Context: The effect of vitamin D on muscle strength in adults is not established.

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1 ORIGINAL Endocrine ARTICLE Research Does Vitamin D Improve Muscle Strength in Adults? A Randomized, Double-blind, Placebo-controlled Trial Among Ethnic Minorities in Norway Kirsten V. Knutsen, Ahmed A. Madar, Per Lagerløv, Mette Brekke, Truls Raastad, Lars C. Stene, and Haakon E. Meyer Department of General Practice (K.V.K., P.L., M.B.), Institute of Health and Society, University of Oslo, Oslo N-0318, Norway; Department of Community Medicine (A.A.M., H.E.M.), Institute of Health and Society, University of Oslo, Oslo N-0318, Norway; Division of Epidemiology (L.C.S., H.E.M.), Norwegian Institute of Public Health, Oslo N-0403, Norway; and Norwegian School of Sport Sciences (T.R.), Oslo N-0806, Norway Context: The effect of vitamin D on muscle strength in adults is not established. Objective: Our objective was to test whether vitamin D supplementation increases muscle strength and power compared with placebo. Design: We conducted a randomized, double-blind, placebo-controlled trial. Setting: The setting was immigrants activity centers. Participants: Two hundred fifty-one healthy adult males and females aged years with non- Western immigrant background performed the baseline test and 86% returned to the follow-up test. Interventions: Sixteen weeks of daily supplementation with 25 g (1000 IU) vitamin D 3,10 g (400 IU) vitamin D 3, or placebo. Main Outcome Measures: Difference in jump height between pre- and postintervention. Secondary outcomes were differences in handgrip strength and chair-rising test. Results: Percentage change in jump height did not differ between those receiving vitamin D (25 or 10 g vitamin D 3 ) and those receiving placebo (mean difference 1.4%, 95% confidence interval: 4.9% to 2.2%, P.44). No significant effect was detected in the subgroup randomized to 25 g vitamin D or in other preplanned subgroup analyses nor were there any significant differences in handgrip strength or the chair-rising test. Mean serum 25-hydroxyvitamin D 3 concentration increased from 27 to 52 nmol/l and from 27 to 43 nmol/l in the 25 and 10 g supplementation groups, respectively, whereas serum 25-hydroxyvitamin D 3 did not change in the placebo group. Conclusions: Daily supplementation with 25 or 10 g vitamin D 3 for 16 weeks did not improve muscle strength or power measured by the jump test, handgrip test, or chair-rising test in this population with low baseline vitamin D status. (J Clin Endocrinol Metab 99: , 2014) Vitamin D has multiple biological effects, some of which appear to impact muscular functioning. The optimal range of vitamin D is still a matter of debate, but ISSN Print X ISSN Online Printed in U.S.A. Copyright 2014 by The Endocrine Society Received June 27, Accepted November 1, First Published Online November 18, 2013 serum 25-hydroxyvitamin D (25-[OH]D) levels of 50 nmol/l or more are considered sufficient by the Institute of Medicine, whereas some experts recommend higher Abbreviation: CI, confidence interval. 194 jcem.endojournals.org J Clin Endocrinol Metab, January 2014, 99(1): doi: /jc

2 doi: /jc jcem.endojournals.org 195 concentrations of 75 nmol/l (1, 2). An intake between 10 and 20 g per day is likely to increase serum levels to at least 50 nmol/l in most individuals (1, 3). Prolonged vitamin D deficiency may lead to osteomalacia in adults and to the classical syndrome of rickets in children with impaired, immature bone development and muscle weakness (rickety myopathy). This myopathy is abolished by vitamin D treatment (4 7). Among older adults, low vitamin D status has been related to low bone density, osteoporotic fractures, and an increased risk of falls. According to recent Cochrane reviews, vitamin D supplementation can reduce the risk of falling in institutionalized elderly and provisional evidence suggests that it may also reduce the risk of falling in noninstitutionalized elderly low in vitamin D (8, 9). The mechanisms behind this observation are not well described, but it is likely that an effect of vitamin D on muscular function is involved (10, 11). Although controversial, such effects are possibly explained by the presence of vitamin D receptors in human muscle and by the improvement of type II muscle atrophy in vitamin D deficient patients after supplementation (12 16). Type II fibers are fast-twitch muscle fibers and their reaction is probably necessary for preventing a fall. Vitamin D also has an important role in the regulation of calcium transport and protein synthesis in muscle cells (17 19). Vitamin D deficiency is prevalent worldwide. In many populations, including in Norway, vitamin D deficiency is particularly prevalent among immigrants from Africa, the Middle East, and Asian countries (20 22). In a prior study from Oslo, we found that 83% of ethnic minorities seeking treatment from a general practitioner for muscular pain, fatigue, or headache had serum 25-(OH)D 3 concentrations less than 50 nmol/l, and one in two women had serum 25-(OH)D 3 concentrations less than 25 nmol/l (23). Randomized