| - Background: Poor vitamin D status is highly prevalent in the Netherlands, particularly among ethnic minorities and the elderly. Both calcium intake and vitamin D status are required for normal insulin secretion and affect parathyroid hormone (PTH) concentrations. High PTH concentrations may reduce insulin sensitivity. - Study objectives: To test the hypothesis that (1) Low calcium intake and poor vitamin D status are associated with type 2 diabetes in the elderly. (2) Calcium intake and vitamin D status affect insulin sensitivity and insulin secretion, and these effects are mediated by concentrations of PTH and 1,25-OH vitamin D. (3) Vitamin D status is associated with hyperglycemia in ethnic minority groups, and vitamin D suppletion can improve glucose metabolism in those with poor vitamin D status. - Approach: To test hypotheses 1 and 2, we will use data from the Hoorn Study. We will study the important steps of the putative causal chain: diet (dietary patterns, calcium and vitamin D intake), vitamin D status, hormones (PTH, 1,25-OH vitamin D), intermediary traits (parameters of insulin sensitivity and secretion), and type 2 diabetes. The association with type 2 diabetes will also be examined in the Longitudinal Aging Study Amsterdam (LASA). For the 3rd hypothesis, we will study vitamin D status in relation to fasting plasma glucose in a cross-sectional study of vitamin D status in 6 ethnic groups, and examine whether vitamin D status partly explains ethnic differences in hyperglycemia. In a subgroup with poor vitamin D status, we will conduct a trial of vitamin D suppletion and hyperglycemia. - Expected results: This study may provide more insight into the role of calcium, vitamin D, and related hormones on the development of type 2 diabetes, and may provide new clues for public health action to reduce the burden of type 2 diabetes in the elderly and ethnic minority groups. |
