾ 3.8 753.5 ⫾ 417.9 75.9 ⫾ 15.5 105.1 ⫾ 60.7 63.28 ⫾ 23.55 1.15 ⫾ 0.37 3.7 ⫾ 1.5
83 70.9 ⫾ 4.0 72.3 26.4 ⫾ 3.7 772.6 ⫾ 318.4 77.3 ⫾ 12.2 120.8 ⫾ 54.1 68.56 ⫾ 31.67 1.14 ⫾ 0.39 4.3 ⫾ 1.8
66 71.5 ⫾ 4.3 56.1 26.7 ⫾ 4.5 714.5 ⫾ 308.1 76.2 ⫾ 14.2 123.7 ⫾ 43.4b 94.16 ⫾ 35.78 1.09 ⫾ 0.36 4.1 ⫾ 1.6
77 71.4 ⫾ 5.2 45.5a 27.2 ⫾ 3.4 691.7 ⫾ 355.0 74.5 ⫾ 13.9 116.3 ⫾ 52.0c 83.15 ⫾ 37d 1.06 ⫾ 0.38 4.1 ⫾ 2.0
Abbreviations: BMI, body mass index; eGFR, estimated glomerular filtration rate; PASE, Physical Activity Scale for the Elderly. a
P ⫽ .007.
b
n ⫽ 65.
c
n ⫽ 76.
d
P ⫽ ⬍ .001.
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doi: 10.1210/jc.2013-3148
Figure 1. Mean serum sclerostin levels by season, expressed as percentage difference from the all-year mean, adjusted for sex. Seasons marked by like letters differ at P ⱕ .002.
and sex did not modify the association of season with sclerostin (P ⫽ .247). Therefore, men and women were combined for the main analyses. Clinical characteristics of the 314 participants by season are shown in Table 1. Unadjusted mean (SEM) serum sclerostin levels were: 34.4 ⫾ 1.7 pmol/L in winter, 30.7 ⫾ 1.7 pmol/L in spring, 26.6 ⫾ 1.9 pmol/L in summer, and 23.0 ⫾ 1.8 in the fall (P ⬍ .001, R2 ⫽ 0.073). Serum sclerostin was not significantly correlated with serum osteocalcin, physical activity, serum PTH, or serum 25OHD (⫺0.10 ⬍ r ⬍ 0.10 for all), and adjustment for these factors and sex did not substantially alter the means. In the fully adjusted model, season remained highly statistically significant (P ⬍ .001). Seasonal levels of sclerostin, expressed as a percentage difference from the all-year mean and adjusted for sex, are shown in Figure 1.
Discussion This cross-sectional study documents dramatic seasonal variation in serum sclerostin levels. The amplitude of the fluctuation, with values 20% above the overall mean in winter and 20% below the mean in the fall, is greater than that reported for other bone turnover markers (2) or for serum PTH (8). Altogether, season accounted for about 7% of the variability in serum sclerostin. In this study, the differences in mean serum osteocalcin, physical activity, and PTH levels across season were in the expected pat-
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terns, but none was statistically significant, perhaps because the sample size was limited. Serum 25OHD did vary with season but in a pattern somewhat different from that of serum sclerostin. There was no evidence that any of these factors accounted for the observed seasonal differences in sclerostin levels in these subjects. A recent report revealed that commonly used sclerostin assays have different reference ranges and that this occurs because they detect different fragments of the sclerostin molecule (17). There are, however, significant correlations between sample cross-measured values by the MesoScale Discovery and the Biomedica and Teco assay kits (17). Thus, measures with the latter assays are also likely to exhibit seasonal variation. An important consideration is whether the changes in serum sclerostin levels with time of year could have resulted from breakdown of sclerostin during storage or other artifact. Several factors argue against this as the explanation. All sclerostin values were in the normal range of the assay (18 –156 ng/L). The samples were batch analyzed for sclerostin, so time lag between the assay of samples for each season was minimal. Finally, samples from the Study of Osteoporotic Fractures, which had been stored under conditions similar to our storage conditions (at ⫺80°C and not previously thawed) and for a similar period of 20 years, were found to have serum sclerostin values in the expected range, consistent with long-term stability of sclerostin in stored serum (18). Our leading explanation for the observed high-amplitude seasonal variation is that it is truly related to some aspect of season, possibly