All ETDs from UAB

Advisory Committee Chair

Barbara A Gower

Advisory Committee Members

Ambika Ashraf

David Calhoun

Betty Darnell

Jose Fernandez

Suzanne Judd

Document Type

Dissertation

Date of Award

2011

Degree Name by School

Doctor of Philosophy (PhD) School of Health Professions

Abstract

Adequate vitamin D status is associated with a reduced risk for type 2 diabetes (T2DM); the relationship may be mediated by insulin sensitizing effects of vitamin D. African Americans (AA) are at a greater risk for T2DM and insulin resistance, as well as vitamin D deficiency, compared to European Americans (EA). The primary objective of this dissertation was to investigate the independent relationships of vitamin D with various measures of insulin sensitivity. Investigations were performed in a series of studies involving adolescent and adult female cohorts. The intravenous glucose tolerance test-derived insulin sensitivity index (SI) or the oral glucose tolerance test-derived Matsuda index were used as indices of whole body insulin sensitivity, and the homeostatic model assessment of insulin resistance (HOMA-IR, a measure derived from fasted values of glucose and insulin) was used as a surrogate for hepatic insulin resistance. In a cohort of premenopausal women, dietary vitamin D was independently, positively associated with insulin sensitivity in AA, but not EA, and the relationship was stronger if HOMA-IR was used. In a cohort of pre- and postmenopausal women, circulating 25(OH)D was independently, positively associated with SI, but not with HOMA-IR. Additionally, adjustment for differences in 25(OH)D concentrations among AA and EA reduced ethnic differences in insulin sensitivity. In a cohort of obese AA adolescent females, participants with vitamin D deficiency (25(OH)D ≤ 15 ng/ml) had a significantly lower Matsuda index, although there was no group difference in HOMA-IR. Finally, in a separate study of obese AA and EA adolescent females, 25(OH)D was independently, inversely associated with fasting glucose, although there was an inverse relationship between 25(OH)D and fasting insulin in EA but not AA. A sub-study (n = 14 AA adolescents) indicated that vitamin D treatment for 8 weeks reduced fasting glucose (P = 0.05). In conclusion, vitamin D was independently associated with insulin sensitivity; however, relationships differed depending on form of vitamin D investigated, the method used to determine insulin sensitivity, and the population being investigated. Supplementation trials investigating vitamin D effects on insulin sensitivity are warranted and may be par-ticularly relevant in AA.

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