Puente Browniano

The 4-week intervention had a positive impact on a number of health markers including body composition, blood pressure and fitness. Albeit small changes, body weight (-0.9 (1.1) kg), waist circumference (-2.3 (3.5) cm) and percent body fat (-0.9 (1.1) %) were significantly lower following the intervention, while fat free mass was maintained. In addition, both systolic (-6.2 (8.4) mmHg) and diastolic (-5.8 (2.2) mmHg) blood pressure were reduced and there was a mean 12.8% increase in VO2max (+4.4 (2.1) ml/kg/min).

Emerging research has pointed to the efficacy of HIIE for improving body composition and health markers compared to other types of exercise [413].

Significant improvements in insulin sensitivity and a 7% increase in VO2max have previously been demonstrated after just two weeks of three HIIE sessions per week in 10 overweight and obese sedentary males [427]. In the present study, VO2max increased by approximately 13% following 4 weeks of exercise. The present findings are consistent with findings of significant increases in VO2max of between 4 and 46% in HIIE programs lasting from 2 to 15 weeks [413]. Given the considerable

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health benefits associated with small increases in cardio-respiratory fitness [433]

(e.g. an increase in physical fitness of 1 MET (relative VO2 3.5 ml/kg/min) has been demonstrated to be associated with a mortality benefit of about 20% [434]) the changes in cardio-respiratory fitness observed with 4 weeks of exercise in the current study are likely associated with many health benefits.

Previously, it has been demonstrated that VO2max, blood pressure and waist circumference significantly improved both in individuals who achieved significant weight loss and in those who achieved a less than expected weight loss following a 12 week intervention [262]. King et al. (2009) [262] concluded that the data demonstrated that meaningful health benefits can be achieved independent of any change in body weight. The present findings add further support to these observations by demonstrating that following 4 weeks of exercise and in the absence of large weight losses, significant improvements in health markers are clearly evident.

5.4.5 Relationships between variables

As others have shown [12, 435-437], individual variability in responses to the exercise intervention were evident in the present study. Weight change ranged from -2.8kg to + 0.8kg over the 4 week intervention. In addition, changes in body composition differed between individuals. However, the changes in body weight and composition were not associated with changes in GE, appetite or EI. It is likely that a larger sample size would be required to detect such associations. Following a 12 week exercise intervention in 15 overweight and obese individuals, Martins et al.

(2010) similarly found no associations between the magnitude of weight change and changes in appetite sensations or appetite related gut peptides, and suggested more power would be required to detect such associations [86].

Similar to the cross sectional study presented in Chapter 4, AEE was negatively correlated with GE times at both pre and post test in the present study. In addition, when changes in physical activity variables and GE were correlated, change in GE ascension time was negatively correlated with change in AEE, steps per day and time in vigorous activity indicating an increase in activity was associated with faster GE. The change in absolute FFM was the only variable associated with change

in EI at the ad libitum lunch test meal. Change in FFM was negatively associated with change in EI which contrasts with evidence of an association of increased FFM with increased EI previously demonstrated in a large cohort of overweight and obese individuals [36]. However, the present findings should be interpreted with caution due to the small sample size for correlations in the present study.

5.4.6 Methodological Considerations

As highlighted in the previous discussion, one limitation of the present study is that it was not powered to detect small but potentially relevant changes in some variables such as certain subjective appetite variables, eating behaviour traits and associations between variables. The findings of the reproducibility study presented in Chapter 3 for example showed that to detect a 10mm change in fasting hunger ratings, 65 participants would be needed whereas 11 participants would be needed to detect a 10mm change in mean ratings. The present study was powered to detect changes in the majority of variables and primarily GE as the primary outcome measure. Although a control group was not included, the study was powered to detect changes that would be considered clinically significant and outside of the normal day to day variability in these measures, based on findings from the reproducibility study presented in Chapter 3. The sample size of 15 participants was sufficient to detect at least an 8% change in GE half and lag times. Therefore, sample size was not a factor explaining the lack of significant change in GE in the present study. However, it is important to acknowledge that in the reproducibility study, the two tests were undertaken seven days apart. This is a shorter duration than between the two test days (pre and post the four-week intervention) in the present study and therefore it is possible the reproducibility of GE over this time frame may be different. Future studies including a control group would be ideal.