light or other circannual influence. Understanding the magnitude, basis, and latitude dependency of seasonal variation in serum sclerostin levels is important for several reasons. The presence of seasonality will influence the design of clinical research and the potential clinical utility of serum sclerostin measurements. We were able to assess the impact of several influencers of serum sclerostin levels, a bone turnover marker, physical activity, and serum PTH, to find that they were not, in fact, the drivers of the observed seasonality in this cross-sectional study. This leaves the explanation, and the opportunity to identify the factor(s) behind the observed variation, open for further investigation. Understanding the seasonal influences on serum sclerostin levels may lead to increased understanding of the regulation of osteocyte function. Finally, one would predict that the amplitude of seasonality is latitude dependent. It will be important to challenge or confirm our findings by describing seasonal excursion in serum sclerostin levels prospectively in other populations and at other latitudes. In conclusion, serum sclerostin levels appear to decline from winter to fall. The seasonality was not explained by
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the seasonal differences in the serum osteocalcin, a biochemical marker of bone formation, by physical activity, or by circulating PTH levels. The physiological basis for the apparent seasonal variation remains to be elucidated. Until the sources of biological variability in the serum sclerostin levels are better understood, the clinical utility of this measure remains uncertain.
J Clin Endocrinol Metab, January 2014, 99(1):E149 –E152
6.
7.
8.
Acknowledgments Address all correspondence and requests for reprints to: Bess DawsonHughes, MD, Jean Mayer United States Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, Massachusetts 02111. E-mail: [email protected]. This study was registered with the clinical trial number NCT00357643. Any opinions, findings, conclusion, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the US Department of Agriculture. This work was supported by Grant AG10353 from the National Institute on Aging, Amgen Inc, and the US Department of Agriculture, Agricultural Research Service, under Agreement 58 –1950-7–707. Disclosure Summary: The authors have nothing to disclose.
9.
10.
11.
12.
13.
14.
References 1. Drake MT, Srinivasan B, Modder UI, et al. Effects of parathyroid hormone treatment on circulating sclerostin levels in postmenopausal women. J Clin Endocrinol Metab. 2010;95:5056 –5062. 2. Woitge HW, Scheidt-Nave C, Kissling C, et al. Seasonal variation of biochemical indexes of bone turnover: results of a population-based study. J Clin Endocrinol Metab. 1998;83:68 –75. 3. Dawson-Hughes B, Harris SS, Krall EA, Dallal GE, Falconer G, Green CL. Rates of bone loss in postmenopausal women randomly assigned to one of two dosages of vitamin D. Am J Clin Nutr. 1995; 61:1140 –1145. 4. Thomsen K, Eriksen EF, Jorgensen JC, Charles P, Mosekilde L. Seasonal variation of serum bone GLA protein. Scand J Clin Lab Invest. 1989;49:605– 611. 5. Lin C, Jiang X, Dai Z, et al. Sclerostin mediates bone response to
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mechanical unloading through antagonizing Wnt/-catenin signaling. J Bone Miner Res. 2009;24:1651–1661. Ardawi MS, Rouzi AA, Qari MH. Physical activity in relation to serum sclerostin, insulin-like growth factor-1, and bone turnover markers in healthy premenopausal women: a cross-sectional and a longitudinal study. J Clin Endocrinol Metab. 2012;97:3691–3699. Gaudio A, Pennisi P, Bratengeier C, et al. Increased sclerostin serum levels associated with bone formation and resorption markers in patients with immobilization-induced bone loss. J Clin Endocrinol Metab. 