Association of Degree of European Genetic Ancestry With Serum Vitamin D Levels in African Americans

Association of Degree of European Genetic Ancestry With Serum Vitamin D Levels in African Americans Abstract Circulating levels of vitamin D are generally lower in African Americans than in US whites, and 1 prior analysis carried out in a small number of African Americans suggested that, within this population, vitamin D levels may be related to the degree of genetic admixture. We assessed the association between percentage of European ancestry and serum vitamin D level (assessed in 2013–2015) among 2,183 African-American women from the Black Women’s Health Study whose DNA had been genotyped for ancestry-informative markers. ADMIXMAP software was used to estimate the percentage of European ancestry versus African ancestry in each individual. In linear regression analyses with adjustment for genotype batch, age, body mass index, supplemental vitamin D use, ultraviolet B radiation flux in the participant’s state of residence, and season of blood draw, each 10% increase in European ancestry was associated with a 0.67-ng/mL increase in serum vitamin D concentration (95% confidence interval: 0.17, 1.17). The association was statistically significant only among women who were not taking vitamin D supplements (for each 10% increase in European ancestry, β = 0.86, 95% confidence interval: 0.14, 1.57). Among African Americans, use of vitamin D supplements may help to reduce vitamin D deficiency associated with genetic ancestry. admixture, African Americans, ancestry, genetics, vitamin D Circulating levels of vitamin D are lower in African Americans than in US whites (1–6), and lower vitamin D levels have been linked to increased risk of a variety of chronic conditions, including cardiovascular disease and cancer (1, 3–5, 7). Thus, vitamin D deficiency may be an important factor contributing to racial disparities in health (1). The primary cause of racial differences in vitamin D levels is thought to be differences in pigmentation, with darker pigmentation reducing vitamin D production in the skin. Differences in diet, including use of vitamin supplements, may also play a role, given the lower dietary intake of vitamin D observed in African Americans compared with whites (1, 3–5). In particular, African Americans have been shown to consume fewer servings of dairy food per day than non–African Americans, possibly due to a higher prevalence of lactose intolerance and corresponding avoidance of dairy products (8–10). Furthermore, in an earlier analysis in the Black Women’s Health Study (BWHS), Wise et al. (9) reported an inverse association between genetically determined percentage of African ancestry and dairy food intake among African Americans. A lower prevalence of vitamin supplement use has also been observed in African Americans compared with whites (11), based on data from the National Health and Nutrition Examination Survey (NHANES). The 30-day prevalence of multivitamin/multimineral use for the 2011–2012 NHANES period was 35% among white adults living in the United States (n = 2,038), as compared with 24% among African-American adults (n = 1,455). The use of vitamin D supplements (excluding multivitamin/multimineral use) was also lower in African Americans: The reported 30-day prevalence for the NHANES 2011–2012 cycle was 23% in US whites compared with only 12% in African Americans. Signorello et al. (1) were the first to study how serum vitamin D levels vary among African Americans based on genetically determined ancestry. These authors estimated European and African admixture using a panel of ancestry-informative markers and tested the association of percentage of European ancestry with circulating vitamin D levels in 379 African-American subjects from the Southern Community Cohort Study (SCCS). They found that a 10% increase in European ancestry was associated with a significant 1.0-ng/mL increase in serum 25-hydroxyvitamin D (25(OH)D) concentration (1). We conducted the present study in an attempt to replicate their findings in a larger sample of African-American women (n = 2,183). METHODS Study population The study population for the current analyses was women enrolled in the BWHS (12). The BWHS is a prospective cohort study of African-American women from across the United States, initiated in 1995 when 59,000 black women aged 21–69 years completed a 14-page postal health questionnaire. Biennial follow-up questionnaires are sent to ascertain a variety of health outcomes and to update covariate data. As of 2013, follow-up had been completed for 88% of the potential years of follow-up for the baseline cohort. The BWHS was granted approval by the institutional review board of Boston University, and informed consent was provided by all study subjects, including written consent for the collection of saliva and blood samples. Vitamin D assessment Blood specimens were collected from BWHS participants from 2013 to 2017. All samples were assayed for serum 25(OH)D level using the liquid chromatography–tandem mass spectrometry method. This method is sensitive and equally specific for measuring both forms of circulating vitamin D, 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3, and the concentrations of these 2 forms are used to calculate total 25(OH)D levels. Vitamin D assays were performed by Quest Diagnostics (Madison, New