A limitation of this study when comparing findings to the cross sectional study presented in Chapter 4 is that participants were instructed to avoid exercise for different intervals of time prior to the GE test as highlighted earlier in this discussion.

In the cross sectional study, participants were instructed to avoid exercise for 24 hours beforehand which is a common protocol used and has previously been used in studies examining appetite control in habitual exercisers [249]. However, in intervention studies examining the effects of exercise training on gut peptides, post

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testing was conducted at 36 hours[211, 212] or > 48hours [86, 214] after the last exercise session. As a result the post GE test took place ≥ 48 hours after the last exercise session in the present study. Although this is a limitation when comparing the findings with the study presented in chapter 4, the strengths of using this protocol means findings are more applicable to compare to previous intervention studies examining changes in gut peptides with exercise [86, 211, 212, 214].

An additional methodological consideration in the present study is the mode of exercise. The exercise intervention consisted solely of indoor cycling on a cycle ergometer to minimise any potential confounding effects of mode of exercise on GE [243]. As a result the present findings cannot be extrapolated to other forms of exercise e.g. resistance training.

Changes in physical activity throughout the week prior to the GE test (including outside of the exercise sessions) could also have influenced the GE responses to the exercise intervention. However, while not statistically significant mean increases in physical activity and AEE were evident between pre and post test suggesting participants did not significantly compensate in terms of reduced activity outside of the intervention. One limitation which is evident retrospectively however is that in the week prior to the GE test, participants completed only 3 exercise sessions. Due to scheduling constrictions, the 4 week exercise program commenced and finished midweek and exercise sessions took place on weekdays. As a result 2 out of the 7 days prior to the GE post test were a weekend and hence exercise sessions were not scheduled and for 2 other days participants were instructed to avoid exercise prior to the GE test. Given the associations between AEE and GE evident in Chapter 4 and in the present study it cannot be discounted that a greater increase in the number of exercise sessions in the 7 days before the GE test may yield different findings. In addition, EI was only assessed prior to and post the exercise intervention, therefore it cannot be discounted that changes in habitual EI during the intervention influenced the findings. This is a common issue in exercise intervention studies as the reduced sensitivity of most available methods of assessing EI represents an ongoing challenge. Underreporting of EI is common in some individuals, regardless of the method [438]. Future studies involving quantitative and objective measurements of EI such as measuring EI over the course of a probe day

where all meals are provided to participants [36] may yield further information on associations between EI, GE and other variables.

The exercise intervention used in the present study (20 sessions over 4 weeks including HIIE) represented a considerable change in lifestyle for sedentary individuals. Although compliance was high in the present study a question that arises is how applicable the intervention would be outside of the laboratory environment.

The optimal protocol for HIIE is still a matter of debate. The most commonly used protocol has been the Wingate test (30 second all out sprints), however it is likely unsuitable for most overweight sedentary individuals [413]. Studies have used various interval protocols ranging from 8 seconds cycle sprint followed by 12 seconds low intensity cycling [439] to a 2 minute cycle sprint followed by 15 seconds of low intensity cycling for a period of 20 minutes [440]. In the present study, the HIIE protocol of alternating between 30 seconds cycling at 100% VO2max and 30 seconds of low intensity cycling for 30 minutes in week 1 was achievable.

However, the participants found the sessions particularly difficult at the beginning of the intervention and would likely struggle to maintain adherence to such an intervention outside of the supervision of the laboratory environment. Others have shown that health benefits can be achieved with less intense protocols to that used in the present study [427], and therefore these protocols may have more ecological validity. Nevertheless, there is no consensus on the optimal protocol for HIIE for overweight and obese sedentary individuals and more research is needed to identify the optimal length and intensity of intervals for achieving varying health outcomes [413]. As the primary aim of the present study was to assess the effects of exercise training on GE and associated variables, a large volume of exercise was necessary to ensure physical activity was the major lifestyle change. In addition, each exercise session was supervised to minimise any confounding effect of adherence on the outcomes. Given the large volume of exercise and that each exercise session was carefully monitored these methodological aspects minimised the influence of other confounding factors on GE and thus increase the validity of the present findings with regard to the primary aim.

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