Epidemiological Assessment of Cardiometabolic Risk

Study participants provided a medical history and a physical examination. Participants self‐reported their demographics and smoking status. Weight was measured using a balance beam scale and height was measured using a Harpenden stadiometer (Holtain Ltd). Body mass index (BMI) was calculated in kg/m². Baseline prevalent diabetes was defined using self‐reported histories, use of antidiabetic agents, fasting plasma glucose concentration >125 mg/dL, or a 2‐h oral glucose tolerance test result >199 mg/dL. Systolic and diastolic blood pressures were measured twice using a conventional mercury sphygmomanometer and averaged. Medications were brought into study visits by participants and categorized using the Iowa Drug Information System.
Serum cystatin C and urine albumin and urine creatinine measurements were available only at year 1 (baseline), so we carried forward these measurements to year 2 when vitamin D metabolites and all other data were collected, consistent with prior studies.^[6, ^11] Urine albumin was measured using a particle‐enhanced turbidimetric inhibition immunoassay allowing for direct albumin quantification (Siemens). Measurement of urine creatinine was done by a modified Jaffé method on a clinical chemistry analyzer (Siemens). Cystatin C was measured at the Health ABC core laboratory (University of Vermont, Burlington, VT, USA) with a BNII nephelometer (Dade Behring Inc.) that used a particle‐enhanced immunonephelometric assay (N Latex Cystatin C). Estimated glomerular filtration rate (eGFR) was calculated using the 2012 CKD Epidemiology Collaboration (CKD‐EPI) cystatin C equation.^[22] Serum calcium and phosphate were measured using direct quantitative colorimetric determination (Stanbio Laboratory). Intact parathyroid hormone (iPTH) was measured in EDTA plasma using a two‐site immunoradiometric assay kit (N‐tact PTHSP; DiaSorin). Fibroblast growth factor‐23 (FGF‐23) was measured using an intact assay (Kainos Laboratories). Serum albumin was measured using the same assay for VDBP with the addition of internal standards for three albumin‐specific peptides, as described previously.^[11]