Jersey; www.QuestDiagnostics.com) at 3 Quest central laboratories. Quest is a national clinical laboratory certified under the Clinical Laboratory Improvement Amendments of 1988 (13). National Institute of Standards and Technology Standard Reference Material for 25(OH)D in human serum was used for quality control. Written informed consent to use the blood samples for health-related research for the entirety of the study period was obtained from participants who provided samples. Ancestry estimation About half of all BWHS subjects had earlier provided saliva samples as a source of genomic DNA. Participants who provided samples were found to be highly representative of all BWHS subjects with regard to a number of characteristics, such as geographic region of residence, education, and body mass index (BMI) (14). More than 16,000 BWHS participants have had their DNA samples genotyped on a set of ancestry-informative markers in one of several BWHS genetic investigations into a variety of health outcomes, including breast cancer, type 2 diabetes, fibroids, and several rare conditions. Among those 16,000 participants, 4,246 had also provided a blood sample. We excluded those who had been selected for genotyping due to breast cancer, type 2 diabetes, or fibroids, leaving 2,183 women available for the present analyses. ADMIXMAP software (15; http://admixmap.sourceforge.net/index.html) was used to determine the percentages of African and European ancestry for each individual. Covariates Covariates included in the analyses were taken from questionnaire data obtained around the time of blood drawing, with the exception of dairy food intake, for which the most recent available data were from 2001. BMI (weight (kg)/height (m)2) was calculated from weight reported on the 2013 questionnaire and adult height. Season of blood draw was defined according to solstice and equinox dates. A covariate for the use of vitamin D supplements was obtained from a yes/no response on the 2013 questionnaire to a question about current use of vitamin D supplements, with or without calcium, at least twice a week. Dairy food intake was assessed in 2001 using a modified version of the National Cancer Institute–Block short-form food frequency questionnaire (16). Total dairy food consumption was calculated by summing the reported number of servings of milk, buttermilk, cream, ice cream, frozen yogurt, yogurt, cheese, and cheese spreads per week. A covariate for ultraviolet B (UVB) radiation flux (in Robertson-Berger units × 10-4) was also generated and was used as a proxy for ambient sun exposure. This covariate was defined as a 4-category variable for level of solar UVB radiation exposure, determined from each participant’s state of residence in 2013 along with the reported average annual amount of UVB radiation in each state (17). Statistical analysis Linear regression, with control for age and genotyping batch, was used to evaluate the association of percentage of European ancestry with total serum 25(OH)D concentration. Models additionally controlled for season of blood draw, BMI, supplemental vitamin D use, and UVB flux. Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry after accounting for the covariates included in each model. Analyses were performed using SAS 9.3 statistical software (SAS Institute, Inc., Cary, North Carolina). Because use of vitamin D supplements greatly affects circulating levels of 25(OH)D, we repeated the analyses separately among women who reported use of vitamin D supplements in the BWHS survey completed during the year before blood drawing and those who reported no use. RESULTS Characteristics of the analytical sample are shown in Table 1. Participants in the higher quartiles of serum 25(OH)D concentration were generally older and had a lower BMI, as reported previously in the BWHS (7). The prevalence of supplemental vitamin D use was markedly higher in each consecutive quartile of serum 25(OH)D level. Table 1. Characteristics of Participants and the Study Sample According to Quartile of Serum 25-Hydroxyvitamin D Concentration, Black Women’s Health Study, 2013–2015 Characteristic  Serum 25(OH)D Quartile  1 (Lowest)  2  3  4 (Highest)  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Serum 25(OH)D level, ng/mL  15.1 (4.0)  15.0    26.9 (2.9)  27.0    35.6 (2.6)  35.0    51.1 (10.0)  48.0    Percentage of European ancestry  20.4 (9.7)  19.0    21.7 (11.6)  19.3    22.2 (11.2)  20.4    22.4 (11.2)  20.8    Age, years  54.3 (9.0)      56.0 (9.0)      58.2 (9.5)      60.4 (8.9)      Body mass indexa  31.0 (7.1)      29.6 (6.2)      29.3 (6.1)      29.5 (6.5)      Reported use of vitamin D supplements      20.3      43.4      60.2      69.5  Highest category of UVB flux (>160 Robertson-Berger units × 10−4)      20.5      17.7      15.5      16.5  Blood drawn during the summer season      19.7      27.8      31.3      27.8  Characteristic  Serum 25(OH)D Quartile  1 (Lowest)  2  3  4 (Highest)  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Serum 25(OH)D level, ng/mL  15.1 (4.0)  15.0    26.9 (2.9)  27.0    35.6 (2.6)  35.0    51.1 (10.0)  48.0    Percentage of European ancestry  20.4 (9.7)  19.0    21.7 (11.6)  19.3    22.2 (11.2)  20.4    22.4 (11.2)  20.8    Age, years  54.3 (9.0)      56.0 (9.0)      58.2 (9.5)      60.4 (8.9)      Body mass indexa  31.0 (7.1)      29.6 (6.2)      29.3 (6.1)      29.5 (6.5)      Reported use