An iconic 1967 study proved that a 7-d ketogenic (low-carbohydrate high-fat [LCHF]) diet impairs performance when eaten by athletes habituated to a high-carbohydrate low-fat (HCLF) diet (1). This suggests I should begin by conceding defeat. Instead, I will present the hidden truth of that study. That trial was not designed to exclude all possible metabolic mechanisms by which the ketogenic LCHF diet might impair exercise performance. The LCHF diet lowers both starting muscle (1) and liver glycogen concentrations (2); the latter increases the probability that exercise-induced hypoglycemia (EIH) will limit subsequent exercise. That trial (1) produced profound EIH in subjects eating the restricted CHO diets (Fig. 4 in [3]). Earlier studies (4,5) established that subjects can develop profound fatigue associated with cerebral symptoms of EIH during prolonged exercise. CHO ingested at exhaustion rapidly reversed the fatigue and the cerebral symptoms, allowing exercise to continue (Fig. 3 in [3]). Christensen and Hansen (5) noted that CHO ingested at exhaustion did not raise the RQ (Figs. 1 and 2 in [5]; Fig. 3 in [3]), so that "… the improved ability to exercise (with glucose ingestion) cannot be explained by the renewed carbohydrate supply to the muscles, because the beneficial effect is only present if this carbohydrate supply forces the blood sugar level to rise. The beneficial effect is thus certainly not a result of the increased carbohydrate depots or a greater percentage of carbohydrate burning in the muscles (current author`s emphasis), but exclusively of the higher blood sugar level and is closely related to the disappearance of the hypoglycemic symptoms. Fatigue must be regarded as a hypoglycemic symptom of cerebral origin" (5, p. 178).
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