Assessing appetite responses to physical activity using functional magnetic resonance imaging

Abstract

The relationship between physical activity, sedentary time, and appetite control has potential implications for weight management. Limited evidence suggests that regular physical activity may influence responsiveness to visual food cues in reward-related brain regions. In the past few years, studies have increasingly focused on neural regulation of appetite and feeding. Despite this, there is scarce research examining how physical activity is associated with brain responses to food cues and the circulating peptides that are implicated in the regulation of appetite within the brain. The first study in this thesis (Chapter 4) is a systematic review of functional magnetic resonance imaging (fMRI) studies examining the effect of physical activity on neural responses to visual food cues in humans. Exercise, both acute and chronic, appears to lower food-cue reactivity in several brain regions, including the insula, hippocampus, orbitofrontal cortex (OFC), postcentral gyrus, and putamen, especially when participants view images of high-energy-density foods. Research suggests that acute exercise may enhance the appeal of low-energy-density food choices. Several cross-sectional studies have demonstrated that individuals with high levels of self-reported physical activity show less reactivity to food cues, particularly cues depicting high-energy-density foods, in the insula, OFC, postcentral gyrus, and precuneus. The results of this review suggest that physical activity may affect food-cue reactivity in motivational, emotional, and reward-related brain regions, possibly indicating a hedonic appetite suppression effect. However, considering methodological variability across a limited number of studies, conclusions should be drawn with caution. The second study reported in this thesis (Chapter 5) used a randomised crossover design to investigate the acute effect of vigorous-intensity exercise (treadmill running) on cerebral blood flow (CBF). This study explored the time course of CBF changes after acute exercise in healthy men using fMRI and its implications for food-cue reactivity paradigms. Overall, differences between trials were evident in grey matter and regional CBF, but the CBF time course was not influenced by exercise, suggesting that food-related blood-oxygen-level-dependent (BOLD) acquisitions after exercise may not be time-sensitive. In future studies, it may be necessary to acquire BOLD and CBF data simultaneously so that differences in CBF between trials that may affect the interpretation of brain food cue reactivity can be considered. ii The third study reported in this thesis (Chapter 6) is a cross-sectional study examining the association of physical activity and sedentary behaviour with neural responses to visual food cues in adults. A negative relationship was found between moderate to vigorous physical activity (MVPA) and food-cue reactivity in the hippocampus, insula, amygdala, middle frontal gyrus, and precentral gyrus. A positive association was identified between MVPA and food-cue reactivity in the striatum, whereas sedentary time was positively associated with food reactivity in the posterior cingulate gyrus and paracingulate gyrus, independently of BMI. Moreover, fasting glucagon-like peptide-1 (GLP-1) concentrations correlated negatively with brain reactivity in areas associated with cognition, emotion, and reward. This suggests that low GLP-1 concentrations may increase hunger and motivation to seek food. Fasting glucose concentrations were negatively associated with brain reactivity in response to food cues in the postcentral gyrus, potentially indicating heightened sensitivity to food cues when glucose concentrations are low. Fasting peptide tyrosine-tyrosine (PYY), a hormone associated with feeling full, was negatively associated with food cue reactivity in the middle frontal gyrus. No relationship was observed between fasting acylated ghrelin concentrations, overall appetite perceptions, and brain responses to food cues. This thesis highlights that physical activity and sedentary behaviour influence brain responses to visual food cues. The three studies emphasise the importance of continued research examining links between physical activity and appetite control. The findings provide a broad understanding of how physical activity and exercise (acute and chronic) affect food-cue and appetite perception, including both the neural and hormonal components involved in appetite perception. Understanding of appetite regulation in the brain has been further enhanced by considering CBF, neural pathways, and appetite related hormones. Using this foundation, further research can be conducted exploring ways to improve lifestyles and eating habits.