C Christine Lee1, Kendra A Young2, Jill M Norris2, Jerome I Rotter3, Yeheng Liu3, Carlos Lorenzo4, Lynne E Wagenknecht5, David E Cole6, Steven M Haffner7, Yii-Der I Chen3, Anthony J Hanley1,8,9. 1. Department of Nutritional Sciences, University of Toronto, Toronto, Ontario M5S 3E2, Canada. 2. Department of Epidemiology, Colorado School of Public Health, University of Colorado Anschutz Medical Campus, Aurora, Colorado 80045. 3. Institute for Translational Genomics and Population Sciences, Los Angeles Biomedical Research Institute and Department of Pediatrics, Harbor-UCLA Medical Center, Torrance, California 90502. 4. Division of Clinical Epidemiology, University of Texas Health Science Center, San Antonio, Texas 78229. 5. Division of Public Health Sciences, Wake Forest School of Medicine, Winston-Salem, North Carolina 27157. 6. Departments of Laboratory Medicine and Pathobiology, Medicine, and Genetics, University of Toronto, Ontario M5S 1A8, Canada. 7. Shavano Park, Texas 78231. 8. Department of Medicine and Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario M5S 1A8, Canada. 9. Leadership Sinai Center for Diabetes, Mount Sinai Hospital, Toronto, Ontario M5T 3L9, Canada.
Abstract
Objectives: We aimed to compare the associations of directly measured plasma free 25-hydroxyvitamin D [25(OH)D] and total 25(OH)D concentrations with insulin sensitivity (SI) and β-cell function in nondiabetic Hispanics and African Americans. We hypothesized that directly measured free 25(OH)D would be more strongly associated with these measures of glucose homeostasis and that associations would differ by race. Design: We studied 1189 nondiabetic participants in the Insulin Resistance Atherosclerosis Study Family Study using data from baseline examinations from 2000 to 2002. SI, acute insulin response, and disposition index (DI) were determined from frequently sampled intravenous glucose tolerance tests. Plasma free and total 25(OH)D concentrations were measured by enzyme-linked immunosorbent assay and radioimmunoassay, respectively. Results: The median concentrations of plasma free 25(OH)D were 3.46 pg/mL for Hispanics and 2.17 pg/mL for African Americans (P < 0.0001), whereas the median concentrations of plasma total 25(OH)D were 16 ng/mL for Hispanics and 10 ng/mL for African Americans (P < 0.0001). Plasma free and total 25(OH)D were both positively associated with SI and DI in generalized estimating equations adjusted for demographic and lifestyle factors. After further adjustment with body mass index, the associations were no longer statistically significant, except for a significant association between plasma free 25(OH)D and SI. There was no effect modification by ethnicity on any of the exposure-outcome associations. Conclusions: Our data showed that plasma free 25(OH)D had a slightly stronger association with SI compared with plasma total 25(OH)D, although the difference was modest and there were no marked differences in the associations between Hispanics and African Americans.
Objectives: We aimed to compare the associations of directly measured plasma free 25-hydroxyvitamin D [25(OH)D] and total 25(OH)D concentrations with insulin sensitivity (SI) and β-cell function in nondiabetic Hispanics and African Americans. We hypothesized that directly measured free 25(OH)D would be more strongly associated with these measures of glucose homeostasis and that associations would differ by race. Design: We studied 1189 nondiabetic participants in the Insulin Resistance Atherosclerosis Study Family Study using data from baseline examinations from 2000 to 2002. SI, acute insulin response, and disposition index (DI) were determined from frequently sampled intravenous glucose tolerance tests. Plasma free and total 25(OH)D concentrations were measured by enzyme-linked immunosorbent assay and radioimmunoassay, respectively. Results: The median concentrations of plasma free 25(OH)D were 3.46 pg/mL for Hispanics and 2.17 pg/mL for African Americans (P < 0.0001), whereas the median concentrations of plasma total 25(OH)D were 16 ng/mL for Hispanics and 10 ng/mL for African Americans (P < 0.0001). Plasma free and total 25(OH)D were both positively associated with SI and DI in generalized estimating equations adjusted for demographic and lifestyle factors. After further adjustment with body mass index, the associations were no longer statistically significant, except for a significant association between plasma free 25(OH)D and SI. There was no effect modification by ethnicity on any of the exposure-outcome associations. Conclusions: Our data showed that plasma free 25(OH)D had a slightly stronger association with SI compared with plasma total 25(OH)D, although the difference was modest and there were no marked differences in the associations between Hispanics and African Americans.
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