Chapter 3 50 Material and methods Study participants Lifelines is a multi-disciplinary prospective population-based cohort study, examining in a unique three-generation design the health and health-related behaviors of 167,729 persons living in the North of The Netherlands. It employs a broad range of investigative procedures in assessing the biomedical, socio-demographic, behavioral, physical and psychological factors which contribute to the health and disease of the general population, with a special focus on multi-morbidity and complex genetics 15. In brief, participants were invited to participate through their general practitioner unless they met any of the following criteria: severe psychiatric or physical illness, limited life expectancy (<5 years), and/or insufficient knowledge of the Dutch language to complete a Dutch questionnaire. For the present study, we used data from the second assessment that took place from 2014 until 2018. Within this second assessment, we collected blood samples from August 2017 until December 2017 in which we measured IEM, plasma magnesium and 24-h urinary magnesium excretion. In total, we included 1669 participants aged >18 years. All participants provided written informed consent. The Lifelines cohort study is conducted according to the principles of the Declaration of Helsinki and approved by the medical ethical committee of the University Medical Center Groningen, The Netherlands. Measurements of clinical parameters All participants visited the Lifelines research facility for a basic medical examination including blood pressure (ten times using a Dinamap automated blood pressure monitor), body height, weight (and body mass index (BMI)) and hip and waist circumference. Participants received oral and written instructions regarding the collection of 24-h urine 16. Participants were instructed to postpone urine collection in case of urinary tract infection or menstruation, and to refrain as far as possible from heavy exercise during the collection period. On the day of collection, they were requested to discard the first morning urine, and to collect the subsequent urine for 24-h including the next morning’s urine. Urine and blood samples were collected for storage and laboratory measurements. Plasma glucose was measured routinely on a Cobas 8000 platform (Roche, Mannheim, Germany) using a hexokinase UV test.Total cholesterol was measured using an enzymatic colorimetric assay, high-density lipoprotein cholesterol using a homogeneous enzymatic colorimetric assay, low-density lipoprotein cholesterol using a direct measurement, homogeneous enzymatic
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