Cognitive function

The effects of breakfast and exercise on cognitive function observed in this programme of studies were not entirely clear. This may not be surprising given that the literature documenting the effect of breakfast alone on cognitive function is generally quite disordered. The positive effect of breakfast on memory in a healthy adult population, in particular episodic memory, has been frequently reported (e.g. Benton & Parker, 1998) and can be viewed with some confidence, whilst data on other cognitive domains is inconclusive. As the RVIP and NBack tasks are both measures of working memory, it is perhaps not surprising that there was no positive influence of breakfast on either task in Chapter 2. In fact, following the test drink, an improvement in RVIP RT was

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decreased FCRT and Stroop accuracy in this study, suggesting a negative impact on decision making and response inhibition. It could be suggested that an increase in serotonin may have caused this impairment in performance, as it has been previously shown that reducing serotonin levels via BCAA supplementation during exercise (Blomstrand, 2001) and via a low CHO/high protein breakfast (Fischer et al., 2002) subsequently improved cognitive performance. However, there are a couple of considerations which suggest that serotonin may not be the mechanism responsible for the cognitive detriments seen in this study. Firstly, a change in serotonin is thought to affect episodic memory only, and not working memory, response inhibition or decision making (Mendelsohn et al., 2009). Serotonin is just one of multiple neurotransmitters which stimulate the pre-frontal cortex, a brain region responsible for attention, working memory and executive functions, and manipulation of serotonin alone may not compromise its function (Robbins, 2005). Also, as previously stated, it is unlikely that the breakfast provided would have led to a significant change in serotonin levels as it contained > 5% protein (Benton, 2002). Instead, it is more probable that the exercise intervention would have caused a stimulation of serotonin production (Kubitz & Mott, 1996), but as previously discussed (section 6.2), the consumption of breakfast prior to exercise attenuated this effect (Davis et al., 1992; Lieberman et al., 2002). Therefore, serotonin concentration may have been highest in the NB E condition in Chapter 2, but was probably relatively unchanged in the other conditions. When it is considered that attentional processes, although normally unaffected by serotonin, have been shown to improve when serotonin is depleted (Mendelsohn et al., 2009) and associatively, memory impairments have also been found following tryptophan loading, which increased serotonin (Sobczak et al., 2003), the effects in Chapter 2 are in fact the opposite of what would be expected if serotonin was responsible for changes in cognitive function in this case. Another possible mechanism for these effects may be cortisol. Raised blood glucose concentrations, such as were likely in the breakfast conditions, can augment cortisol secretion during challenging cognitive tasks, leading to impaired cognitive performance (Gibson, 2007; Buchanan, Tranel & Adolphs, 2006).

In Chapter 4, breakfast consumption prior to exercise did not affect post-exercise cognitive task performance. This could be because cognition was measured in a field setting, but there were effects observed when this same method was used in Chapter 5. In this chapter, a detrimental effect of the larger breakfast on mid-afternoon NBack and RVIP performance, again the two tasks which

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measure working memory, were identified. However, these negative effects were not apparent when a smaller breakfast was consumed suggesting that this may be a preferable practice; this is discussed further later. These results somewhat mirror those found by Nabb and Benton (1996), who reported that memory score negatively correlated with the caloric content of the breakfast administered, although Michaud et al. (1991) previously reported improved episodic memory following a high caloric breakfast.

As mentioned, overall the results from this thesis do not suggest that breakfast has an entirely positive effect on cognitive function, in particular on working memory. A recent article by Zilberter and Zilberter (2013) suggests that the belief that consuming breakfast is beneficial for cognitive function may be incorrect. They agree that a stable blood glucose profile appears to contribute positively to cognitive function, but highlight the fact that the most stable post-prandial profile is seen following a high-fat, rather than high-CHO, breakfast (Fischer, 2001). They suggest that the metabolic response seen following consumption of a high-fat breakfast more closely mimics the effects of skipping breakfast, and imply that this practice may in fact produce the same neuroprotective effects as those seen when following a ketogenic (low-CHO) or intermittent fasting diet. Of course, when considering whether or not consuming breakfast is beneficial, it must be considered in context. The authors do mention that the persistent hunger often associated with following these types of diet is often difficult to manage (Stote et al., 2007); naturally, this can lead to detriments in mood (Fischer, et al., 2004) and potentially diminish cognitive function due to the distraction of hunger sensations. Even though fasting may lead to a more stable metabolic response acutely, there is evidence that insulin sensitivity and glucose responses are improved when the next meal after breakfast is consumed both acutely (e.g. Astbury et al., 2011) and regularly (Pereira et al., 2011). It is interesting to reconsider whether consuming breakfast may indeed be the best practice with regards to improving cognitive function. However, the long term health benefits of breakfast consumption are well documented (Pasman, Blokdijk, Bertina, Hopman & Hendriks, 2003; Smith, 1998, 1999; Smith, 2003; Benton & Brock, 2010; Odegaard et al., 2013), and even though breakfast may not have a consistent, robust, positive effect on cognition acutely, its beneficial role for general health and wellbeing is clear and should not be disregarded.

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Interestingly, the negative effect of breakfast consumption on RVIP RT and Stroop accuracy did appear to be alleviated by exercise in the active male sample in Chapter 2. The literature does suggest that exercise is stimulating (Lambourne & Tomporowski, 2010) and an increase in arousal post-exercise can improve RT responses (Etnier et al., 1997) and decision making speed (Adam et al., 1997; McMorris & Graydon, 1996). Whilst data from this thesis do not support the hypothesis that breakfast prior to exercise can enhance post-exercise cognitive

performance, they do suggest that in habitually active males, exercise can reverse any negative cognitive consequence of breakfast consumption.

Cognitive performance and mood are frequently measured together within the same study. It has been suggested that cognitive function may be moderated by mood state (Chepenik et al., 2007), although this has been recently challenged (Hopkins et al., 2012). Still, a possible mechanism responsible for the cognitive detriments following breakfast may be the increased feelings of tension associated with consuming breakfast prior to rest, given the documented relationship between mood and cognitive performance. However, tension was not higher when breakfast was consumed prior to exercise, possibly due to increased pre-frontal cortex activity following exercise, which correlates with feelings of calmness (Hall et al., 2007). Findings from the field study (Chapter 4) also provide some support for the cognition/mood mediation theory. Feeling more tense and jittery was associated with more ART errors and NBack task performance appeared particularly sensitive to changes in mood; higher fatigue was associated with a decline in

performance, whilst a better mood and higher alertness coincided with an improvement in performance on this task. Contradictory, when the same mobile phone field-setting methodology was used in Chapter 5 there was an increase in mood mid-afternoon when breakfast was consumed, but a decrease in cognitive performance at the same time point, the opposite of what would be expected. However, there was little evidence of this relationship when cognitive performance and mood were measured in a laboratory environment. Feeling more relaxed and less mentally fatigued did not appear to coincide with enhanced cognitive performance; a recent reported showed that changes in cognition following exercise do not correlate with mood, suggesting that separate neural systems may facilitate the effects on these parameters (Hopkins et al., 2012), but it is difficult to ascertain whether or not this may be correct from the evidence presented in this thesis.

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Poorer appetite control post-exercise (a higher desire to eat and lower satisfaction)

positively correlated with task difficultly ratings in the field study (Chapter 4). However, despite a higher post-exercise EI being associated with higher alertness and lower fatigue, this was also associated with slower RVIP RT and ART. Indeed, RT is one aspect of cognition which is often shown to improve when hunger is higher through lower EI (Fischer, et al., 2004; Hopkins et al., 2012).