of vitamin D supplements      20.3      43.4      60.2      69.5  Highest category of UVB flux (>160 Robertson-Berger units × 10−4)      20.5      17.7      15.5      16.5  Blood drawn during the summer season      19.7      27.8      31.3      27.8  Abbreviations: 25(OH)D, 25-hydroxyvitamin D; SD, standard deviation; UVB, ultraviolet B. a Weight (kg)/height (m)2. Table 1. Characteristics of Participants and the Study Sample According to Quartile of Serum 25-Hydroxyvitamin D Concentration, Black Women’s Health Study, 2013–2015 Characteristic  Serum 25(OH)D Quartile  1 (Lowest)  2  3  4 (Highest)  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Serum 25(OH)D level, ng/mL  15.1 (4.0)  15.0    26.9 (2.9)  27.0    35.6 (2.6)  35.0    51.1 (10.0)  48.0    Percentage of European ancestry  20.4 (9.7)  19.0    21.7 (11.6)  19.3    22.2 (11.2)  20.4    22.4 (11.2)  20.8    Age, years  54.3 (9.0)      56.0 (9.0)      58.2 (9.5)      60.4 (8.9)      Body mass indexa  31.0 (7.1)      29.6 (6.2)      29.3 (6.1)      29.5 (6.5)      Reported use of vitamin D supplements      20.3      43.4      60.2      69.5  Highest category of UVB flux (>160 Robertson-Berger units × 10−4)      20.5      17.7      15.5      16.5  Blood drawn during the summer season      19.7      27.8      31.3      27.8  Characteristic  Serum 25(OH)D Quartile  1 (Lowest)  2  3  4 (Highest)  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Serum 25(OH)D level, ng/mL  15.1 (4.0)  15.0    26.9 (2.9)  27.0    35.6 (2.6)  35.0    51.1 (10.0)  48.0    Percentage of European ancestry  20.4 (9.7)  19.0    21.7 (11.6)  19.3    22.2 (11.2)  20.4    22.4 (11.2)  20.8    Age, years  54.3 (9.0)      56.0 (9.0)      58.2 (9.5)      60.4 (8.9)      Body mass indexa  31.0 (7.1)      29.6 (6.2)      29.3 (6.1)      29.5 (6.5)      Reported use of vitamin D supplements      20.3      43.4      60.2      69.5  Highest category of UVB flux (>160 Robertson-Berger units × 10−4)      20.5      17.7      15.5      16.5  Blood drawn during the summer season      19.7      27.8      31.3      27.8  Abbreviations: 25(OH)D, 25-hydroxyvitamin D; SD, standard deviation; UVB, ultraviolet B. a Weight (kg)/height (m)2. Overall, a 10% increase in European ancestry was associated with a statistically significant age-adjusted 0.80-ng/mL increase in serum 25(OH)D concentration (95% confidence interval (CI): 0.27, 1.32) (Table 2). With additional adjustment for BMI, supplemental vitamin D use, UVB flux, and season of blood draw, the β coefficient was 0.67 (95% CI: 0.17, 1.17) (Table 2). Further control for energy-adjusted dairy food consumption produced little change in the results (data not shown). Table 2. Association of Percentage of European Ancestry With Serum 25-Hydroxyvitamin D Level, Overall and According to Use of Vitamin D Supplements, Black Women’s Health Study, 2013–2015 Analysis Sample  No. of Women  Age-Adjusted βa  95% CI  Multivariable βb  95% CI  Overall  2,183  0.80  0.27, 1.32  0.67  0.17, 1.17  Vitamin D supplement use             Nonuser  1,129  0.88  0.16, 1.60  0.86  0.14, 1.57   User  1,054  0.51  −0.18, 1.21  0.47  −0.23, 1.17  Analysis Sample  No. of Women  Age-Adjusted βa  95% CI  Multivariable βb  95% CI  Overall  2,183  0.80  0.27, 1.32  0.67  0.17, 1.17  Vitamin D supplement use             Nonuser  1,129  0.88  0.16, 1.60  0.86  0.14, 1.57   User  1,054  0.51  −0.18, 1.21  0.47  −0.23, 1.17  Abbreviations: CI, confidence interval; 25(OH)D, 25-hydroxyvitamin D. a Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry, adjusting for genotyping batch and age. b Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry, adjusting for genotyping batch, age, body mass index, supplemental vitamin D use, ultraviolet B radiation flux, and season of blood draw. Table 2. Association of Percentage of European Ancestry With Serum 25-Hydroxyvitamin D Level, Overall and According to Use of Vitamin D Supplements, Black Women’s Health Study, 2013–2015 Analysis Sample  No. of Women  Age-Adjusted βa  95% CI  Multivariable βb  95% CI  Overall  2,183  0.80  0.27, 1.32  0.67  0.17, 1.17  Vitamin D supplement use             Nonuser  1,129  0.88  0.16, 1.60  0.86  0.14, 1.57   User  1,054  0.51  −0.18, 1.21  0.47  −0.23, 1.17  Analysis Sample  No. of Women  Age-Adjusted βa  95% CI  Multivariable βb  95% CI  Overall  2,183  0.80  0.27, 1.32  0.67  0.17, 1.17  Vitamin D supplement use             Nonuser  1,129  0.88  0.16, 1.60  0.86  0.14, 1.57   User  1,054  0.51  −0.18, 1.21  0.47  −0.23, 1.17  Abbreviations: CI, confidence interval; 25(OH)D, 25-hydroxyvitamin D. a Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry, adjusting for genotyping batch and age. b Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry, adjusting for genotyping batch, age, body mass index, supplemental vitamin D use, ultraviolet B radiation flux, and season of blood draw. Among nonusers of vitamin D supplements (52% of the sample), there was a strong, statistically significant association of increasing 25(OH)D level with increasing percentage of European ancestry (Table 2). A 10% increase in European ancestry was associated with a 0.86-ng/mL increase in serum 25(OH)D concentration (95% CI: 0.14, 1.57). By contrast, there was not a statistically significant association among supplement users, and the β coefficient for a 10% increase in European ancestry in that group was 0.47 (95% CI: −0.23, 1.17). DISCUSSION