studies of the effect of vitamin D supplementation on muscle function in adults have primarily focused on subjects older than 50 years of age (24, 25). Meta-analyses of randomized, controlled trials have shown a probable effect of vitamin D supplementation (daily doses of 20 to 25 g) on strength and balance among subjects over 60 years of age (25, 26). A limited number of studies have demonstrated an increase in hip muscular strength when baseline serum 25-(OH)D was less than 25 nmol/l (24 26). Clinically, we have observed that some patients with low vitamin D levels report improvement in physical functioning after supplementation with vitamin D. Based on this clinical observations and the lack of available randomized studies among adults, we designed a randomized, double-blind, placebo-controlled study to investigate the potential effects of vitamin D supplementation in adults, aged 18 to 50 years (23, 24). The objective of the present study was to test whether 16 weeks of daily oral vitamin D 3 supplementation (25 or 10 g) compared with placebo would improve muscle strength and power among ethnic minority groups aged 18 to 50 years with a presumed low level of vitamin D. Muscle strength and power were measured by a jump test, as well as a chair-rising test and a handgrip test. Materials and Methods Trial design The study was a block randomized, double-blind, placebocontrolled, parallel-group trial with three equally sized groups. Participants Eligible participants were healthy adult immigrants, men and women, aged years, who lived in Oslo but were born in or had parents born in the Middle East, Africa, or South Asia. They were recruited through local immigrant organizations web sites or through local radio. Informational meetings took place at adult education schools, local activity centers for immigrants, and mosques. Those who agreed to participate provided written consent. Participants were excluded if they regularly used vitamin D containing supplements, were receiving treatment for vitamin D deficiency, were pregnant or breastfeeding, had known malabsorption, used medication interfering with vitamin D metabolism (eg, thiazides, antiepileptic drugs, prednisolone, or hormone replacement therapy), had kidney disease, cancer, tuberculosis, sarcoidosis, osteoporosis, or a recent fracture, or used strong painkillers. Those who fulfilled the eligibility criteria and consented to participate were enrolled in the study. Study setting The study took place at 11 different local immigrants activity centers in Oslo, Norway. Recruitment and baseline assessment took place from January 20 to March 1, After 16 weeks (range 15 19, median 16), follow-up assessments were conducted from May 12 to June 21, The same data collection team visited all the centers and performed the baseline and follow-up data collection. Interpreters were used when necessary, but most of the study participants were able to communicate in the Norwegian language. Interventions Participants were randomly assigned to one of three groups for daily oral intake of tablets containing either 25 g of vitamin D 3,10 g of vitamin D 3, or placebo (no vitamin D) over 16 weeks. The tablets were manufactured by BioPlus Life Sciences Pvt Ltd and were identical in color, size, taste, and packaging. Each participant received a box with 120 tablets and we instructed the participants to take two tablets the following day if they forgot to take one tablet. We included an intervention group with 10 g per day of vitamin D 3 because this is the recommended daily intake according to official Norwegian guidelines (27). Participants were contacted with a mobile phone text message twice a week during the study period to maximize adherence. Close to the end of the intervention period, the participants re-

3 196 Knutsen et al Does Vitamin D Improve Muscle Strength? J Clin Endocrinol Metab, January 2014, 99(1): ceived a short text message or/and a telephone call to remind them of the follow-up appointment. Participants were advised to maintain their usual dietary pattern during the study period and were further advised to contact the study staff by telephone if they had questions related to the study. Outcomes Muscular strength and power were assessed using three different tests that measured different aspects of muscular function. The primary outcome was the mean difference between the combined intervention groups and the placebo group in percentage increase in maximum jump height from baseline to the end of the study. Jump height was measured using a counter movement jump test, with participants on a platform (FP4; HUR Labs OY), similar to prior research (28). This test specifically measures the strength and power in the proximal muscles. Participants were instructed to start in an upright position, with their hands on the hips, then to flex the hips and knees to a self-selected depth, and jump as high as possible. Jump heights were calculated from the vertical reaction force impulse during takeoff, and the average of the two highest among a total of five jumps was used in the data analysis. Secondary outcomes were corresponding