Prolonged (>90 min), strenuous exercise depletes endogenous carbohydrate (CHO) stores and is associated with a transient state of immunodepression and increased illness risk. Commencing exercise with low CHO availability accelerates fatigue and exacerbates perturbations to several in vitro immune components, particularly inflammatory T-lymphocyte (T-cell) cytokine production to an immune challenge (i.e. stimulation). Recurrent illness impairs training availability and performance; therefore, CHO fuelling strategies are recommended to support both endurance performance and immune function. Ketone bodies are an additional energetic substrate derived exogenously via ingesting ketone or ketogenic supplements or endogenously via hepatic ketogenesis following conformity to a very low-CHO, ketogenic diet (KD). Ketone supplementation and adaptation to a KD increase blood KB concentrations (minutes vs. days to weeks, respectively) from 0.1-0.2 to >0.5 mmol/L (i.e. nutritional ketosis or hyperketonaemia); albeit, exert disparate effects on CHO and fat metabolism. Therefore, the aim of this thesis was to examine the effect nutritional ketosis – via exogenous and endogenous origin – on endurance performance and immune function.

To identify the optimal dose of the racemic ketogenic supplement, R,S-1,3-butanediol (BD), the first experiment (Chapter 4) explored dose response effects (0 + 0 g/kg; 0.5 + 0 g/kg; 0.7 + 0 g/kg; 0.35 + 0.35 g/kg BD; boluses separated by 1.5 h) on blood D--hydroxybutyrate (D-bHB) concentration and T-cell related interleukin (IL)-4, IL-10 and interferon (IFN)-g gene expression within Staphylococcal Enterotoxin B (SEB)-stimulated peripheral blood mononuclear cells (PBMC) at rest. BD ingestion increased blood D-bHB concentration up to ~1 mmol/L; however, there was no was no effect on T-cell related cytokine gene expression within SEB-stimulated PBMCs compared to placebo (PLA). The second (Chapter 5) and third (Chapter 6) experiments examined the effect of BD ingestion compared to PLA on a preloaded (85 min at 85% second ventilatory threshold) ~30 min cycling time-trial (TT) performance and T-cell related IL-4, IL-10 and IFN-g gene expression within SEB-stimulated PBMCs, respectively. BD ingestion increased blood D-bHB concentration to 0.4-0.8 mmol/L during exercise and 1.38 ± 0.35 mmol/L at 1-h post-TT; however, blood glucose and lactate concentrations, exercise efficiency (i.e. oxygen uptake) and TT performance were unaltered (PLA, 28.50 ± 3.59; BD, 28.72 ± 3.23 min), with participants reporting gastrointestinal distress and minor symptoms of nausea, euphoria and dizziness. BD ingestion increased T-cell related gene expression throughout the trial; whereas, IL-4 and IL-10 gene expression were unaltered. These findings suggest blood D-bHB concentrations up to ~1 mmol/L do not benefit high-intensity endurance performance, but may transiently amplify the initiation of a pro-inflammatory type-1 T-cell cytokine response to an immune challenge during and 1-h following exercise.

The fourth (Chapter 7) and fifth (Chapter 8) experiments examined the effect of a 31-day KD (13.7 MJ, 4% [0.5 g/kg/day] CHO and 78% [4 g/kg/day] fat) compared to the participants habitual, mixed diet (13.1 MJ, 43% [4.6 g/kg/day] CHO and 38% [1.8 g/kg/day] fat) on moderate-intensity (70% maximal oxygen uptake [VO2max]) running capacity and T-cell related IL-4, IL-10 and IFN-g gene expression within multi-antigen-stimulated PBMCs and salivary secretory immunoglobulin A (SIgA), respectively. Adaptation to the KD increased fat oxidation (2- to 3-fold) and blood D-bHB concentration to 0.6-1.6 mmol/L during exercise and 2.8 mmol/L at 1-h post-exhaustion, whilst reducing CHO oxidation. Despite impairing exercise efficiency, as evidenced by increased oxygen uptake and energy expenditure, mean time-to-exhaustion was unaffected following the KD (pre-HD, 237 ± 44 vs. post-HD, 231 ± 35 min; pre-KD, 239 ± 27 vs. post-KD, 219 ± 53 min). T-cell related IFN-g gene expression transiently increased immediately after exhaustion; whereas, IL-4 and IL-10 gene expression were unaltered following the KD. Diet and exercise had no effect on SIgA concentration and secretion rate. These findings suggest conforming to a KD can preserve submaximal exercise capacity and may transiently amplify the initiation of a pro-inflammatory type-1 T-cell cytokine response to an immune challenge immediately following exercise; however, it does not alter mucosal immunity.

In conclusion, the findings of this thesis demonstrate the unique effects of ketone supplementation and adaptation to a KD on exercise metabolism, endurance performance and immune function. BD ingestion does not improve high-intensity endurance performance and should be avoided due to adverse gastrointestinal and systemic effects. Whereas, adaptation to a KD can preserve submaximal exercise capacity and may be considered a viable dietary option for select individuals. Moreover, elevated blood D-HB concentration – via BD ingestion and adaptation to a KD – can transiently increase pro-inflammatory circulatory type-1 T-cell cytokine gene expression to an immune challenge following exercise onset and cessation; however, the clinical implications for immune function are uncertain.