The Metabolic Effects of Ultra-endurance Exercise and Training Under Low or High Glycaemic Index-carbohydrate Diets.

Inclusion Criteria: * Informed consent obtained * Male or female aged 18-65 years (both inclusive) * Otherwise healthy (as judged by premedical questionnaire) and participating in regular training (volume in training \>10 h training per week) and participating in endurance events and/or VO2max \> 55 ml.kg-1.min-1. Exclusion Criteria: * Receipt of any investigational medicinal product within 1 month prior to screening in this trial * Haemoglobin \<8.0 mmol/L (male) or \<7.0 mmol/L (female). * Systemic (oral or i.v.) corticosteroids, monoamine oxidase (MAO) inhibitors, non-selective beta-blockers, growth hormone and non-routine vitamins and herbal products. Furthermore, thyroid hormones are not allowed unless the use of these has been stable during the past 3 months. * Suffer from or history of a life-threatening disease (i.e. cancer judged not to be in full remission except basal cell skin cancer or squamous cell skin cancer), or clinically severe diseases that directly influence the study results, as judged by the Investigator. This does not prohibit the participation of patients taking medications that influences their metabolism (e.g. statin) or cardio- respiratory system (e.g. asthma spray) as long as the therapy is stable and is not adapted throughout the run of the trial. Furthermore, it does not exclude patients how have coeliac disease (or similar diseases or allergies), as long as the disease is stable, and patients are able to stay on their specific (e.g.) gluten- free diet. * Known cardiac problems defined as decompensated heart failure (New York Heart Association (NYHA) class III and IV) 10 at any time and/or angina pectoris within the last 12 months and/or acute myocardial infarction at any time * Supine blood pressure at screening (after resting for 5 min in supine position) outside the range of 90-140 mmHg for systolic or 50-90 mmHg for diastolic (excluding white-coat hypertension; therefore, if a repeated measurement on a second screening Visit shows values within the range, the participant can be included in the trial). This exclusion criterion also pertains to participants being on anti-hypertensives. * History of clinically significant abnormal ECG issues, as judged by the Investigator. * Severe retinopathy or maculopathy and/or severe neuropathy, in particular autonomic neuropathy, as judged by the Investigator. * Any chronic disorder or severe disease which, in the opinion of the Investigator might jeopardise participant's safety or compliance with the protocol. * History of allergies and/or intolerances to drugs or foods or a history of severe anaphylactic reaction e.g. fructose intolerance. * Significant history of alcoholism or drug/chemical abuse as per Investigator's judgement or a positive result in the urine drug/alcohol screen at the screening Visit. * Smoker (defined as a participant who is smoking more than 5 cigarettes or the equivalent per day). * Not able or willing to refrain from smoking or use of nicotine substitute products during the monitoring period. * Participant with mental incapacity or language barriers precluding adequate understanding or cooperation or who, in the opinion of their general practitioner or the Investigator, should not participate in the trial. * Participating in any other research trial which may interfere with the current study. * Potentially non-compliant or uncooperative during the trial, as judged by the Investigator. * Any condition that would interfere with trial participation or evaluation of results, as judged by the Investigator. * Any known history of diabetes mellitus, or the use of any anti- hyperglycaemic drug or insulin to treat diabetes and related conditions.