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.