Consistent ethnic specific differences in diabetes risk and vitamin D status in the National Health and Nutrition Examination Surveys

- Vitamin D status not associated with diabetes in blacks, but is in non-blacks.
- Confirms previous results from earlier US surveys.
- Consistency suggests this is unlikely due to chance.
- Suggests a covariate maybe modifying association between vitamin D and diabetes.
- Covariate maybe absent or reduced in non-Hispanic blacks.

Previous findings from the Third National Health and Nutrition Examination Survey (NHANES III), a representative sample of the US population carried out in 1988-1994, showed an inverse association between diabetes risk and serum concentrations of 25-hydroxyvitamin D (25(OH)D) in non-Hispanic whites and Mexican-Americans but not in non-Hispanic blacks. The study aim was to determine if this same pattern in ethnic variation occurred in more recent NHANES surveys. Cross-sectional data came from the NHANES carried out from 1988 to 1994 (NHANES III) and from 2001 to 2006 (NHANES 01-06). The analysis included 11,331 people (5641 non-Hispanic white, 2714 non-Hispanic black and 2976 Mexican American) without known diabetes mellitus, fasting for ≥8 h and aged ≥20 years, with available measurements of 25(OH)D, fasting glucose, fasting insulin and body mass index (BMI). Adjusting for age, gender, BMI, leisure time physical activity and season, higher levels of 25(OH)D were associated with decreased fasting glucose, decreased fasting insulin, and decreased diabetes risk in both non-Hispanic whites and Mexican Americans for both surveys and when combined. When combining NHANES III and NHANES 01-06 the odds ratio (95% confidence interval) for having diabetes was 0.28 (0.19, 0.41) in the highest 25(OH)D quartile compared to the lowest quartile in non-Hispanic whites, and 0.13 (0.06, 0.28) in Mexican Americans (both p < 0.0001); but 1.54 (0.62, 3.82) in non-Hispanic blacks, among whom 25(OH)D was not associated with fasting glucose, fasting insulin, or diabetes risk (p > 0.05). There was a significant interaction between non-Hispanic whites and Mexican Americans combined, compared with non-Hispanic blacks, when 25(OH)D was regressed against fasting glucose (p = 0.016) but not against fasting insulin (p > 0.05).The major finding in both NHANES surveys of consistent inverse associations between serum 25(OH)D concentrations and diabetes risk in non-Hispanic whites and Mexican Americans, but not in non-Hispanic blacks, suggests this finding is unlikely due to chance.

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