differences in improvement in a chair-rising test and a handgrip test between the combined intervention groups vs placebo. The chair-rising test was used as a measure of proximal leg muscle strength and power (29). Participants were instructed to rise from a sitting position and to sit down again five times without using the arms, as fast as possible. This was repeated and the time (seconds) spent in the faster of the two trials was used in the analysis. Handgrip strength was tested using a hand-held dynamometer in the dominant hand (type baseline 90 kg; Chattanoog Group), in a standing position, with the arm parallel to the trunk (30). The test was performed twice and the highest value in kilograms was used in the analysis. Blood sampling and handling Nonfasting blood samples were collected at both the baseline and the follow-up examination. Blood for serum was collected in serum-separator gel tubes and centrifuged after 30 minutes to 2 hours. Blood for plasma was collected in EDTA tubes and centrifuged within 30 minutes at room temperature at the study site. Serum and plasma were separated and frozen in several aliquots at 20 C the same day and within 1 2 weeks frozen at 80 C. All samples were stored at 80 C until they were analyzed in one batch. Laboratory assays All laboratory analyses were done at Fürst Medical Laboratory in Oslo, Norway ( This laboratory is accredited by the International Organization for Standardization and is part of vitamin D quality assessment scheme, DEQAS. Serum 25-hydroxyvitamin D was measured using HPLC tandem mass spectrometry, with Waters Acquity UPLC and Waters triple quadrupole mass spectrometry instruments. Both 25- (OH)D 2 and -D 3 were measured, but -D 2 levels are generally negligible among adults in Norway and D 3 was used for analysis. The within batch coefficient of variation for serum 25-(OH)D 3 was 4.8% at higher concentrations (186 nmol/l) and 7.2% within lower concentrations (55 nmol/l). Plasma PTH was analyzed with Advia Centaur XP (Siemens HMSD) with a coefficient of variation of 8.2%. Serum levels of calcium, albumincorrected calcium, and alkaline phosphates were measured using Advia 2400 (Siemens). Other measurements and variables Body weight was measured with a Bosogramm 3000 scale (loading capacity 150 kg) to the nearest 100 g with participants in indoor clothing without shoes. Height was measured to the nearest centimeter with a rigid meter standard. The same devices were used both at baseline and at follow up. Background information about age and ethnicity was collected at baseline. Information about compliance and acceptability/experience of the study subjects was collected at follow-up. Randomization We chose a computer-generated block randomization to ensure a good balance of the number in each group during the trial and randomly varying the block size between 3 and 6. Blinding Group allocation was unknown to participants, research staff, investigators, and data collectors. Data analyses were also blinded. The tablet boxes were numbered according to the randomization list by an external pharmacy (the Hospital Pharmacy at Oslo University Hospital, Rikshospitalet, Sykehusapotekene HF). The group allocation list was stored at this pharmacy with a copy in a sealed envelope. Each participant was consecutively numbered and received a prepackaged tablet box with the corresponding number. The analyses of the primary outcome measures and the evaluation of the physical performance tests were done before the randomization list was opened. At the end, the results of serum 25-(OH)D and plasma PTH analyses at follow-up were unmasked. Registration The Norwegian Medicine Agency authorized the conduct of this study as a clinical trial. The study has been registered at EudraCT ( ). Tablets were manufactured by BioPlus Life Sciences Pvt Ltd, DMA certificated for good manufacturing practice, and the ingredients met the requirements of British Pharmacopé (ClinicalTrials.gov Identifier: NCT ). Statistical analysis Before the study started, the power calculations were performed to estimate the sample size needed to obtain at least 80% power with an level of 5% to detect a five percentage point increase in the jump test in the two combined equally sized groups receiving 25 or 10 g vitamin D per day compared with placebo. Based on prior research, we assumed that the SD for the difference in percentage increase in jump height was 12% (31). A total sample size of 210 was estimated as necessary to provide 81% power for the primary analysis. As we assumed a certain loss to follow-up, we aimed to recruit at least 250 participants. The statistical analyses were done using t tests, bivariate correlation, ANOVA, and linear regression using SPSS (IBM SPSS Statistics, version 20). An level of P.05 was considered statistically significant. Analyses were performed for participants who completed the follow-up, regardless of the level of compliance with supplements. The participants who did not par-

