It seems that “new” ideas in nutrition and sports nutrition are constantly recycled. There have been at least 3 distinct periods of challenge to the contemporary guidelines for the carbohydrate (CHO)-focused diet for training support and competition performance by the low carbohydrate, high fat (LCHF) diet. The first involved the 1983 study of a ketogenic (<20 g/d) CHO diet in trained cyclists by Phinney and colleagues. Although the results of this study are challenged by methodological issues, they suggest that 4 weeks of LCHF achieved adaptations that sustain exercise capacity at moderate intensity cycling compared with a high-CHO diet but at the expense of high-intensity exercise. A second phase of interest from 1995 to 2005, focused on adaptation to a non-ketogenic LCHF diet (∼25% CHO energy and 60% fat energy). As little as 5 days of training with LCHF was shown to retool the muscle to enhance fat-burning capacity with robust changes that persist despite acute strategies to restore carbohydrate availability (e.g. glycogen supercompensation, carbohydrate intake during exercise). However, the failure to detect clear performance benefits during endurance/ultra-endurance protocols, combined with evidence of impaired performance of high-intensity exercise via a down-regulation of carbohydrate metabolism led to the conclusion that “fat adaptation” strategies were not useful for competitive athletes in conventional sports. Recent re-emergence of interest in LCHF diets, coupled with testimonials from sportspeople who (apparently) follow them, has created a need to re-examine the potential benefits of this eating style. Recent AIS studies of the LCHF diet in world class athletes have focussed attention on the bioenergetics of substrate use, reminding us that we have known for a century that CHO oxidation is more economical (produces more ATP for a given amount of oxygen) than fat oxidation. A reduction in exercise economy is likely to be detrimental to the performance of exercise in higher intensity domains (e.g. >80% VO2max), confirming that LCHF is not likely to assist the performance of competitive endurance athletes where success requires sustained or critical outputs at such workloads. Notwithstanding the outcomes of future research and the enthusiastic debate on social media around the “keto lifestyle”, there is a need for better awareness of current sports nutrition guidelines which promote an individualised and periodised approach to fuel availability during training, allowing the athlete to prepare for competition performance with metabolic flexibility and optimal utilisation of all muscle substrates. Nevertheless, there may be a few scenarios where LCHF diets are of benefit, or at least are not detrimental, for sports performance.