| Summary: | This thesis investigated three commonly used nutritional interventions that are often claimed to alter substrate metabolism and improve exercise performance: green tea extract (GTE), coffee and vitamin D. GTE and caffeine have been hypothesized to increase fat oxidation at rest and during exercise, thereby lowering the reliance on skeletal muscle glycogen and improving endurance exercise capacity. We observed that 7 days GTE supplementation resulted in an increase in metabolites related to fat and energy metabolism at rest but not during moderate intensity exercise. The current thesis also found that endurance exercise performance can be improved to the same extent by either using coffee or caffeine. However, these improvements in endurance exercise performance were independent of changes to fat oxidation during exercise. We also demonstrate that athletes living in Birmingham, United Kingdom, display a high prevalence of vitamin D deficiency during the winter and thus require nutritional support. However, despite the high prevalence of vitamin D deficiency, there was no association between vitamin D status and skeletal muscle function or exercise performance. Short term vitamin D supplementation at doses above the current recommended daily allowance was highly effective in correcting vitamin D deficiency to sufficiency. But supplementation did not alter any measure of performance.
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