4 doi: /jc jcem.endojournals.org 197 Table 1. Baseline Characteristic for the Three Study Arms Among the 251 Participants Who Completed the Baseline Tests, Number (%), and Mean (SD) Characteristic Vitamin D 3, 25 g (n 84) Vitamin D 3, 10 g (n 85) Placebo (n 82) Sex, female 58 (69%) 61 (72%) 63 (77%) Age, y 36 (8.2) 37 (7.6) 39 (7.6) Female 35 (7.5) 36 (7.8) 38 (7.6) Male 40 (9.1) 40 (6.6) 39 (7.8) Body mass index 27.0 (5.2) 27.5 (5.2) 27.8 (5.0) Female 27.7 (5.3) 27.9 (5.7) 28.9 (5.0) Male 25.0 (4.6) 26.3 (3.0) 24.5 (3,3) Living in Norway, y 13 (6.7) 13 (6.8) 14 (6.7) Female 12 (6.0) 12 (6.4) 13 (6.0) Male 16 (7.6) 16 (7.1) 16 (8.3) Region of origin South Asia 31 (37%) 31 (36%) 33 (40%) Middle East and North Africa 15 (18%) 9 (11%) 12 (15%) Africa South of Sahara 38 (45%) 45 (53%) 37 (45%) Serum 25-(OH)D 3, nmol/l 27 (16) 26 (15) 27 (15) Female 27 (17) 25 (15) 26 (15) Male 28 (15) 27 (15) 29 (16) Plasma PTH, pmol/l b 7.5 (3.7) 7.3 (3.4) 8.0 (4.0) Serum corrected calcium, mmol/l a 2.31 (0.09) 2.30 (0.07) 2.30 (0.08) Serum calcium, mmol/l 2.37 (0.09) 2.36 (0.08) 2.36 (0.10) Serum alkaline phosphatase, U/L 83 (28) 77 (22) 76 (24) Jump height, cm b 18.3 (6.0) 17.3 (6.5) 16.8 (6.8) Female c 15.6 (4.3) 14.6 (4.8) 14.2 (4.4) Male c 24.4 (4.7) 23.9 (5.2) 25.4 (6.3) Chair rising test, s b,d (2.73) (2.66) (2.33) Female c (3.15) (2.92) (2.45) Male c (1.43) (1.47) (1.88) Handgrip strength, kg 29 (10) 28 (9) 28 (9) Female c 25 (7) 24 (5) 25 (5) Male c 39 (8) 40 (8) 40 (8) Reference range: serum 25-(OH)D nmol/l, plasma PTH: pmol/l, serum calcium mmol/l, serum albumin corrected calcium mmol/l, serum alkaline phosphatase 105 U/L. a Serum calcium corrected for albumin. b Missing data for three to five participants. c Test difference at baseline between female and male for: Jump height and handgrip strength, P.0001, chair-rising test, P.042. d Seconds to complete five rises from a chair. ticipate in the follow-up were few (14%) and equally distributed between the three groups. All 251 participants were included in the baseline analyses presented in Table 1. Subgroup analyses prespecified in the protocol were supplemental dose, gender, region of origin, age groups, and baseline serum 25-(OH)D 3 above or below 25 nmol/l. Ethics The study was approved by the Regional Committee for Medical and Health Research Ethics (study code: 2010/1982). Participation was based on written consent. Results Of the 251 participants, 215 (86%) attended the follow-up examination, and 202 (81%) had valid jump tests both at baseline and at follow up (Figure 1). Baseline analysis revealed no difference between those lost to follow-up and those who completed the study. There were no substantial differences in baseline characteristics between the three study groups (Table 1). Mean concentration of serum 25-(OH)D 3 was 26 nmol/l (range 5 87 nmol/l). A total of 8% had a serum concentration 50 nmol/l and 42% had a serum concentration 25 nmol/l (female 41% and male 46%). The concentration of serum 25-(OH)D 2 was negligible (mean 2.5 nmol/l). Median plasma PTH level was within reference values; however, 31% (76/246) were above the upper reference value ( pmol/l). At follow-up, 84% (180/215) of the participants returned the tablet box (which had contained 120 tablets at baseline). Assuming that not returned tablets in the tablet boxes had been consumed, median tablet intake was 108 (range 2 120). Eighty percent had consumed 80% of the

5 198 Knutsen et al Does Vitamin D Improve Muscle Strength? J Clin Endocrinol Metab, January 2014, 99(1): Figure 1. Flow diagram. Effect of vitamin D supplementation on physcial performance among ethnic minorities in Norway. Randomized, doubleblind, placebo-controlled study. tablets and 69% had consumed 90% of the tablets (of a total of 112 tablets for the 16-week study period). During the 16-week intervention, serum 25-(OH)D 3 increased and plasma PTH and serum alkaline phosphatase decreased in the intervention groups but not in the placebo group, and there was no detectable change in serum calcium level for any of the groups (Table 2). The percentage of participants who had a serum 25-(OH)D 3 level 50 Table 2. Serum Concentrations, Mean (SD) of 25(OH)D 3, PTH, Calcium, and Alkaline Phosphatase in the Two Treatment groups and in the Placebo Group Before and After Intervention (n 215) Baseline 16 weeks Change Serum 25(OH)D 3, nmol/l 25 g Vitamin D 27 (17) 52 (20) 25 (22) a 10 g Vitamin D 27 (15) 43 (17) 16 (20) a Placebo 27 (15) 25 (12) 2 (10) Plasma PTH, pmol/l 25 g Vitamin D 7.6 (3.9) 6.1 (2.5) 1.5 (3.2) a 10 g Vitamin D 7.1 (3.5) 6.1 (2.1) 1.1 (2.7) b Placebo 7.8 (3.3) 8.2 (4.0) 0.4 (3.0) Serum corrected calcium, mmol/l d 25 g Vitamin D 2.32 (0.09) 2.35 (0.09) 0.03 (0.07) 10 g Vitamin D 2.31 (0.06) 2.32 (0.11) 0.02 (0.11) Placebo 2.30 (0.08) 2.34 (0.09) 0.03 (0.06) Serum calcium, mmol/l 25 g Vitamin D 2.37 (0.09) 2.38 (0.10) 0.01 (0.09) 10 g Vitamin D 2.37 (0.08) 2.35 (0.13) 0.01 (0.13) Placebo 2.36 (0.10) 2.37 (0.11) 0.01 (0.08) Serum alkaline phosphatase, U/L 25 g Vitamin D 82 (28) 79 (27) 3 (10) c 10 g Vitamin D 75 (21) 71 (19) 4 (8) b Placebo 75 (23) 76 (27) 1 (11) Two hundred fifteen came to follow-up; analysis is missing for one to five persons. Treatment compared with placebo: a P.001; b P.01; c P.05. d Corrected for serum albumin.

