Abstract

Dietary choices have a direct impact on the gut microbiome, which in turn influences several body functions. In recent years, polyphenols, which are plant secondary compounds, have been shown to have prebiotic-like effects and associated with multiple health benefits. Cranberries are native to North America and have the highest polyphenol content and antioxidant capacity among the commonly consumed fruits and vegetables. Furthermore, cranberries stand out due to their high levels of the rare A-type proanthocyanidin (PAC-1), which is believed to be the main contributor to the beneficial effects. Recently, the concept that a link between skeletal muscle and the host’s gut microbiota exists was put forward, with several research groups proposing that supplementation with polyphenols could promote improved muscle function, and consequently, improve exercise performance. One proposed mechanism for the positive effects of polyphenols on muscle function is through improved mitochondrial capacity. Mitochondria are the main producers of ATP, and their ability to generate energy as efficiently as possible is directly related to performance, especially in endurance athletes. The effect of cranberry polyphenols on exercise performance and skeletal mitochondrial function has not been explored before. This dissertation consists of five chapters. Chapter 1 introduces key concepts to provide background information and states the rationale, objectives, and hypotheses of the dissertation. Chapter 2 describes a rodent study that aimed to investigate the effects of cranberry A-type proanthocyanidins combined with HIIT training on maximal running speed and skeletal muscle mitochondrial function. Chapter 3 consists of a systematic review with meta-analyses that synthesizes the current literature on the effects of polyphenol-rich berries on exercise performance, inflammation, and muscle damage. Chapter 4 is a clinical trial done with competitive/elite endurance runners that investigated the effects of a polyphenol-rich freeze-dried cranberry powder on running performance, lactate production, and skeletal muscle oxygenation. Chapter 5 is a follow-up study to the one described in Chapter 4 that aimed to explore the effects of the same cranberry powder on skeletal muscle mitochondrial capacity using near-infrared spectroscopy in healthy active adults. Finally, Chapter 6 discusses the findings from chapters 2-5 and provides general limitations and future research directions.