Vitamin D levels have been demonstrated to be lower in African Americans than in US whites (1–6). Our findings indicate that among African Americans, vitamin D levels are related to the degree of genetic admixture, with higher levels observed among those in the upper percentiles of European ancestry. Notably, the association was present only among women who were not taking vitamin D supplements. We postulate that in the absence of supplementation, factors related to genetic ancestry, including genes related to skin pigmentation and metabolism, will likely explain more of the variability in vitamin D levels. The direction of our findings is consistent with results reported by Signorello et al. (1) based on 379 African Americans in the SCCS. The β coefficient among nonusers of supplements in the BWHS (β = 0.86) was consistent with the magnitude of the comparable estimate from the SCCS (β = 1.00). The SCCS report did not provide data on what proportion of the 379 women were using vitamin D supplements, but it is likely to have been lower than the 48% in the BWHS. First, use of vitamin D supplements has increased over time (11), and the SCCS blood draws were conducted approximately 10 years before the BWHS blood collection. Second, in an earlier SCCS paper, Egan et al. (5) reported that only 33% of African Americans in the study reported taking multivitamins (which may or may not have included vitamin D). The SCCS was limited to residents of 10 southeastern states, whereas the BWHS included residents from all regions of the mainland United States. The mean percentage of European ancestry in the SCCS sample was 7.1%, whereas in the BWHS the mean was 21.7%. Much of the association between percentage of European ancestry and vitamin D status may be driven by differences in skin pigmentation, given that skin pigmentation has been shown to decrease with decreasing percentage of African ancestry among African Americans (18). It would be necessary to have a measure of skin pigmentation in order to parse out this segment of the ancestry–vitamin D association and determine what portion of the association is explained by other factors. Other explanatory factors may include diet and inherent differences in the ability to absorb or synthesize vitamin D that are unrelated to melanin levels (1). We were able to account for dietary intake of vitamin D in our analyses by adjusting for dairy food consumption. Control for dairy consumption produced little change in our results; while dairy consumption was positively associated with European ancestry in our study sample, it was not associated with serum 25(OH)D level. Our study was limited by a lack of data on skin pigmentation, which prevented us from deciphering how much this factor was responsible for the ancestry–vitamin D association we observed. In addition, the most recent dairy food consumption data available were from 2001, and changes in dietary patterns since that time could not be accounted for. Also, we did not have quantitative information on the amount of vitamin D in supplements taken by those women who reported supplement use. A strength of the study was the large sample of African-American women with both genetic ancestry estimates and vitamin D measurements. In summary, we observed a significant positive association between percentage of European ancestry and serum 25(OH)D level in African-American women. Importantly, the association was most evident among women who were not taking supplemental vitamin D. Vitamin D supplement use appeared to be an effective means of reducing disparities in 25(OH)D levels conferred by genetic ancestry. ACKNOWLEDGMENTS Author affiliations: Slone Epidemiology Center at Boston University, Boston, Massachusetts (Stephen A. Haddad, Edward A. Ruiz-Narváez, Yvette C. Cozier, Hanna Gerlovin, Lynn Rosenberg, Julie R. Palmer); and Department of Biostatistics, School of Public Health, Boston University, Boston, Massachusetts (Hanna Gerlovin). This work was funded by National Institutes of Health grants R01 CA058420, R01 CA098663, and UM1 CA164974. Conflict of interest: none declared. Abbreviations BMI body mass index BWHS Black Women’s Health Study CI confidence interval NHANES National Health and Nutrition Examination Survey 25(OH)D 25-hydroxyvitamin D SCCS Southern Community Cohort Study UVB ultraviolet B REFERENCES 1 Signorello LB, Williams SM, Zheng W, et al.  . Blood vitamin D levels in relation to genetic estimation of African ancestry. Cancer Epidemiol Biomarkers Prev . 2010; 19( 9): 2325– 2331. Google Scholar CrossRef Search ADS PubMed  2 Zadshir A, Tareen N, Pan D, et al.  . The prevalence of hypovitaminosis D among US adults: data from the NHANES III. Ethn Dis . 2005; 15( 4 suppl 5): S5-97– S5-101. 3 Holick MF. High prevalence of vitamin D inadequacy and implications for health. Mayo Clin Proc . 2006; 81( 3): 353– 373. Google Scholar CrossRef Search ADS PubMed  4 Harris SS. Vitamin D and African Americans. J Nutr . 2006; 136( 4): 1126– 1129. Google Scholar CrossRef Search ADS PubMed  5 Egan KM, Signorello LB, Munro HM, et al.  . 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New York, NY: Oxford University Press; 1996: 355– 372. 18 Shriver MD, Parra EJ, Dios S, et al.  . Skin pigmentation, biogeographical ancestry and admixture mapping. Hum Genet . 2003; 112( 4): 387– 399. Google Scholar PubMed  © The Author(s) 2018. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png American Journal of Epidemiology Oxford University Press

