Randomized, single-blind, trial to determine whether a high-protein, low calorie diet is more effective than a control low calorie diet in improving the metabolic and histologic abnormalities in patients with Non-alcoholic Fatty Liver Disease undergoing bariatric surgery.

Inclusion Criteria:

• Morbidly obese subjects with a BMI >40 kg/m2 who are eligible for bariatric surgery and have accepted to undergo RYGB
• Weight stable for at least 6 months before the study (+/- 5 kg within the previous 6 months)
• Stable medication
• Provision of informed consent

Exclusion Criteria:

• Patients not eligible for laparoscopic RYGB
• Incapacity to give a valid informed consent or unwilling to give the consent
• Pregnancy or lactating
• Type 2-diabetes mellitus
• Significant illness within the two weeks preceding surgery, as judged by the physician.
• Obvious infection (bacteria, virus etc)
• Major cardiovascular disease
• Major gastrointestinal, respiratory, or any hormonal disorders
• History of drug addiction and/or alcohol use
• Suspected or confirmed poor compliance
• Alcohol consumption of > 20 g per day in the case of women and > 30 g per day in the case of men for at least 3 consecutive months during the previous 5 years
• Presence of hepatitis C or frank cirrhosis.