6 doi: /jc jcem.endojournals.org 199 Table 3. Jumping Height, Handgrip Strength, and Time on the Chair-Rising Test in the Two Treatment Groups and in the Placebo Group at Baseline and at the End of Intervention Physical Performance, Absolute Figures (cm, kg, or s) Physical Performance, Percent Change N Baseline Mean (SD) 16 wk Mean (SD) Difference Mean (SD) Mean (SD) Change Difference (95%CI) Compared with Placebo P Value a Jumping height, cm 25 g Vitamin D (6.8) 19.6 (6.8) 0.8 (1.7) 4.6 (11.3) 0.4 ( 5.1 to 4.4) g Vitamin D (6.3) 17.8 (6.3) 0.3 (1.5) 2.6 (10.4) 2.4 ( 7.2 to 2.4).24 Placebo (6.3) 17.0 (6.8) 0.6 (1.7) 5.0 (12.6) Reference Reference Handgrip strength, kg 25 g Vitamin D (10) 28 (9) 1.03 (3.84) 1.78 (13.72) 0.21 ( 4.42 to 4.85) g Vitamin D (9) 26 (9) 1.51 (3.56) 4.56 (11.75) 2.57 ( 7.00 to 1.86).25 Placebo (9) 27 (8) 0.85 (3.81) 1.99 (14.63) Reference Reference Chair test, s 25 g Vitamin D (2.83) (2.90) 1.45 (1.88) (12.87) 0.02 ( 3.97 to 4.00) g Vitamin D (2.74) 10.74) (2.19) 1.59 (1.54) (9.98) 0.53 ( 4.09 t0 3.04).77 Placebo (2.43) 10.49) (2.06) 1.47 (1.54) (10.82) Reference Reference Jump height test: Of the 215 who came to follow-up, 5 had become pregnant and did not perform this test, and 8 others had problems with performing it. Chair-rising test: Of the 215 who came to follow-up, 5 had become pregnant and did not perform this test, and 5 others had problems with performing it. a P value for difference in physical test performance in percentage in supplement groups compared with placebo (the reference). nmol/l and 25 nmol/l at the end of the study was 57% and 85% in the 25- g supplementation group, 38% and 87% in the 10- g supplementation group, and 5.6% and 42% in the placebo group, respectively. There was a negative correlation between jump height and levels of plasma PTH at baseline (r 0.19, P.003) and baseline and follow-up combined (r 0.16, P.001), but there was no correlation between change in jump height and change in PTH levels before and after the intervention. There was no correlation between jump height and serum 25-(OH)D 3 level at baseline (r 0.09, P.16) or baseline and follow-up combined (r 0.02, P.62). Primary outcome There was no significant effect of vitamin D 3 supplements compared with placebo on jump height (difference in percentage change: 1.4%, 95% confidence interval [CI]: 4.9% to 2.2%, P.44). No significant effects were found in the preplanned subgroup analyses (see Table 3 and Figure 2), except for a nonsignificant tendency toward a positive effect of supplements among men (difference in percentage increase: 4.9%, 95% CI: 2.3% to 12.1%, P.170) and a nonsignificant negative effect in women ( 3.6%, 95% CI: 7.7% to 0.6%, P.095); the P value for interaction between intervention and gender was P.035. Figure 2. Forest plot. Difference in mean (95% CI) jumping height between supplement groups (25 g and 10 g vitamin D 3 /d) and placebo after 16 weeks. Secondary outcomes There were no significant effects of the intervention on performance on the chair-rising test (mean difference 0.25%, 95% CI: 3.49% to 3.00%, P.88) or handgrip test (mean difference 1.12%, 95% CI: 5.16% to 2.91%, P.58) (Table 3). We also analyzed the secondary outcomes separately by gender. Specifically, the difference in percentage change for chair rising was 0.44% for men (95% CI: 7.90% to 8.78%, P.91) and 0.27% for women (95% CI: 3.96% to 3.43%, P.89). Results for handgrip strength for men was 2.52%

7 200 Knutsen et al Does Vitamin D Improve Muscle Strength? J Clin Endocrinol Metab, January 2014, 99(1): (95% CI: 10.48% to 5.43%, P.52) and 0.51% for women (95% CI: 5.28% to 4.26%, P.83). There were no significant interactions between intervention and gender for the chair-rising test (P.86) or for the handgrip test (P.66). Harms Reported adverse events were few, mild, and equally distributed between the supplementation groups and the placebo group. At follow-up, three persons reported a brief admission to hospital during the study period (one in the placebo group and two in the 25 g vitamin D group), but the symptoms were unrelated to the intervention. Discussion In this randomized, double-blind, placebo-controlled trial among adults with an ethnic minority background and low serum concentrations of 25-hydroxyvitamin D, we found no increase in muscle strength or power after 16 weeks of supplementation with either 25 or 10 g daily vitamin D 3. We selected healthy participants aged 18 to 50 years due to the relative paucity of research investigating this age group. Strength and weaknesses The study was performed and executed strictly as a randomized, controlled trial according to CONSORT guidelines ( The strengths of this