Association of Degree of European Genetic Ancestry With Serum Vitamin D Levels in African Americans

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Abstract

Abstract Circulating levels of vitamin D are generally lower in African Americans than in US whites, and 1 prior analysis carried out in a small number of African Americans suggested that, within this population, vitamin D levels may be related to the degree of genetic admixture. We assessed the association between percentage of European ancestry and serum vitamin D level (assessed in 2013–2015) among 2,183 African-American women from the Black Women’s Health Study whose DNA had been genotyped for ancestry-informative markers. ADMIXMAP software was used to estimate the percentage of European ancestry versus African ancestry in each individual. In linear regression analyses with adjustment for genotype batch, age, body mass index, supplemental vitamin D use, ultraviolet B radiation flux in the participant’s state of residence, and season of blood draw, each 10% increase in European ancestry was associated with a 0.67-ng/mL increase in serum vitamin D concentration (95% confidence interval: 0.17, 1.17). The association was statistically significant only among women who were not taking vitamin D supplements (for each 10% increase in European ancestry, β = 0.86, 95% confidence interval: 0.14, 1.57). Among African Americans, use of vitamin D supplements may help to reduce vitamin D deficiency associated with genetic ancestry. admixture, African Americans, ancestry, genetics, vitamin D Circulating levels of vitamin D are lower in African Americans than in US whites (1–6), and lower vitamin D levels have been linked to increased risk of a variety of chronic conditions, including cardiovascular disease and cancer (1, 3–5, 7). Thus, vitamin D deficiency may be an important factor contributing to racial disparities in health (1). The primary cause of racial differences in vitamin D levels is thought to be differences in pigmentation, with darker pigmentation reducing vitamin D production in the skin. Differences in diet, including use of vitamin supplements, may also play a role, given the lower dietary intake of vitamin D observed in African Americans compared with whites (1, 3–5). In particular, African Americans have been shown to consume fewer servings of dairy food per day than non–African Americans, possibly due to a higher prevalence of lactose intolerance and corresponding avoidance of dairy products (8–10). Furthermore, in an earlier analysis in the Black Women’s Health Study (BWHS), Wise et al. (9) reported an inverse association between genetically determined percentage of African ancestry and dairy food intake among African Americans. A lower prevalence of vitamin supplement use has also been observed in African Americans compared with whites (11), based on data from the National Health and Nutrition Examination Survey (NHANES). The 30-day prevalence of multivitamin/multimineral use for the 2011–2012 NHANES period was 35% among white adults living in the United States (n = 2,038), as compared with 24% among African-American adults (n = 1,455). The use of vitamin D supplements (excluding multivitamin/multimineral use) was also lower in African Americans: The reported 30-day prevalence for the NHANES 2011–2012 cycle was 23% in US whites compared with only 12% in African Americans. Signorello et al. (1) were the first to study how serum vitamin D levels vary among African Americans based on genetically determined ancestry. These authors estimated European and African admixture using a panel of ancestry-informative markers and tested the association of percentage of European ancestry with circulating vitamin D levels in 379 African-American subjects from the Southern Community Cohort Study (SCCS). They found that a 10% increase in European ancestry was associated with a significant 1.0-ng/mL increase in serum 25-hydroxyvitamin D (25(OH)D) concentration (1). We conducted the present study in an attempt to replicate their findings in a larger sample of African-American women (n = 2,183). METHODS Study population The study population for the current analyses was women enrolled in the BWHS (12). The BWHS is a prospective cohort study of African-American women from across the United States, initiated in 1995 when 59,000 black women aged 21–69 years completed a 14-page postal health questionnaire. Biennial follow-up questionnaires are sent to ascertain a variety of health outcomes and to update covariate data. As of 2013, follow-up had been completed for 88% of the potential years of follow-up for the baseline cohort. The BWHS was granted approval by the institutional review board of Boston University, and informed consent was provided by all study subjects, including written consent for the collection of saliva and blood samples. Vitamin D assessment Blood specimens were collected from BWHS participants from 2013 to 2017. All samples were assayed for serum 25(OH)D level using the liquid chromatography–tandem mass spectrometry method. This method is sensitive and equally specific for measuring both forms of circulating vitamin D, 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3, and the concentrations of these 2 forms are used to calculate total 25(OH)D levels. Vitamin D assays