Kidney function declines with age and this could affect phosphate balance after an acute phosphate load (increased phosphate intake). In daily life, we regularly experience acute phosphate loads through our diet.

The aim of this clinical study is to determine whether the body's response to increased phosphate intake changes with age and whether there are sex differences in this response.

Forty subjects (males and females aged 18-25 or 63-70) will ingest an oral phosphate load, after which their body's response will be assessed by collecting blood and urine samples over a period of ten hours.

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