Abstract Introduction Limited evidence exists regarding pre-operative “liver shrinking” diet, but a two-week period of low calorie diet (LCD) is mandated before bariatric surgery, with weight outcomes rarely measured. Longer periods of LCD can ameliorate obesity, diabetes and metabolic-associated-fatty-liver disease (MAFLD). Primary aim to determine if response to LCD is predictive of long-term weight loss. Secondary aim to determine if alteration in liver function, and MAFLD, correlate with weight change after diet/ in follow up. Methodology Patients undergoing sleeve gastrectomy recruited and weight, blood plasma and 16S rRNA faecal microbiome profile recorded at baseline, following two-week LCD and at six-months, with liver biopsy at surgery (6). Results First nine patients had median baseline BMI 47.3kg/m2, and 34.7 kg/m2 at 6-months (-12.3kg/m2). Median excess weight loss was 8.6% (6.35kg) after LCD and 47.57% (34.65kg) at 6-months. Spearman’s Rank Correlation was weakly positive (0.5) for absolute weight loss between LCD and follow up, and was 0.25 using excess weight loss (non-significant). Change in liver function and MAFLD grade did not correlate with weight loss following LCD and at 6-months. Conclusions Weight loss response to LCD correlated poorly with long-term weight loss (analysing by absolute/ excess weight). Liver health and MAFLD did not predict 6-month weight loss. A dramatic response to LCD may not manifest in longer term metabolic improvement after surgery. We will also report on faecal microbiome; it appears more complex mechanisms underpin weight change and response to dietary manipulation in the surgical cohort than can be extrapolated from population studies.