were performed by Quest Diagnostics (Madison, New Jersey; www.QuestDiagnostics.com) at 3 Quest central laboratories. Quest is a national clinical laboratory certified under the Clinical Laboratory Improvement Amendments of 1988 (13). National Institute of Standards and Technology Standard Reference Material for 25(OH)D in human serum was used for quality control. Written informed consent to use the blood samples for health-related research for the entirety of the study period was obtained from participants who provided samples. Ancestry estimation About half of all BWHS subjects had earlier provided saliva samples as a source of genomic DNA. Participants who provided samples were found to be highly representative of all BWHS subjects with regard to a number of characteristics, such as geographic region of residence, education, and body mass index (BMI) (14). More than 16,000 BWHS participants have had their DNA samples genotyped on a set of ancestry-informative markers in one of several BWHS genetic investigations into a variety of health outcomes, including breast cancer, type 2 diabetes, fibroids, and several rare conditions. Among those 16,000 participants, 4,246 had also provided a blood sample. We excluded those who had been selected for genotyping due to breast cancer, type 2 diabetes, or fibroids, leaving 2,183 women available for the present analyses. ADMIXMAP software (15; http://admixmap.sourceforge.net/index.html) was used to determine the percentages of African and European ancestry for each individual. Covariates Covariates included in the analyses were taken from questionnaire data obtained around the time of blood drawing, with the exception of dairy food intake, for which the most recent available data were from 2001. BMI (weight (kg)/height (m)2) was calculated from weight reported on the 2013 questionnaire and adult height. Season of blood draw was defined according to solstice and equinox dates. A covariate for the use of vitamin D supplements was obtained from a yes/no response on the 2013 questionnaire to a question about current use of vitamin D supplements, with or without calcium, at least twice a week. Dairy food intake was assessed in 2001 using a modified version of the National Cancer Institute–Block short-form food frequency questionnaire (16). Total dairy food consumption was calculated by summing the reported number of servings of milk, buttermilk, cream, ice cream, frozen yogurt, yogurt, cheese, and cheese spreads per week. A covariate for ultraviolet B (UVB) radiation flux (in Robertson-Berger units × 10-4) was also generated and was used as a proxy for ambient sun exposure. This covariate was defined as a 4-category variable for level of solar UVB radiation exposure, determined from each participant’s state of residence in 2013 along with the reported average annual amount of UVB radiation in each state (17). Statistical analysis Linear regression, with control for age and genotyping batch, was used to evaluate the association of percentage of European ancestry with total serum 25(OH)D concentration. Models additionally controlled for season of blood draw, BMI, supplemental vitamin D use, and UVB flux. Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry after accounting for the covariates included in each model. Analyses were performed using SAS 9.3 statistical software (SAS Institute, Inc., Cary, North Carolina). Because use of vitamin D supplements greatly affects circulating levels of 25(OH)D, we repeated the analyses separately among women who reported use of vitamin D supplements in the BWHS survey completed during the year before blood drawing and those who reported no use. RESULTS Characteristics of the analytical sample are shown in Table 1. Participants in the higher quartiles of serum 25(OH)D concentration were generally older and had a lower BMI, as reported previously in the BWHS (7). The prevalence of supplemental vitamin D use was markedly higher in each consecutive quartile of serum 25(OH)D level. Table 1. Characteristics of Participants and the Study Sample According to Quartile of Serum 25-Hydroxyvitamin D Concentration, Black Women’s Health Study, 2013–2015 Characteristic  Serum 25(OH)D Quartile  1 (Lowest)  2  3  4 (Highest)  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Serum 25(OH)D level, ng/mL  15.1 (4.0)  15.0    26.9 (2.9)  27.0    35.6 (2.6)  35.0    51.1 (10.0)  48.0    Percentage of European ancestry  20.4 (9.7)  19.0    21.7 (11.6)  19.3    22.2 (11.2)  20.4    22.4 (11.2)  20.8    Age, years  54.3 (9.0)      56.0 (9.0)      58.2 (9.5)      60.4 (8.9)      Body mass indexa  31.0 (7.1)      29.6 (6.2)      29.3 (6.1)      29.5 (6.5)      Reported use of vitamin D supplements      20.3      43.4      60.2      69.5  Highest category of UVB flux (>160 Robertson-Berger units × 10−4)      20.5      17.7      15.5      16.5  Blood drawn during the summer season      19.7      27.8      31.3      27.8  Characteristic  Serum 25(OH)D Quartile  1 (Lowest)  2  3  4 (Highest)  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Serum 25(OH)D level, ng/mL  15.1 (4.0)  15.0    26.9 (2.9)  27.0    35.6 (2.6)  35.0    51.1 (10.0)  48.0    Percentage of European ancestry  20.4 (9.7)  19.0    21.7 (11.6)  19.3    22.2 (11.2)  20.4    22.4 (11.2)  20.8    Age, years  54.3 (9.0)      56.0 (9.0)      58.2 (9.5)      60.4 (8.9)      Body mass indexa  31.0 (7.1)      29.6 (6.2)      29.3 (6.1)      29.5 (6.5)      Reported use of vitamin D supplements      20.3      43.4      60.2      69.5  Highest category of UVB flux (>160 Robertson-Berger units × 10−4)      20.5      17.7      15.5      16.5  Blood drawn during the summer season      19.7      27.8      31.3      27.8  Abbreviations: 25(OH)D, 25-hydroxyvitamin D; SD, standard deviation; UVB, ultraviolet B. a Weight (kg)/height (m)2. Table 1. Characteristics of Participants and the Study Sample According to Quartile of Serum 25-Hydroxyvitamin D Concentration, Black Women’s Health Study, 2013–2015 Characteristic  Serum 25(OH)D Quartile  1 (Lowest)  2  3  4 (Highest)  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Serum 25(OH)D level, ng/mL  15.1 (4.0)  15.0    26.9 (2.9)  27.0    35.6 (2.6)  35.0    51.1 (10.0)  48.0    Percentage of European ancestry  20.4 (9.7)  19.0    21.7 (11.6)  19.3    22.2 (11.2)  20.4    22.4 (11.2)  20.8    Age, years  54.3 (9.0)      56.0 (9.0)      58.2 (9.5)      60.4 (8.9)      Body mass indexa  31.0 (7.1)      29.6 (6.2)      29.3 (6.1)      29.5 (6.5)      Reported use of vitamin D supplements      20.3      43.4      60.2      69.5  Highest category of UVB flux (>160 Robertson-Berger units × 10−4)      20.5      17.7      15.5      16.5  Blood drawn during the summer season      19.7      27.8      31.3      27.8  Characteristic  Serum 25(OH)D Quartile  1 (Lowest)  2  3  4 (Highest)  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Mean (SD)  Median  %  Serum 25(OH)D level, ng/mL  15.1 (4.0)  15.0    26.9 (2.9)  27.0    35.6 (2.6)  35.0    51.1 (10.0)  48.0    Percentage of European ancestry  20.4 (9.7)  19.0    21.7 (11.6)  19.3    22.2 (11.2)  20.4    22.4 (11.2)  20.8    Age, years  54.3 (9.0)      56.0 (9.0)      58.2 (9.5)      60.4 (8.9)      Body mass indexa  31.0 (7.1)      29.6 (6.2)      29.3 (6.1)      29.5 (6.5)      Reported use of vitamin D supplements      20.3      43.4      60.2      69.5  Highest category of UVB flux (>160 Robertson-Berger units × 10−4)      20.5      17.7      15.5      16.5  Blood drawn during the summer season      19.7      27.8      31.3      27.8  Abbreviations: 25(OH)D, 25-hydroxyvitamin D; SD, standard deviation; UVB, ultraviolet B. a Weight (kg)/height (m)2. Overall, a 10% increase in European ancestry was associated with a statistically significant age-adjusted 0.80-ng/mL increase in serum 25(OH)D concentration (95% confidence interval (CI): 0.27, 1.32) (Table 2). With additional adjustment for BMI, supplemental vitamin D use, UVB flux, and season of blood draw, the β coefficient was 0.67 (95% CI: 0.17, 1.17) (Table 2). Further control for energy-adjusted dairy food consumption produced little change in the results (data not shown). Table 2. Association of Percentage of European Ancestry With Serum 25-Hydroxyvitamin D Level, Overall and According to Use of Vitamin D Supplements, Black Women’s Health Study, 2013–2015 Analysis Sample  No. of Women  Age-Adjusted βa  95% CI  Multivariable βb  95% CI  Overall  2,183  0.80  0.27, 1.32  0.67  0.17, 1.17  Vitamin D supplement use             Nonuser  1,129  0.88  0.16, 1.60  0.86  0.14, 1.57   User  1,054  0.51  −0.18, 1.21  0.47  −0.23, 1.17  Analysis Sample  No. of Women  Age-Adjusted βa  95% CI  Multivariable βb  95% CI  Overall  2,183  0.80  0.27, 1.32  0.67  0.17, 1.17  Vitamin D supplement use             Nonuser  1,129  0.88  0.16, 1.60  0.86  0.14, 1.57   User  1,054  0.51  −0.18, 1.21  0.47  −0.23, 1.17  Abbreviations: CI, confidence interval; 25(OH)D, 25-hydroxyvitamin D. a Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry, adjusting for genotyping batch and age. b Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry, adjusting for genotyping batch, age, body mass index, supplemental vitamin D use, ultraviolet B radiation flux, and season of blood draw. Table 2. Association of Percentage of European Ancestry With Serum 25-Hydroxyvitamin D Level, Overall and According to Use of Vitamin D Supplements, Black Women’s Health Study, 2013–2015 Analysis Sample  No. of Women  Age-Adjusted βa  95% CI  Multivariable βb  95% CI  Overall  2,183  0.80  0.27, 1.32  0.67  0.17, 1.17  Vitamin D supplement use             Nonuser  1,129  0.88  0.16, 1.60  0.86  0.14, 1.57   User  1,054  0.51  −0.18, 1.21  0.47  −0.23, 1.17  Analysis Sample  No. of Women  Age-Adjusted βa  95% CI  Multivariable βb  95% CI  Overall  2,183  0.80  0.27, 1.32  0.67  0.17, 1.17  Vitamin D supplement use             Nonuser  1,129  0.88  0.16, 1.60  0.86  0.14, 1.57   User  1,054  0.51  −0.18, 1.21  0.47  −0.23, 1.17  Abbreviations: CI, confidence interval; 25(OH)D, 25-hydroxyvitamin D. a Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry, adjusting for genotyping batch and age. b Beta coefficients represent the difference in mean 25(OH)D concentration (ng/mL) for a 10% increase in European ancestry, adjusting for genotyping batch, age, body mass index, supplemental vitamin D use, ultraviolet B radiation flux, and season of blood draw. Among nonusers of vitamin D supplements (52% of the sample), there was a strong, statistically significant association of increasing 25(OH)D level with increasing percentage of European ancestry (Table 2). A 10% increase in European ancestry was associated with a 0.86-ng/mL increase in serum 25(OH)D concentration (95% CI: 0.14, 1.57). By contrast, there was not a statistically significant association among