OBJECTIVE—To determine the association between serum 25-hydroxyvitamin D (25OHD) and diabetes risk and whether it varies by ethnicity. RESEARCH DESIGN AND METHODS—We performed an analysis of data from participants who attended the morning examination of the Third National Health and Nutrition Examination Survey (1988–1994), a cross-sectional survey of a nationally representative sample of the U.S. population. Serum levels of 25OHD, which reflect vitamin D status, were available from 6,228 people (2,766 non-Hispanic whites, 1,736 non-Hispanic blacks, and 1,726 Mexican Americans) aged ≥20 years with fasting and/or 2-h plasma glucose and serum insulin measurements. RESULTS—Adjusting for sex, age, BMI, leisure activity, and quarter of year, ethnicity-specific odds ratios (ORs) for diabetes (fasting glucose ≥7.0 mmol/l) varied inversely across quartiles of 25OHD in a dose-dependent pattern (OR 0.25 [95% CI 0.11–0.60] for non-Hispanic whites and 0.17 [0.08–0.37] for Mexican Americans) in the highest vitamin D quartile (25OHD ≥81.0 nmol/l) compared with the lowest 25OHD (≤43.9 nmol/l). This inverse association was not observed in non-Hispanic blacks. Homeostasis model assessment of insulin resistance (log_e) was inversely associated with serum 25OHD in Mexican Americans (P = 0.0024) and non-Hispanic whites (P = 0.058) but not non-Hispanic blacks (P = 0.93), adjusting for confounders. CONCLUSIONS—These results show an inverse association between vitamin D status and diabetes, possibly involving insulin resistance, in non-Hispanic whites and Mexican Americans. The lack of an inverse association in non-Hispanic blacks may reflect decreased sensitivity to vitamin D and/or related hormones such as the parathyroid hormone.

The report by Hyppönen and Power in this issue of the Journal (1) highlights a frustrating and regrettable situation for nutrition researchers. In the early 1970s, the same serum 25-hydroxyvitamin D [25(OH)D] concentrations reported by Hyppönen and Power were thought to be indicative of “healthy” white adults in the United Kingdom (2). However, during those early years after the discovery of 25(OH)D, the adequacy of its serum concentration was based simply on whether the concentration was enough to prevent osteomalacia or rickets. Three decades later, we know that 25(OH)D concentrations relate to many other aspects of health, including fracture risk, bone density, colon cancer, and even tooth attachment (3); we also know that much higher concentrations of 25(OH)D are needed to prevent adverse outcomes. Indeed, in the 1958 British birth cohort, lower 25(OH)D is associated with a higher percentage of hemoglobin A_1C (a measure of long-term glucose concentration), which further emphasizes the need to maintain optimal 25(OH)D concentrations (4).

Populations with low vitamin D status, such as blacks living in the US or UK, have increased blood pressure (BP) compared with whites. We analyzed the association between serum 25-hydroxyvitamin D (25OHD) and BP to determine whether low 25OHD explains any of the increased BP in blacks. The Third US National Health and Nutrition Examination Survey (NHANES III) is a cross-sectional survey representative of the US civilian population during 1988 to 1994. Analyses were restricted to 12,644 people aged > or =20 years with measurements of BP and 25OHD, after excluding those on hypertensive medication. Adjusted mean serum 25OHD was lowest in non-Hispanic blacks (49 nmol/L), intermediate in Mexican Americans (68 nmol/L), and highest in non-Hispanic whites (79 nmol/L). When participants were divided into 25OHD quintiles, mean (standard error) systolic BP was 3.0 (0.7) mm Hg lower (P = .0004) and diastolic BP was 1.6 (0.6) mm Hg lower (P = .011) for participants in the highest quintile (25OHD > or =85.7 nmol/L) compared with the lowest (25OHD < or =40.4 nmol/L), adjusting for age, sex, ethnicity, and physical activity. Further adjustment for body mass index (BMI) weakened the inverse association between 25OHD and BP, which remained significant for systolic BP (P < .05). The inverse association between 25OHD and systolic BP was stronger in participants aged > or =50 years than younger (P = .021). Ethnic differences in 25OHD explained about half of the increased hypertension prevalence in non-Hispanic blacks compared with whites. Vitamin D status, which is amenable to intervention by safely increasing sun exposure or vitamin D supplementation, was associated inversely with BP in a large sample representative of the US population.

The possibility that UV radiation, through a mechanism that involves vitamin D, may protect against cardiovascular disease (CVD) was initially proposed in 1981.¹ Results from recent meta-analyses^2,3 have confirmed that people with low vitamin D status have increased risk of CVD. Several pathophysiologic mechanisms could explain this inverse association.⁴

Scragg R (Department of Community Health, University of Auckland, Private Bag, Auckland, New Zealand), Jackson R, Holdaway I M, Lim T and Beaglehole R. Myocardial infarction is inversely associated with plasma 25-hydroxyvitamin D3 levels: A community-based study. International Journal of Epidemiology 1990; 19: 559–563. The relation between the plasma level of 25-hydroxyvitamin D3, the main metabolite of sun-induced vitamin D, and myocardial infarction (MI) was investigated in a community-based case-control study. Some 179 MI patients presenting to hospital within 12 hours of the onset of symptoms were individually matched with controls by age, sex and date of blood collection. MI patients had significantly lower mean 25-hydroxyvitamin D3 levels than controls (32.0 versus 35.5 nmol/L; p = 0.017), with the case-control differences being greatest in winter and spring. The relative risk of MI for subjects with 25-hydroxyvitomin D3 levels equal to or above the median was 0.43 (95% confidence limits = 0.27, 0.69) compared to subjects below the median. The decrease in MI risk associated with raised vitamin D3 levels was observed in all seasons. These results provide support for the hypothesis that increased exposure to sunlight is protective against coronary heart disease.