Acute kidney injury (AKI) occurs in approximately one-third of patients undergoing cardiac surgery (CS), and represents one of the most significant negative predictors of patient outcome in this population. In the healthy adult, a high protein meal is known to enhance glomerular filtration rate and is mediated by an increase in renal blood flow. The investigators hypothesized that preoperative oral protein load may precondition the kidneys for upcoming insults and reduce the rate of postoperative AKI and long-term renal outcome.

Acute kidney injury (AKI) occurs in approximately one-third of patients undergoing cardiac surgery (CS), and represents one of the most significant negative predictors of patient outcome in this population. To date, there is no therapy to prevent AKI. In the healthy adult, a high protein meal is known to enhance glomerular filtration rate (GFR) and is mediated by an increase in renal blood flow. The investigators hypothesized that preoperative oral protein load may induce an adaptive response of the kidneys, and precondition the kidneys for upcoming insults. In the present study, the investigators aimed to compare the prevalence and severity of AKI in patients undergoing oral high protein load of the 'Preoperative Renal Functional Reserve Predict Risk of AKI after Cardiac Operation' study to age- and gender-matched "controls" who had a standard preoperative care the day prior to surgery within the same period (November 2014-October 2015) at San Bortolo Hospital, Vicenza, Italy. Both groups were followed 1 year post-discharge to evaluate long-term renal outcomes.

Inclusion Criteria:

1. Subjects older than 18 years undergoing elective cardiac surgery
2. Subjects older than 18 years with an estimated eGFR >60 ml/min/1.73 m2 (CKD-EPI)
3. Subjects who signed informed consent forms

Exclusion Criteria:

1. Pregnancy
2. Chronic kidney disease ≥ stage III (KDIGO criteria)
3. Preexisting acute kidney injury (as determined by all available serum creatinine values from hospital and outpatient medical records within the previous 90 days)
4. Solitary kidney
5. Diabetes mellitus type 1
6. Recent cardiac arrest or myocardial infarction up to 7 days before surgery
7. Liver failure or cirrhosis
8. Total parenteral nutrition
9. Haemoglobin <11 g/dl
10. Sepsis
11. History of malabsorption, chronic inflammatory bowel disease, short bowel, or pancreatic insufficiency
12. Transplant donor or recipient
13. Active autoimmune disease with renal involvement
14. Rhabdomyolysis
15. Prostate hypertrophy with International Prostate Symptom Score ≥20
16. Transcatheter aortic valve implantation
17. Active neoplasm
18. Patients who did not pause angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers or received non-steroidal anti-inflammatory drugs a minimum of 48 hours before hospital admission.
19. Patients who received intravenous radiocontrast agents within the 72 hours before hospital admission.

Adult patients with estimated GFR ≥60 ml/min/1.73 m2 (CKD-Epidemiology Collaboration equation) undergoing elective cardiac surgery (coronary artery bypass, valve replacements, combined or other surgery, with cardiopulmonary bypass) between November 2014 and October 2015 at the San Bortolo Hospital, Vicenza, Italy

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