study include a low rate of loss to follow-up (14%) and inclusion of a group of participants with low vitamin D status. In addition, the blood samples were analyzed in one batch. All of the physical performance tests, both at baseline and at follow-up, were supervised and registered by the same researcher. Assessments were performed during winter and spring, a time with minimal impact of sun exposure. We selected jumping height as the main outcome to measure muscle function (fast-twitch muscle fibers) that may be most affected by vitamin D and also applied by another comparable study (13, 28). When planning the study, we aimed to detect an increase in jumping height of at least 5% (confer power calculation). The narrow CI of our main analysis (95%: 4.9% to 2.2%) indicates that it is unlikely that we missed an effect greater than 5% due to type 2 error. However, the sample was underpowered for testing genderspecific effects because of high female enrollment and unexpected gender-specific effects. The duration of this study (16 wk) was sufficient to raise serum 25-(OH)D levels significantly in the intervention groups, but 43% in the 25- g supplementation group and 62% in the 10- g supplementation group did not reach serum 25-(OH)D levels 50 nmol/l. Very few participants reached 75 nmol/l, a concentration advocated by many professional societies. The limited study period probably contributed to the low loss to follow-up. We cannot however exclude the possibility that a longer intervention period might have produced an effect. Most previous studies of this type have had intervention periods of 2 to 24 (average 8) months (24). We did not observe any effect of vitamin D supplementation on jumping height despite a low baseline level of mean serum 25-(OH)D. A study among postmenarchal adolescent girls (n 69), given single doses of vitamin D 2 (corresponding to 10 g/d) every third month for 1 year, similarly found no effect in jumping height compared with placebo, despite low levels of baseline serum 25-(OH)D (mean 18 nmol/l) (28). Another study among young male athletes (n 25) with baseline levels of mean serum 25- (OH)D slightly greater than 50 nmol/l showed no improvement in jumping height after receiving weekly supplementation with vitamin D 3 for 12 weeks (three study groups:143 g/d, 71 g/d, or placebo) (32). Collectively, these studies, which respectively incorporated designs using a longer duration and a higher supplemental dose of vitamin D, are consistent with our findings. Nevertheless, we unintentionally recruited more women than men and therefore we cannot formally exclude the possibility that the lack of a significant effect in jump height in men was due to limited statistical power. The hypothesis that vitamin D has an effect in men was not, however, supported in the gender-specific analyses of our two secondary endpoints (chair-rising test and handgrip test). A systematic review found a weak to moderate relationship between high serum 25-(OH)D levels and increased muscle strength in healthy adults (age y), but these studies were few and the CIs were wide (33). We do not know if a similar study would have given positive results in an older population. Two randomized studies have investigated handgrip strength among young adults. No effect on handgrip strength was found, except for men when reanalyzed and stratified for gender. However, the number of men (n 24) in the study was small (34, 35). In our study, a greater number of men participated (n 57 at follow-up), but we failed to detect any effect on handgrip strength. New studies have shown an increase in serum testosterone levels after vitamin D supplementation in healthy men (36, 37). Future studies should examine the effect of vitamin D supplementation in both genders separately and ideally include measurements of sex hormones. Metabolism of vitamin D 3 is complex. Its metabolism may depend on the dosage used. Thus in our low-dosage study we might have reached steady state at an earlier stage

