The aim of the study is to demonstrate that a ketoacid supplement in patients with stage III to IV chronic renal failure, reduces the excretion of urinary urea nitrogen

Patients with an estimated glomerular filtration rate of less than 25 ml/kg/min, not on renal substitution therapy, without important concomitant diseases and aged between 40 and 70 years, will be invited to participate in the study. They will be randomly assigned to a group that will receive receive a supplement of ketoacids (Ketosteril) 1 tablet containing 630 mg of ketoacids every 5 kg of body weight and a diet containing 25 to 35 kcal/kg and 0.3 g/protein per day or to a group receiving placebo and a diet containing 25 to 35 kcal/kg and 0.6 g/protein per day. The intervention will last 16 weeks. At baseline and the end of the intervention a blood sample will be obtained to measure creatinine, urea nitrogen and cystatin C. Also a spot urine sample will be obtained to measure urea nitrogen and creatinine excretion. The compliance with the dietary prescription and the ketoacid or placebo supplement will be assessed every 2 weeks.

Inclusion Criteria:

• Estimated glomerular filtration rate of less tan 25/ml/min/1.73m2
• Not on renal substitution therapy
• Absence of severe life threatening concomitant diseases

Exclusion Criteria:

• Malignant or renovascular hypertension
• Use of systemic steroids or immunosuppressant drugs
• Alcohol or illicit drug abuse
• A body mass index of less tan 18 kg/m2

There will be no masking of the treatment arm for researchers