2010;95:2248 –2253. Krall EA, Sahyoun N, Tannenbaum S, Dallal GE, Dawson-Hughes B. Effect of vitamin D intake on seasonal variations in parathyroid hormone secretion in postmenopausal women. N Engl J Med. 1989; 321:1777–1783. Bellido T, Ali AA, Gubrij I, et al. Chronic elevation of parathyroid hormone in mice reduces expression of sclerostin by osteocytes: a novel mechanism for hormonal control of osteoblastogenesis. Endocrinology. 2005;146:4577– 4583. Seger RL, Cross RA, Rosen CJ, et al. Investigating the mechanism for maintaining eucalcemia despite immobility and anuria in the hibernating American black bear (Ursus americanus). Bone. 2011;49: 1205–1212. Dawson-Hughes B, Harris SS, Krall EA, Dallal GE. Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older. N Engl J Med. 1997;337:670 – 676. Fuleihan GE, Gundberg CM, Gleason R, et al. Racial differences in parathyroid hormone dynamics. J Clin Endocrinol Metab. 1994; 79:1642–1647. Garcia-Martin A, Rozas-Moreno P, Reyes-Garcia R, et al. Circulating levels of sclerostin are increased in patients with type 2 diabetes mellitus. J Clin Endocrinol Metab. 2012;97:234 –241. Gennari L, Merlotti D, Valenti R, et al. Circulating sclerostin levels and bone turnover in type 1 and type 2 diabetes. J Clin Endocrinol Metab. 2012;97:1737–1744. Preece MA, O’Riordan JL, Lawson DE, Kodicek E. A competitive protein-binding assay for 25-hydroxycholecalciferol and 25-hydroxyergocalciferol in serum. Clin Chim Acta. 1974;54:235–242. Washburn RA, Smith KW, Jette AM, Janney CA. The Physical Activity Scale for the Elderly (PASE): development and evaluation. J Clin Epidemiol. 1993;46:153–162. Durosier C, Van Lierop A, Ferrari S, Chevalley T, Papapoulos S, Rizzoli R. Association of circulating sclerostin with bone mineral mass, microstructure, and turnover biochemical markers in healthy elderly men and women. J Clin Endocrinol Metab. 2013;98(9): 3873–3883. Arasu A, Cawthon PM, Lui LY, et al. Serum sclerostin and risk of hip fracture in older Caucasian women. J Clin Endocrinol Metab. 2012;97:2027–2032.
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R e p o r t — E n d o c r i n e
O N L I N E
R e s e a r c h
Serum Sclerostin Levels Vary With Season Bess Dawson-Hughes, Susan S. Harris, Lisa Ceglia, and Nancy J. Palermo Jean Mayer United States Department of Agriculture Human Nutrition Research Center on Aging (B.D.H., S.S.H., L.C., N.J.P.), Tufts University, and Division of Endocrinology, Diabetes, and Metabolism (B.D.H., L.C.), Tufts Medical Center, Boston, Massachusetts 02111
Context: To establish the clinical utility of serum sclerostin levels, it is important to know whether there is seasonal variation in the measurements. Objective: This study was done to determine whether serum sclerostin levels vary by season in healthy older men and women. Methods: Serum sclerostin levels were measured in archived serum of 314 healthy men and women aged 65 years and older and examined for seasonal variation. Several factors known to vary by season and previously reported to be associated with serum sclerostin levels, including serum osteocalcin, physical activity, and serum PTH levels, were also measured in these subjects. Sex did not modify the association of season with sclerostin, so the men and women were analyzed together. Results: Serum sclerostin levels varied significantly by season (P ⬍ .001, after adjustment for sex). Sclerostin levels in the wintertime were 20% higher than the all-year mean, the levels gradually declined through the spring and summer, and by the fall, they were 20% below the all-year mean. Adjustment for serum osteocalcin, physical activity, and serum PTH did not alter the seasonal means. Seasonal differences in serum osteocalcin, physical activity, and serum PTH were not statistically significant. Conclusions: This study documents marked seasonal variation in serum sclerostin levels. It is important to recognize this source of biological variability when considering the potential clinical utility of sclerostin measurements. (J Clin Endocrinol Metab 99: E149 –E152, 2014)