8 doi: /jc jcem.endojournals.org 201 than when using higher doses. In a Finnish study applying daily dosages at our levels (5 20 g/d) they reached equilibrium state at 6 weeks (38), whereas Heaney et al (39, 40) using higher doses reported that steady-state level was not achieved until 16 weeks. There is also some evidence that those with low baseline levels of 25-(OH)D have a steeper increase in 25-(OH)D levels when supplied with vitamin D than those within higher levels of 25-(OH)D (38, 41). However, a study period of more than 16 weeks may be necessary to obtain 25-(OH)D levels 50 nmol/l, for a sufficiently long period. Most human studies on vitamin D have been done in white participants, but there is emerging evidence that the association between 25-(OH)D and health outcomes might differ across ethnic groups. For example, in the Women s Health Initiative Observational Study higher concentration of 25-(OH)D was associated with lower risk of fractures in white women, whereas the opposite was observed in black and Asian women (42). In the Multi-Ethnic Study of Atherosclerosis, a higher concentration of 25-(OH)D was associated with decreased risk of coronary heart disease in white and Chinese participants, whereas no association was found in black and Hispanic participants (43). In our study only ethnic minorities participated. Thus we cannot extrapolate our results to other ethnic groups. However, in our study an effect of vitamin D on muscle function was not indicated in any of the regional subgroups based on ethnicity (Figure 2). Conclusions Daily supplementation of 25 g or10 g vitamin D 3 for 16 weeks among healthy immigrants from the Middle East, Africa, or South Asia aged years with low 25-(OH)D levels did not have an effect on muscular strength or power in this study, despite the significant increase in serum 25-hydroxyvitamin D and decrease in PTH levels. Acknowledgments The authors thank Eva Kristensen and Morten Ariansen for their help with data collection, Ingvild Dalen for statistical consultation, Anne Karen Jenum for the biobank support, Marie Buchmann and Anne-Lise Sund at Fürst Medical Laboratory for facilitating the laboratory analyses, and Svein Gjelstad for his assistance in data management. Finally, we extend our gratitude to the study participants who participated in this 16-week trial. Address all correspondence and requests for reprints to: Kirsten V. Knutsen, Department of General Practice, Institute of Health and Society, University of Oslo, P.O. Box 1130 Blindern, N-0318 Oslo, Norway. k.i.v.knutsen@medisin.uio.no. This work was supported by Institute of Health and Society, University of Oslo, Oslo, Norway and the Norwegian Women s Public Health Association. The study was also supported by Fürst Medical Laboratory and by Nycomed Pharma AS, including free trial drugs. None of the supporting bodies had any influence on the performance of the trial, analyses of data, writing, or the publication of the results. ClinicalTrials.gov Identifier: NCT Disclosure Summary: The authors have nothing to disclose. References 1. Rosen CJ, Abrams SA, Aloia JF, et al. IOM committee members respond to Endocrine Society vitamin D guideline. J Clin Endocrinol Metab. 2012;97(4): Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endodrinol Metab. 2011; 96(7): German Nutrition Society. New reference values for Vitamin D. Ann Nutr Metab. 2012;60(4): Al-Said YA, Al-Rached HS, Al-Qahtani HA, Jan MM. Severe proximal myopathy with remarkable recovery after vitamin D treatment. Can J Neurol Sci. 2009;36(3): Ladhani S, Srinivasan L, Buchanan C, Allgrove J. Presentation of vitamin D deficiency. Arch Dis Child. 2004;89(8): Soliman AT, El-Dabbagh M, Adel A, et al. Clinical responses to a mega-dose of vitamin D3 in infants and toddlers with vitamin D deficiency rickets. J Trop Pediatr. 2010;56(1): Hazell TJ, DeGuire JR, Weiler HA. Vitamin D: an overview of its role in skeletal muscle physiology in children and adolescents. Nutr Rev. 2012;70(9): Cameron ID, Gillespie LD, Robertson MC, et al. Interventions for preventing falls in older people in care facilities and hospitals. Cochrane Database Syst Rev. 2012;12:CD Gillespie LD, Robertson MC, Gillespie WJ, et al. Interventions for preventing falls in older people living in the community. Cochrane Database Syst Rev. 2012;9:CD Bischoff HA, Borchers M, Gudat F, et al. In situ detection of 1,25- dihydroxyvitamin D3 receptor in human skeletal muscle tissue. Histochem J. 2001;33(1): Glerup H, Mikkelsen K, Poulsen L, et al. Hypovitaminosis D myopathy without biochemical signs of osteomalacic bone involvement. Calcif Tissue Int. 2000;66(6): Sato Y, Iwamoto J, Kanoko T, Satoh K. Low-dose vitamin D prevents muscular atrophy and reduces falls and hip fractures in women after stroke: a randomized controlled trial. Cerebrovasc Dis. 2005; 20(3): Bischoff-Ferrari HA. Relevance of vitamin D in muscle health. Rev Endocrin Metab Disord. 2012;13(1): Wang Y, Zhu J, DeLuca HF. Where is the vitamin D receptor? Arch Biochem Biophys. 2012;523(1): Bouillon R, Verstuyf A. Vitamin D, mitochondria, and muscle. J Clin Endocrinol Metab. 2013;98(3): Srikuea R, Zhang X, Park-Sarge OK, Esser KA. VDR and CYP27B1 are expressed in C2C12 cells and regenerating skeletal muscle: potential role in suppression of myoblast proliferation. Am J Physiol Cell Physiol. 2012;303(4):C396 C Pfeifer M, Begerow B, Minne HW. Vitamin D and muscle function. Osteoporos Int. 2002;13(3): Hamilton B. Vitamin D and human skeletal muscle. Scand J Med Sci Sports. 2010;20(2): Dirks-Naylor AJ, Lennon-Edwards S. The effects of vitamin D on skeletal muscle function and cellular signaling. J Steroid Biochem Mol Biol. 2011;125(3 5):

