This thesis investigates the effect of body composition on insulin resistance and the impact of supplementation with nutritional support or carbohydrate treatment. Insulin resistance occurs as a response to a number of stressors, including surgery. However, the mechanism underlying the development of insulin resistance is as yet unclear. Adipose tissue distribution appears to play a role in the development of insulin resistance and obesity-related complications. In obese and non-obese patients undergoing open abdominal surgery who received preoperative carbohydrate or placebo, there was a significant fall in perioperative insulin sensitivity and changes in the expression of genes relating to carbohydrate and fat oxidation. There was no influence of perioperative carbohydrate or obesity on change in insulin sensitivity. Patients undergoing neoadjuvant chemotherapy for oesophageal cancer underwent pre and post chemotherapy assessment of insulin sensitivity and body composition. There was a significant reduction in insulin sensitivity despite minimal change in body composition and adequate nutritional intake. These studies have provided further information about the optimal methods for assessment of insulin sensitivity and body composition as well as an insight into mechanisms underlying the association between body composition and insulin sensitivity.