This project consists of four substudies: a cohort study (A) and a sodium intake intervention (B) and a sodium excretion intervention (C) and a water intervention study (D). The main objective of the cohort study (A) is to investigate the prognostic implications of tissue sodium accumulation in CKD patients. The primary objective of the sodium intervention studies is to study the effect of high and low sodium intake (B) and increased renal sodium excretion (C) on tissue sodium content. The main objective of the water intervention study (D) is to investigate the effect of increased and habitual water intake on tissue sodium content and transepidermal water loss.

Cohort study (A): 60 CKD patients (eGFR 15 - 60 ml/min/1.73m2) are included in this cohort study. At the start of the study the association between tissue sodium content and micro-and macrovascular function will be evaluated. These patients will be followed up to investigate the association between tissue sodium content, quality of life and renal and cardiovascular events. Sodium intake intervention (B): A subgroup of 14 CKD patients (eGFR 15 - 60 ml/min/1.73m2) will be randomized to a 2-week low sodium diet and a 2-week high sodium diet in a cross-over study. Sodium excretion intervention (C): A subgroup of 12 CKD patients (eGFR 30 - 60 ml/min/1.73m2) with hypertension will be randomized to receive 6 weeks of treatment with hydrochlorothiazide, spironolactone and lercanidipine in a randomized open-label cross-over trial. Water intake intervention (D) A subgroup of 12 CKD patients (with eGFR 15-30 ml/min/1.73m2, hypertension and fasting morning urine osmolality \< 425 mOsm/kg for men and \< 400 mOsm/kg for women) will be randomized to a 4-week habitual water intake and 4-week increased water intake in a cross-over study.