clerostin is a glycoprotein produced by osteocytes. Serum sclerostin levels have been strongly correlated with bone marrow plasma sclerostin in postmenopausal women (1). Sclerostin is best known as an inhibitor of bone formation. Bone formation, as indicated by serum osteocalcin levels, has been shown to vary with season. Season influences several factors that have been associated with serum sclerostin levels; however, it is not known whether serum sclerostin levels vary with time of year. Although not observed consistently (2), in healthy postmenopausal women studied in Boston, Massachusetts, serum osteocalcin levels were 4%– 6% lower in winter/ spring than in summer/fall, whereas 24-hour urinary deoxypyridinoline, a marker of bone resorption, did not
S
vary with season (3). A similar seasonal pattern of osteocalcin was observed in young adults at the higher latitude of Denmark (4). If serum sclerostin tracked inversely with bone formation markers, as might be expected of an inhibitor of bone formation, and then sclerostin levels would be higher in winter than in the summer. Physical activity is known to influence serum sclerostin levels. Sclerostin levels decrease in response to mechanical strain (5, 6) and increase during bed rest (7). Any wintertime decreases in physical activity might therefore be expected to result in higher wintertime sclerostin levels. Serum PTH varies with season, with levels higher in winter than in summer, at least in subjects with vitamin D insufficiency. In a cross-sectional analysis of healthy post-
ISSN Print 0021-972X ISSN Online 1945-7197 Printed in U.S.A. Copyright © 2014 by The Endocrine Society Received August 13, 2013. Accepted November 5, 2013. First Published Online November 18, 2013
Abbreviation: 25OHD, 25-hydroxyvitamin D.
doi: 10.1210/jc.2013-3148
J Clin Endocrinol Metab, January 2014, 99(1):E149 –E152
jcem.endojournals.org
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menopausal women studied in Boston, latitude 42 degrees, mean serum PTH levels varied from 3.8 pmol/L in late winter to 3.0 pmol/L in late summer (8). PTH is a negative regulator of serum sclerostin in ambient conditions (9) and when exogenous PTH is administered (1). Thus, wintertime increases in PTH would be expected to suppress serum sclerostin levels during the winter season. Currently it is unknown whether serum sclerostin levels in humans fluctuate with season and, if they do, in what pattern. There is one cross-sectional study comparing serum sclerostin levels in American black bears during two seasons (10). Sclerostin levels were higher in bears in the wintertime (during hibernation) than in bears measured in the spring. It is unclear whether the higher levels resulted from reduced mechanical loading, from seasonal influences, or other factors. The objective of this study was to measure serum sclerostin levels in a group of healthy older men and women to determine whether levels varied by season. It is important to understand the scope of seasonal variation in serum sclerostin because such variation needs to be considered in the design and interpretation of clinical studies in which it is measured. Additionally, understanding the cause of seasonal variation could provide some insight into the mechanisms of osteocyte function.
Materials and Methods This study was carried out using existing baseline data and new sclerostin measurements in serum archived at baseline from 314 healthy men and women aged 65 years and older who participated in our STOP/IT (Studies to stop bone loss from the hip) trial (11). Of 389 who completed the study, we excluded 14 nonwhite subjects because they have different PTH homeostasis (12), 21 with diabetes because they have higher sclerostin levels (13, 14) and subjects with no stored serum (n ⫽ 40). Subjects were asked
Table 1.
