The Effect of Bariatric Surgery on Insulin Sensitivity and Energy Metabolism

Insulin resistance strongly relates to ectopic lipid deposition in skeletal muscle and the liver, which correlate with insulin resistance. Lipid metabolites accumulating in skeletal muscle and the liver are thought to impair insulin signalling and thereby reduce glucose uptake and glycogen storage. Insulin resistant humans frequently present with decreased mitochondrial function in skeletal muscle which might contribute to lipid accumulation and the development of insulin resistance. Metabolic dysfunction-associated steatotic liver disease comprises steatosis, steatohepatitis and cirrhosis. MASLD correlates with insulin resistance, increased risk for cardiovascular diseases and type 2 diabetes. The mechanisms leading from steatosis to steatohepatitis and insulin resistance in the liver are yet unclear. Bariatric surgery aims at profound reduction of body weight. Also, it frequently and rapidly leads to normalization of glucose tolerance even before the onset of body weight reduction. The underlying mechanisms are yet unclear. In this study we aim to explore the mechanisms underlying the onset of insulin resistance and steatohepatitis in patients with steatosis and to identify the mechanisms leading to improved glucose tolerance in humans after bariatric surgery. We test the following hypotheses: increased lipid availability leads to (i) increased lipid oxidation and oxidative stress (ii) accumulation of lipid metabolites that impair insulin signalling (iii) bariatric surgery improves insulin sensitivity by increasing lipid oxidation. This study will contribute to the understanding of MASLD and will help to identify new targets for the therapy of diabetes.