9 202 Knutsen et al Does Vitamin D Improve Muscle Strength? J Clin Endocrinol Metab, January 2014, 99(1): Mithal A, Wahl DA, Bonjour JP, et al. Global vitamin D status and determinants of hypovitaminosis D. Osteoporos Int. 2009;20(11): Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3): Madar AA, Stene LC, Meyer HE. Vitamin D status among immigrant mothers from Pakistan, Turkey and Somalia and their infants attending child health clinics in Norway. Br J Nutr. 2009;101(7): Knutsen KV, Brekke M, Gjelstad S, Lagerløv P. Vitamin D status in patients with musculoskeletal pain, fatigue and headache: a crosssectional descriptive study in a multi-ethnic general practice in Norway. Scand J Prim Health Care. 2010;28(3): Rejnmark L. Effects of vitamin D on muscle function and performance: a review of evidence from randomized controlled trials. Ther Adv Chronic Dis. 2011;2(1): Stockton KA, Mengersen K, Paratz JD, Kandiah D, Bennell KL. Effect of vitamin D supplementation on muscle strength: a systematic review and meta-analysis. Osteoporos Int. 2011;22(3): Muir SW, Montero-Odasso M. Effect of vitamin D supplementation on muscle strength, gait and balance in older adults: a systematic review and meta-analysis. J Am Geriatr Soc. 2011;59(12): Nasjonalt råd for ernæring: Meyer H, Brunvand L, Brustad M, Holvik K, Johansson L, Paulsen JE. Tiltak for å sikre en god vitamin D-status i befolkningen (Proposals to secure a good vitamin D-status in the population), Rapport IS-1408, Oslo, Norwegian Nutrition Council; Ward KA, Das G, Roberts SA, et al. A randomized, controlled trial of vitamin D supplementation upon musculoskeletal health in postmenarchal females. J Clin Endocrinol Metab. 2010;95(10): Bohannon RW. Test-retest reliability of the five-repetition sit-tostand test: a systematic review of the literature involving adults. J Strength Cond Res. 2011;25(11): Reuter SE, Massy-Westropp N, Evans AM. Reliability and validity of indices of hand-grip strength and endurance. Aust Occup Ther J. 2011;58: Ward KA, Das G, Berry JL, et al. Vitamin D status and muscle function in post-menarchal adolescent girls. J Clin Endocrinol Metab. 2009;94(2): Close GL, Leckey J, Patterson M, et al. The effects of vitamin D3 supplementation on serum total 25[OH]D concentration and physical performance: a randomised dose-response study. Br J Sports Med. 2013;47(11): Redzic M, Lewis RM, Thomas DT. Relationship between 25-hydoxyvitamin D, muscle strength, and incidence of injury in healthy adults: a systematic review. Nutr Res. 2013;33(4): Gupta R, Sharma U, Gupta N, et al. Effect of cholecalciferol and calcium supplementation on muscle strength and energy metabolism in vitamin D-deficient Asian Indians: a randomized, controlled trial. Clin Endocrinol (Oxf). 2010;73(4): Goswami R, Vatsa M, Sreenivas V, et al. Skeletal muscle strength in young asian indian females after vitamin D and calcium supplementation: a double-blind randomized controlled clinical trial. J Clin Endocrinol Metab. 2012;97(12): Pilz S, Frisch S, Koertke H, et al. Effect of vitamin D supplementation on testosterone levels in men. Horm Metab Res. 2011;43(3): Wehr E, Pilz S, Boehm BO, März W, Obermayer-Pietsch B. Association of vitamin D status with serum androgen levels in men. Clin Endocrinol (Oxf). 2010;73(2): Viljakainen HT, Palssa A, Kärkkäinen M, Jakobsen J, Lamberg- Allardt C. How much vitamin D3 do the elderly need? J Am Coll Nutr. 2006;25(5): Heaney RP, Davies KM, Chen TC, Holick MF, Barger-Lux MJ. Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol. Am J Clin Nutr. 2003;77: Heaney RP, Recker RR, Grote J, Horst RL, Armas LA. Vitamin D(3) is more potent than vitamin D(2) in humans. J Clin Endocrinol Metab. 2011;96(3):E447 E Holvik K, Madar AA, Meyer HE, Lofthus CM, Stene LC. A randomised comparison of increase in serum 25-hydroxyvitamin D concentration after 4 weeks of daily oral intake of 10 microg cholecalciferol from multivitamin tablets or fish oil capsules in healthy young adults. Br J Nutr. 2007;98(3): Cauley JA, Danielson ME, Boudreau R, et al. Serum 25-hydroxyvitamin D and clinical fracture risk in a multiethnic cohort of women: the Women s Health Initiative (WHI). J Bone Miner Res. 2011; 26(10): Robinson-Cohen C, Hoofnagle AN, Ix JH, et al. Racial differences in the association of serum 25-hydroxyvitamin D concentration with coronary heart disease events. JAMA. 2013;310(2):