This study presents a real-life insight into the lives of five seasonal endurance athletes who by virtue of their own curiosity wished to experience the effects of a ketogenic diet on their sports performance. Overall, participants were able to increase the substrate utilisation of free fatty acids, reduce body fat and experience positive health benefits, but their maximal aerobic performance was compromised.
The reduced body fat can likely be explained by a resultant calorie deficit created by the diet, as participants reported enhanced feelings of satiety and a reduction in overall food intake. This outcome was unsurprising and comparable to findings in previous research on both strength and endurance athletes [13,14,15]. Initial weight reduction can be associated with a loss in body water through glycogen depletion, [16, 17], and this was also likely the case in this study; however fat loss was evident as per skinfold changes. A further theory relating to weight loss, which is as yet, rigorously tested, is an increased drive for fat breakdown rather than storage as circulating levels of insulin remain low during ketogenic diets . Perhaps a combination of all three mechanisms can explain the weight loss. A limitation of the study was a lack of energy comparison prior to, and during the study, which would have provided some clarity about these mechanisms.
All participants had a greater fatty acid oxidation at a higher given intensity at the end of the trial compared to baseline. This finding of enhanced fat utilisation aligns with those of several other groups that have incorporated ketogenic and non-ketogenic dietary protocols [5, 19, 20]. Furthermore, this substrate utilisation alteration can be attributed to the change in diet as training was kept relatively consistent throughout the intervention. Our participants also had a higher oxygen cost at sub-maximal workloads due to the higher use of fat as an energy substrate. However, this did not benefit exercise capacity.
On the whole, maximal aerobic performance was reduced, another comparable outcome to similar research [21,22,23], the exception being one athlete in Phinney et al.’s 4-week cycling study , and Zajac et al.’s eight off-road cyclists , who all showed performance increases. The performance decrement in our study, and others, is likely due to changes in metabolic pathways that impair glycogen metabolism at higher exercise intensities [24,25,26]. Specifically, a down-regulation of the carbohydrate oxidative enzyme, pyruvate dehydrogenase (PDH), which via conversion of pyruvate to acetyl-coenzyme A, links the glycolytic pathway with the Krebs cycle . PDH is said to be reduced rapidly through a reduction in circulating insulin and an increase in circulating levels of free fatty acids . Evidence suggests that PDH is upregulated upon carbohydrate reintroduction ; however, there is little insight into its fate along with other mitochondrial enzymes in the context of low carbohydrate availability. Despite similarities in findings with other studies, some of these studies are limited by short-duration low carbohydrate diets [28,29,30]. Future research with chronically fat-adapted athletes is needed to investigate these micro-level mechanisms alongside performance outcomes. Recently, Volek et al.’s work  with chronically fat-adapted ultra-endurance athletes (>6 months) not only demonstrated a 2.3 times greater fat oxidation rate in the LCHF group compared with the mainstream dietary group, but also demonstrated no difference in resting and replete muscle glycogen stores between groups. Authors suggest a homeostatic muscle glycogen repletion mechanism arising from hepatic gluconeogenesis, which might serve to provide clues into why many athletes report optimal performance, anecdotally, when having eaten in an LCHF manner for extended periods of time. While this is a plausible speculation, a similar study by Webster et al.,  showed no difference in gluconeogenic rates during exercise in fasting LCHF and mixed-diet athletes. In fact, glucose was produced endogenously to a greater extent in the mixed diet group, and was attributed to greater rates of hepatic glycogenolysis. Researchers concluded that gluconeogenesis during exercise may remain stable across a range of dietary regimes after an overnight fast, but that hepatic glycogenolysis is influenced by dietary carbohydrate. Further exploration of fuel contributions to gluconeogenesis and the effect of different feeding protocols on endogenous glucose producing mechanisms is warranted. It is important to note that both of these studies did not incorporate a performance measure, leaving the questions to this key issue unanswered [19, 20].
This is one of the few studies to report specifically on endurance athletes’ experiences of undertaking a ketogenic diet. Athletes reported similar negative physiological experiences to those reported by athletes in comparable ketogenic diet studies [13, 14, 31]. However, they also reported experiencing benefits throughout the trial. One of these benefits was enhanced recovery; possibly, the rise in blood ketones had some influence, as beta-hydroxybutyrate has been associated with upregulating antioxidant gene expression and decreasing reactive oxygen species . However, further research is required to substantiate this within athletic populations.
From a physical well-being perspective, the cases of improved skin, and the resolution of an ongoing prostate issue, were major points of discussion of benefits experienced. We speculate that it is the reduction of systemic inflammation as a result of a lower total sugar  and Omega 6 fatty acid intake, thereby rebalancing the Omega 6:3 fatty acid ratio in an anti-inflammatory direction  that gave rise to these outcomes. All participants were consuming high-Omega 6 industrial seed oils prior to the study (used as cooking fat and derived from processed foods). During the study these fats were replaced with coconut oil, butter and olive oil; i.e., fats containing minimal Omega 6 fatty acid content.
Being a translational study, we followed up participants informally 12 months after the study concluded. They were all still competing in endurance events, and while not eating a ketogenic diet, none of them had returned to their previous high carbohydrate, low fat style of eating. Collectively, they reported that once the study concluded they gradually increased their carbohydrate intake until the point at which they felt their performance at high intensities return. They were still restricting carbohydrate and eating more fat than mainstream guidelines recommend, and reported having discovered the optimal macronutrient ratio that satisfied a performance, body composition and a health goal.
This study had several limitations: Its design as a pilot case study, with no standardisation of training prevents any inference from being made to athletic populations. However, it is still relevant to both the researcher and the practitioner as it provides insights into what is considered important for athletes, particularly those in the 40+ age range. i.e., alongside improving performance, they are also more cognisant of their overall health and well-being. A lesson learned from undertaking this research, and a key consideration for researchers and practitioners, is to encourage a reduction in athlete training intensity and volume in the early weeks of embarking on a ketogenic diet. This will likely induce less early fatigue and other negative symptoms related to training, and allow for metabolic adaptations to occur in a lower stress milieu.