J Clin Endocrinol Metab, January 2014, 99(1):E149 –E152
not to take any supplemental calcium or vitamin D for 2 months prior to entry into the study. Subjects were enrolled at approximately an even rate over a 13-month period, from February 1992 through February 1993. For purposes of categorizing study visit dates by season, winter was defined as January through March, spring as April through June, summer as July through September, and fall as October through December. The study was approved by the Tufts Medical Center Human Investigation Review Committee, and all participants gave written informed consent. Criteria for exclusion were bone-altering conditions or medications, kidney or liver disease, and current cancer (see Reference 11 for a detailed list). Blood was collected between 7:00 and 9:00 AM after the subjects had fasted for at least 8 hours. Serum osteocalcin was measured during the study by immunoradiometric assay (Nichols Institute) with a coefficient of variation of 5.6 %-7.7%. Serum 25-hydroxyvitamin D (25OHD) was measured by the method of Preece et al (15) with coefficients of variation of 5.6%–7.7%. Serum sclerostin was batch analyzed in 2013 in serum archived at ⫺80°C for 20 years and not previously thawed. The samples were assayed on a MesoScale Discovery, using a proprietary combination of electrochemiluminescence detection and patterned arrays. The reference range for this assay is 18 –156 ng/L, the mean coefficient of variation of this assay is 4%, and the lower level of detection (defined as 2.5 SD above the background) is 1.1 ng/L. Leisure, household, and occupational activity was estimated with use of the Physical Activity Scale for the Elderly questionnaire (16).
Statistical methods Analyses were conducted with SPSS version 21.0 (IBM Corp). Baseline characteristics (Table 1) were compared across seasons with ANOVA (continuous variables) and 2 tests (categorical variables). Adjusted mean sclerostin values (Figure 1) were computed and compared with the Lsmeans option in the General Linear Models command. Values of P ⬍ .05 were considered to indicate statistical significance.
Results Mean serum sclerostin levels did not differ in the men and the women (30.1 and 28.1 ng/L, respectively, P ⫽ .290),
Clinical characteristics of the 314 subjects (Mean ⫾ SD)
N Age, y Females, % BMI, kg/m2 Calcium intake, mg/d eGFR, mL/min per 1.73 m2 PASE score Serum 25OHD, nmol/L Serum osteocalcin, nmol/L Serum PTH, pmol/L
Winter
Spring
Summer
Fall
88 70.9 ⫾ 4.9 55.7 26.3 ⫾ 3.8 753.5 ⫾ 417.9 75.9 ⫾ 15.5 105.1 ⫾ 60.7 63.28 ⫾ 23.55 1.15 ⫾ 0.37 3.7 ⫾ 1.5
83 70.9 ⫾ 4.0 72.3 26.4 ⫾ 3.7 772.6 ⫾ 318.4 77.3 ⫾ 12.2 120.8 ⫾ 54.1 68.56 ⫾ 31.67 1.14 ⫾ 0.39 4.3 ⫾ 1.8
66 71.5 ⫾ 4.3 56.1 26.7 ⫾ 4.5 714.5 ⫾ 308.1 76.2 ⫾ 14.2 123.7 ⫾ 43.4b 94.16 ⫾ 35.78 1.09 ⫾ 0.36 4.1 ⫾ 1.6
77 71.4 ⫾ 5.2 45.5a 27.2 ⫾ 3.4 691.7 ⫾ 355.0 74.5 ⫾ 13.9 116.3 ⫾ 52.0c 83.15 ⫾ 37d 1.06 ⫾ 0.38 4.1 ⫾ 2.0
Abbreviations: BMI, body mass index; eGFR, estimated glomerular filtration rate; PASE, Physical Activity Scale for the Elderly. a
P ⫽ .007.
b
n ⫽ 65.
c
n ⫽ 76.
d
P ⫽ ⬍ .001.
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doi: 10.1210/jc.2013-3148
Figure 1. Mean serum sclerostin levels by season, expressed as percentage difference from the all-year mean, adjusted for sex. Seasons marked by like letters differ at P ⱕ .002.
and sex did not modify the association of season with sclerostin (P ⫽ .247). Therefore, men and women were combined for the main analyses. Clinical characteristics of the 314 participants by season are shown in Table 1. Unadjusted mean (SEM) serum sclerostin levels were: 34.4 ⫾ 1.7 pmol/L in winter, 30.7 ⫾ 1.7 pmol/L in spring, 26.6 ⫾ 1.9 pmol/L in summer, and 23.0 ⫾ 1.8 in the fall (P ⬍ .001, R2 ⫽ 0.073). Serum sclerostin was not significantly correlated with serum osteocalcin, physical activity, serum PTH, or serum 25OHD (⫺0.10 ⬍ r ⬍ 0.10 for all), and adjustment for these factors and sex did not substantially alter the means. In the fully adjusted model, season remained highly statistically significant (P ⬍ .001). Seasonal levels of sclerostin, expressed as a percentage difference from the all-year mean and adjusted for sex, are shown in Figure 1.