supplement users, and the β coefficient for a 10% increase in European ancestry in that group was 0.47 (95% CI: −0.23, 1.17). DISCUSSION Vitamin D levels have been demonstrated to be lower in African Americans than in US whites (1–6). Our findings indicate that among African Americans, vitamin D levels are related to the degree of genetic admixture, with higher levels observed among those in the upper percentiles of European ancestry. Notably, the association was present only among women who were not taking vitamin D supplements. We postulate that in the absence of supplementation, factors related to genetic ancestry, including genes related to skin pigmentation and metabolism, will likely explain more of the variability in vitamin D levels. The direction of our findings is consistent with results reported by Signorello et al. (1) based on 379 African Americans in the SCCS. The β coefficient among nonusers of supplements in the BWHS (β = 0.86) was consistent with the magnitude of the comparable estimate from the SCCS (β = 1.00). The SCCS report did not provide data on what proportion of the 379 women were using vitamin D supplements, but it is likely to have been lower than the 48% in the BWHS. First, use of vitamin D supplements has increased over time (11), and the SCCS blood draws were conducted approximately 10 years before the BWHS blood collection. Second, in an earlier SCCS paper, Egan et al. (5) reported that only 33% of African Americans in the study reported taking multivitamins (which may or may not have included vitamin D). The SCCS was limited to residents of 10 southeastern states, whereas the BWHS included residents from all regions of the mainland United States. The mean percentage of European ancestry in the SCCS sample was 7.1%, whereas in the BWHS the mean was 21.7%. Much of the association between percentage of European ancestry and vitamin D status may be driven by differences in skin pigmentation, given that skin pigmentation has been shown to decrease with decreasing percentage of African ancestry among African Americans (18). It would be necessary to have a measure of skin pigmentation in order to parse out this segment of the ancestry–vitamin D association and determine what portion of the association is explained by other factors. Other explanatory factors may include diet and inherent differences in the ability to absorb or synthesize vitamin D that are unrelated to melanin levels (1). We were able to account for dietary intake of vitamin D in our analyses by adjusting for dairy food consumption. Control for dairy consumption produced little change in our results; while dairy consumption was positively associated with European ancestry in our study sample, it was not associated with serum 25(OH)D level. Our study was limited by a lack of data on skin pigmentation, which prevented us from deciphering how much this factor was responsible for the ancestry–vitamin D association we observed. In addition, the most recent dairy food consumption data available were from 2001, and changes in dietary patterns since that time could not be accounted for. Also, we did not have quantitative information on the amount of vitamin D in supplements taken by those women who reported supplement use. A strength of the study was the large sample of African-American women with both genetic ancestry estimates and vitamin D measurements. In summary, we observed a significant positive association between percentage of European ancestry and serum 25(OH)D level in African-American women. Importantly, the association was most evident among women who were not taking supplemental vitamin D. Vitamin D supplement use appeared to be an effective means of reducing disparities in 25(OH)D levels conferred by genetic ancestry. ACKNOWLEDGMENTS Author affiliations: Slone Epidemiology Center at Boston University, Boston, Massachusetts (Stephen A. Haddad, Edward A. Ruiz-Narváez, Yvette C. Cozier, Hanna Gerlovin, Lynn Rosenberg, Julie R. Palmer); and Department of Biostatistics, School of Public Health, Boston University, Boston, Massachusetts (Hanna Gerlovin). This work was funded by National Institutes of Health grants R01 CA058420, R01 CA098663, and UM1 CA164974. Conflict of interest: none declared. Abbreviations BMI body mass index BWHS Black Women’s Health Study CI confidence interval NHANES National Health and Nutrition Examination Survey 25(OH)D 25-hydroxyvitamin D SCCS Southern Community Cohort Study UVB ultraviolet B REFERENCES 1 Signorello LB, Williams SM, Zheng W, et al.  . Blood vitamin D levels in relation to genetic estimation of African ancestry. Cancer Epidemiol Biomarkers Prev . 2010; 19( 9): 2325– 2331. Google Scholar CrossRef Search ADS PubMed  2 Zadshir A, Tareen N, Pan D, et al.  . The prevalence of hypovitaminosis D among US adults: data from the NHANES III. Ethn Dis . 2005; 15( 4 suppl 5): S5-97– S5-101. 3 Holick MF. High prevalence of vitamin D inadequacy and implications for health. Mayo Clin Proc . 2006; 81( 3): 353– 373. Google Scholar CrossRef Search ADS PubMed  4 Harris SS. Vitamin D and African Americans. J Nutr . 2006; 136( 4): 1126– 1129. Google Scholar CrossRef Search ADS PubMed  5 Egan KM, Signorello LB, Munro HM, et al.  . 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American Journal of EpidemiologyOxford University Press

Published: Jan 30, 2018

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