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Adolescents with type 1 diabetes (T1DM) are given nutritional education, but the knowledge about their adherence to the food recommendations and associations between dietary intake and metabolic control is poor. Gastrointestinal symptoms are more prevalent in adults with T1DM than in healthy controls, which may be due to disturbed gastrointestinal motility. The meal content affects the gastric emptying rate and the postprandial glycaemia in healthy adults and adults with type 2 diabetes. Meal ingestion also elicits several postprandial hormonal changes of importance for gastrointestinal motility and glycaemia. Eating disorders are more prevalent in young females with T1DM than in healthy females, and are associated with poor metabolic control. The prevalence of eating disorders in adolescent boys with T1DM is not known. This thesis focuses on eating and gastrointestinal function in adolescents with T1DM. Three population-based, cross-sectional studies demonstrated that adolescents with T1DM consume healthy foods more often and have a more regular meal pattern than age- and sex-matched controls. Yet both boys and girls are heavier than controls. The intake of saturated fat is higher and the intake of fibre is lower than recommended in adolescents with T1DM. Patients with poor metabolic control consume more fat and less carbohydrates than patients with better metabolic control. Gastrointestinal symptoms are common in adolescents with T1DM, but the prevalence is not increased compared with controls. Gastrointestinal symptoms in patients are associated with female gender, daily cigarette smoking, long duration of diabetes, poor metabolic control during the past year, and an irregular meal pattern. Adolescent boys with T1DM are heavier and have higher drive for thinness than healthy boys, but do not differ from them in scales measuring psychopathology associated with eating disorders. In a randomized, cross-over study, we found that a meal with a high fat and energy content reduces the initial (0–2 hours) postprandial glycaemic response and delays gastric emptying in adolescents with T1DM given a fixed prandial insulin dose compared with a low-fat meal. The glycaemic response is significantly associated with the gastric emptying rate. Both a high- and a low-fat meal increase the postprandial concentrations of glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) and suppress the postprandial ghrelin levels in adolescents with T1DM. The postprandial changes of these hormones are more pronounced after the high-fat meal. Insulin-like growth factor binding-protein (IGFBP) –1 concentrations decrease after insulin administration irrespective of meal ingestion. The GLP-1 response is negatively associated with the gastric emptying rate. The fasting ghrelin levels are negatively associated with the postprandial glycaemic response, and the fasting IGFBP-1 levels are positively associated with the fasting glucose levels. We conclude that nutritional education to adolescents with T1DM should focus more on energy intake and expenditure to prevent and treat weight gain. It should also focus on fat quality and fibre intake to reduce the risk of macrovascular complications and improve glycaemia. Gastrointestinal symptoms in adolescents with T1DM should be investigated and treated as in other people irrespective of having diabetes. However, adolescents with long duration of diabetes, poor metabolic control, and symptoms from the upper gut should have their gastric emptying rate examined during euglycaemia. There may be an increased risk for development of eating disorders in adolescent males with T1DM since they are heavier than healthy boys and have higher drive for thinness. This should be investigated in future, larger studies. For the first time, we showed that a fat-rich meal delays gastric emptying and reduces the initial glycaemic response in patients with T1DM. The action profile of the prandial insulin dose to a fat-rich meal may need to be postponed and prolonged compared with the profile to a low-fat meal to reach postprandial normoglycaemia. Circulating insulin levels affect postprandial GIP, GLP-1, and ghrelin, but not IGFBP-1, responses less than the meal content. The pronounced GIP-response to a fat- and energy-rich meal may promote adiposity, since GIP stimulates lipogenesis. Such an effect would be disadvantageous for adolescents with T1DM since they already have increased body fat mass and higher weights compared with healthy adolescents. Adolescents with T1DM may have subnormal postprandial ghrelin suppression, which may be due to their increased insulin resistance or elevated growth hormone levels. This needs to be investigated in future, controlled studies.