Discussion This cross-sectional study documents dramatic seasonal variation in serum sclerostin levels. The amplitude of the fluctuation, with values 20% above the overall mean in winter and 20% below the mean in the fall, is greater than that reported for other bone turnover markers (2) or for serum PTH (8). Altogether, season accounted for about 7% of the variability in serum sclerostin. In this study, the differences in mean serum osteocalcin, physical activity, and PTH levels across season were in the expected pat-
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terns, but none was statistically significant, perhaps because the sample size was limited. Serum 25OHD did vary with season but in a pattern somewhat different from that of serum sclerostin. There was no evidence that any of these factors accounted for the observed seasonal differences in sclerostin levels in these subjects. A recent report revealed that commonly used sclerostin assays have different reference ranges and that this occurs because they detect different fragments of the sclerostin molecule (17). There are, however, significant correlations between sample cross-measured values by the MesoScale Discovery and the Biomedica and Teco assay kits (17). Thus, measures with the latter assays are also likely to exhibit seasonal variation. An important consideration is whether the changes in serum sclerostin levels with time of year could have resulted from breakdown of sclerostin during storage or other artifact. Several factors argue against this as the explanation. All sclerostin values were in the normal range of the assay (18 –156 ng/L). The samples were batch analyzed for sclerostin, so time lag between the assay of samples for each season was minimal. Finally, samples from the Study of Osteoporotic Fractures, which had been stored under conditions similar to our storage conditions (at ⫺80°C and not previously thawed) and for a similar period of 20 years, were found to have serum sclerostin values in the expected range, consistent with long-term stability of sclerostin in stored serum (18). Our leading explanation for the observed high-amplitude seasonal variation is that it is truly related to some aspect of season, possibly light or other circannual influence. Understanding the magnitude, basis, and latitude dependency of seasonal variation in serum sclerostin levels is important for several reasons. The presence of seasonality will influence the design of clinical research and the potential clinical utility of serum sclerostin measurements. We were able to assess the impact of several influencers of serum sclerostin levels, a bone turnover marker, physical activity, and serum PTH, to find that they were not, in fact, the drivers of the observed seasonality in this cross-sectional study. This leaves the explanation, and the opportunity to identify the factor(s) behind the observed variation, open for further investigation. Understanding the seasonal influences on serum sclerostin levels may lead to increased understanding of the regulation of osteocyte function. Finally, one would predict that the amplitude of seasonality is latitude dependent. It will be important to challenge or confirm our findings by describing seasonal excursion in serum sclerostin levels prospectively in other populations and at other latitudes. In conclusion, serum sclerostin levels appear to decline from winter to fall. The seasonality was not explained by
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Serum Sclerostin and Season
the seasonal differences in the serum osteocalcin, a biochemical marker of bone formation, by physical activity, or by circulating PTH levels. The physiological basis for the apparent seasonal variation remains to be elucidated. Until the sources of biological variability in the serum sclerostin levels are better understood, the clinical utility of this measure remains uncertain.
J Clin Endocrinol Metab, January 2014, 99(1):E149 –E152
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Acknowledgments Address all correspondence and requests for reprints to: Bess DawsonHughes, MD, Jean Mayer United States Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, Massachusetts 02111. E-mail: [email protected]. This study was registered with the clinical trial number NCT00357643. Any opinions, findings, conclusion, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the US Department of Agriculture. This work was supported by Grant AG10353 from the National Institute on Aging, Amgen Inc, and the US Department of Agriculture, Agricultural Research Service, under Agreement 58 –1950-7–707. Disclosure Summary: The authors have nothing to disclose.
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