Assessing How the Body Responds to Increased Phosphate Intake, and How This Response Depends on Age and Sex.

Inclusion Criteria: * Healthy volunteers 18-25 and 63-70 years old * 18.5 ≥ BMI \< 26 kg/m2 * 0.84 ≥ phosphate in plasma ≤ 1.45 mM * 2.20 ≥ calcium in plasma ≤ 2.50 mM * 15 ≥ PTH in plasma ≤ 65 pg/ml * eGFR ≥ 60 ml/min/1-73 m2 * 90 ≥ Systolic blood pressure ≤ 140 mmHg * 60 ≥ Diastolic blood pressure ≤ 85 mmHg * No hematuria and no acute urinary tract infection * Ability and willingness to participate in the study * Voluntary signed inform consent Exclusion Criteria: * Diabetes mellitus * Pregnancy * History of kidney stones * History of parathyroidectomy * History of anorexia nervosa * History of bulimia * History of solid organ transplantation. * Nephrolithiasis * Sarcoidosis * Chronic pancreatitis * Chronic diarrhea * Chronic liver disease * Complete distal renal acidosis (dRTA) * Cystinuria * Hypo- or hyperaldosteronism * Hyper- or hypotension * Hypernatremia * Gastrointestinal disorders * Known hypersensitivity reactions to azoic dyes, acetylsalicylic acid, as well as antirheumatic drugs and painkillers (prostaglandin synthesis inhibitors). Intake of vitamin D and calcium supplements, phosphate, bisphosphonates, cinacalcet, denusomab, teriparatide, systemic glucocorticoids or mineralcorticoids, antiepileptics, carboanydrase inhibitors or diuretics