Breakfast consumption, breakfast composition and exercise : the effects on adolescents' cognitive function

The studies described in this thesis were undertaken to examine the factors affecting adolescents cognitive function across the school morning. Specifically, the effects of breakfast consumption, breakfast glycaemic index (GI) and a mid-morning bout of exercise were examined, whilst the final experimental chapter examined their combined effects. The battery of cognitive function tests used in the present study was administered via a laptop computer and took approximately 15 min to complete. Across all experimental chapters, the visual search test (assessing visual perception), the Stroop test (assessing attention) and the Sternberg paradigm (assessing working memory) were used. Furthermore, in chapter V the Flanker task (also assessing attention) was added to the testing battery. The first experimental study (chapter IV) examined the effects of consuming a self-selected breakfast on cognitive function, compared to breakfast omission. Ninety-six adolescents (12 to 15 years old) completed two experimental trials (breakfast consumption and breakfast omission), scheduled seven days apart, in a randomised crossover design. Following breakfast consumption, accuracy on the more complex level of the visual search test was higher than following breakfast omission (p = 0.021). Similarly, accuracy on the Stroop test was better maintained across the morning following breakfast consumption when compared with breakfast omission (p = 0.022). Furthermore, responses on the Sternberg paradigm were quicker later in the morning following breakfast consumption, on the more complex levels (p = 0.012). Breakfast consumption also produced higher self-report energy and fullness, lower self-report tiredness and hunger, and higher blood glucose concentrations, compared with breakfast omission (all p < 0.001). Overall, the findings suggested that breakfast consumption enhanced adolescents cognitive function, when compared with breakfast omission. The second experimental study (chapter V) examined the effects of consuming a high GI breakfast, a low GI breakfast and breakfast omission on cognitive function. Forty-one adolescents (12 to 14 years old) completed three experimental trials, each scheduled seven days apart, in a randomised crossover design. There was a greater improvement in response times across the morning following a low GI breakfast, compared to breakfast omission on the complex level of the Stroop test (p = 0.009) and both levels of the Flanker task (p = 0.041), and compared to following a high GI breakfast on the complex level of the visual search test (p = 0.025) and all levels of the Sternberg paradigm (p = 0.013). Furthermore, accuracy was enhanced following a low GI breakfast, compared to breakfast omission on the more complex levels of the visual search test (p = 0.032), Sternberg paradigm (p = 0.051) and Flanker task (p = 0.001), and compared to following a high GI breakfast on both levels of the Stroop test (p = 0.033) and the more complex levels of the Sternberg paradigm (p = 0.002) and Flanker task (p = 0.014). Furthermore, participants exhibited a lower glycaemic response following the low GI breakfast (p < 0.001), though there was no difference in the insulinaemic response (p = 0.063), compared to following the high GI breakfast. Overall, the findings suggest that a low GI breakfast is the most beneficial for adolescents cognitive function, compared with a high GI breakfast and breakfast omission. The third experimental study (chapter VI) examined the effects of a mid-morning bout of exercise, following a self-selected breakfast, on cognitive function. Forty-five adolescents (12 to 13 years old) completed two experimental trials (exercise and resting), scheduled seven days apart, in a randomised crossover design. There was a greater improvement in response times across the morning following the mid-morning bout of exercise on all levels of the Sternberg paradigm (p = 0.010). There was also a greater improvement in response times across the morning on the visual search test following the exercise (p = 0.009), but this improved speed was combined with a greater decrease in accuracy following the exercise (p = 0.044). This suggests that following exercise, the adolescents exhibited a speed-accuracy trade-off, whereby they responded quicker, but this was to the detriment of accuracy. Overall, the findings suggest that whilst the mid-morning bout of exercise improved some components of cognitive function (e.g. response times on the Sternberg paradigm), it did not affect other components (e.g. Stroop test performance). The final experimental study (chapter VII) examined the combined effects of breakfast GI and a mid-morning bout of exercise on adolescents cognitive function. Forty-two adolescents (11 to 13 years old) were allocated to matched high GI (n = 22) and low GI (n = 20) breakfast groups. Within the matched groups, participants completed two experimental trials (exercise and resting) in a randomised, crossover design. The findings indicate that, for the complex level of the Stroop test, following the high GI breakfast there was a greater improvement in response times across the morning on the resting trial, whereas following the low GI breakfast response times improved across the morning on both the exercise and resting trials, though the magnitude of the improvement was greatest on the exercise trial (p = 0.012). On the Sternberg paradigm, response times improved across the morning following the low GI breakfast regardless of exercise, whereas following the high GI breakfast response times improved across the morning on the exercise trial, though remained similar across the morning on the resting trial (p = 0.019). Overall, the findings suggest that the effects of the mid-morning bout of exercise were dependent upon the breakfast GI and the component of cognitive function being examined and that, for the Stroop test, the beneficial effects of the low GI breakfast and mid-morning bout of exercise were additive. Overall, the results from this thesis suggest that breakfast consumption is more beneficial than breakfast omission and more specifically, that a low GI breakfast is more beneficial than both a high GI breakfast and breakfast omission, for adolescents cognitive function across the school morning. However, the effects of exercise appear to be more variable, with the effect of exercise depending upon the component of cognitive function examined and the GI of the breakfast consumed. Overall, the findings presented in this thesis suggest that the nutritional effects on adolescents cognitive function (i.e. the effects of breakfast consumption and GI) were stronger and more consistent than the exercise induced effects.