The effect of a ketogenic diet (KD) - a very low carbohydrate and high-fat diet - without caloric restrictions on endurance capacity in healthy adults

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The ketogenic diet (KD) is a very low carbohydrate, high-fat and adequate-protein diet that without limiting calories, induces a metabolic condition called ketosis, a state of elevated levels of ketone bodies in the blood. There is one traditional diet known to have been essentially free of carbohydrates, namely that of the Inuits, their diet consisted over generations mainly of mammals, fish and birds and nearly no vegetables. A KD was first introduced in our society to treat epilepsy in the 1920s and has become quite popular in recent decades as a weight-loss diet. A KD inhibits insulin secretion most straightforwardly via the ingestion of very few carbohydrates. An insulin-inhibition diet was proven effective in experimental animal models targeting tumour energy metabolism. Although there is too little scientific evidence currently to recommend a KD in cancer patients (larger trials are ongoing), an increasing number of cancer patients put themselves on a KD outside of clinical trials and often without medical supervision in response to frequent press reports and health books about the positive potential of KDs. Regular physical exercise during and after anti-cancer therapy has been associated with many health benefits, ranging from improvements in quality of life and cancer-related fatigue, to the reduction in cancer recurrences and mortality among several cancers. Therefore, any diet that may compromise physical performance and the patient s capacity to adhere to an exercise regime would be of great concern. Furthermore, there is already evidence of the potential for KDs to deplete muscle and liver glycogen stores and to trigger symptomatic side-effects of weakness and fatigue. In the literature to date, there is limited data focusing on the effect of a KD outside weight-loss studies (caloric restriction) and involving a long enough intervention duration to overcome the metabolic adaption phase to chronically low-carbohydrate intake (1-2 week) on physical performance. Therefore, the major objective of the proposed study is to firstly assess whether the application of a KD without caloric restriction leads to a loss of physical performance (endurance capacity and muscle strength) in healthy adults. The experimental intervention of this before-and-after comparison trial in 50 healthy adults consists of a KD lasting 6 weeks with a previous preparation period consisting of detailed instructions during regular classes over a 3-week period on how to follow a KD. The primary outcome measure will be the maximum rate of oxygen consumption to exhaustion as measured during graded bicycle exercise. VO2max reflects an individual s aerobic physical fitness and is a key determinant of endurance capacity. Key secondary outcome measures are strength, body composition and metabolic parameters.