The aim of this study is to assess the effect of a diet prior to cardiac surgery on the common postoperative decline of renal function. Until now, there is no known drug or procedure to preserve the kidneys from this impairment. Patients with a known kidney disease are especially at risk. A potential beneficial effect of a diet prior to surgery has been shown in investigations in mammals, therefore this study will investigate if a preoperative diet in patients with known kidney disease and scheduled heart surgery can attenuate or prevent a postoperative loss of kidney function.

Patients with cardiothoracic surgery are at risk for postoperative acute renal failure which is associated with significant morbidity and mortality. There is no drug or procedure known to prevent this loss of renal function. Experimental data suggests, that a preoperative caloric restriction might provide kidney protection in this context. This clinical trial investigates if this phenomenon is also applicable in humans. Patients with a increased risk for a postoperative renal failure due to known chronic kidney disease are randomized in 2 groups. Patients of the diet group receive a calorie restriction to 60% of the calculated daily energy rate from day -7 until day -1 (included) pre-surgery (day 0 corresponds to day of surgery). Patients of the control group receive alimentation ad libitum.

Primary objective is the increase of serum creatinine in mg/dl 24 h after induction of ischemia ("cross clamping") in comparison to baseline value obtained in the morning of the day of surgery (day 0) in order to analyse if a preoperative calorie restriction as a preventive strategy leads to a attenuation of postoperative kidney injury. Hypothesis: A seven day calorie restriction reduces the increase of serum creatinine after cardiac surgery in patients with known chronic kidney disease.

Inclusion Criteria:

1. Men and women 18 years of age or older

2. Caucasian origin

3. Scheduled cardiothoracic operation with employment of cardio-pulmonary bypass and a lead time of 11 days minimum.

4. Indication for cardiac surgery is determined by the cardiothoracic specialist

5. Patient and/or legal guardian must be willing and able to give written informed consent

6. one of the following risk factors:

   1. serum creatinine >1,1 mg/dl in male and > 0,9 mg/dl in female
   2. type 2 diabetes
   3. peripheral artery occlusive disease
   4. heart failure with NYHA III-IV (ejection fraction < 50 %)
   5. combined CABG and heart valve surgery
   6. further surgery after CABG or heart valve surgery

Exclusion Criteria:

1. End-stage renal disease (patient on dialysis)
2. Indwelling kidney transplant
3. Malnutrition (BMI < 18,5 kg/m2)
4. Body weight < 46 kg in male and < 51 kg in female
5. BMI > 35 kg/m2 or body weight > 120 kg
6. Catabolic state (serum albumine < 25 g/l)
7. Diet within the previous 4 weeks
8. Inappetence
9. Weight loss > 1 kg within the previous 2 weeks, if not explained by use of diuretics
10. Hospital stay during the last 7 days prior to cardiac surgery
11. Consuming underlying disease
12. Uncontrolled local or systemic infection
13. Contraindication for enteral nutrition.
14. Known allergy against or incompatibility with ingredients of the employed formula-diet
15. Pregnancy or breast feeding
16. Participation in other interventional clinical trials.
17. Missing safe method of contraception or missing occurence of menopause (in female)
18. Professional or private relationship between subject and the investigators or dependence on the investigators.
19. Placement in an institution based on official orders.