Changes in insulin sensitivity following HIIT

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Chapter 4. Results
Participant Characteristics

There were 29 participants (10 male and 19 female) recruited for this study. Ten participants were randomised to the CMIET and control groups and nine randomised to the HIIT group. Nine individuals withdrew from the study before completion due to either a change in work or study commitments (5 participants), prolonged illness of a family member (2 participants), personal reasons (1 participant) and injury obtained outside of the study (1 participant). Mean values and standard deviations for participant characteristics before (pre) and after (post) the 12 week intervention are detailed in Table 4-1. All subjects satisfied the inclusion criteria for this study. Two participants in the HIIT group were on anti-hypertensive medications (beta-blocker, ACE inhibitor and a calcium channel blocker) and one of these participants was also taking a lipid modifying medication (Simvastatin). Both participants had been taking these medications without a change in dosage for more than six months, and their prescription did not change during the study period.

Exercise Training

Exercise training data was collected every session over the 12 week period, for each participant and mean and standard deviation values are summarised in Table 4-2. The exercise intervention groups were designed so both groups had similar exercise caloric expenditure, frequency, type and duration; the only major difference between the groups, was the single weekly interval training session in the HIIT group. Table 4-2 shows that despite the HIIT group performing a single weekly interval training bout, relative energy expenditure for each exercise session was similar between the HIIT and CMIET groups. Similarly, both the HIIT and CMIET groups had similar treadmill and cycle exercise intensities for their respective CMIET sessions. Adherence was high in both exercise groups for those participants whom completed the entire 12 week intervention. The highest adherence was for the HIIT sessions. Adherence was similar for the CMIET group and the HIIT group’s CMIET sessions.

Maximal Oxygen Consumption

All participants completed the VO_2max test until voluntary exhaustion pre and post intervention, with all participants satisfying at least two criteria outlined in section 3.5.5, for a true maximum. Figure 4-1 and Figure 4-2 highlight the changes in relative and absolute VO_2max following the 12 week intervention. Table 4-1 shows absolute VO_2max increased by 11.1% and 3.7% in the HIIT and CMIET groups and decreased by 4.2% in the control group. When adjusting VO_2max for any changes in body mass during the 12 week intervention, the magnitude of the changes were still similar (Table 4-1). Relative VO_2max increased 10.1% in the HIIT group and 3.9% in the CMIET group, but decreased by 5.7% in the sedentary control group. In order to make inferences about the effect of the interventions, the uncertainty in the effect, as expressed by 90% confidence intervals were calculated and the likelihood that the true value of the effect presents substantial change was determined; these are detailed in Table 4-3, Table 4-4 and Table 4-5. When comparing pre and post relative VO_2max in the HIIT group, relative to the control group, there was a mean difference of 4.9 mL·kg^-1·min^-1 and it was deemed to be ‘likely beneficial’ (Table 4-4). When comparing pre and post relative VO_2max in the CMIET group, relative to the control group, there was a mean difference of 3.0 mL·kg^-1·min^-1 and CMIET was deemed ‘possibly beneficial’ (Table 4-5). The primary research aim of the study was to assess changes in VO_2max following 12 weeks of CMIET combined with HIIT, compared to CMIET only. When comparing pre and post relative VO_2max values in the HIIT group, relative to the CMIET group, there was a mean difference of 1.9 mL·kg^-1·min^-1 and it remains ‘unclear’ whether one group was more beneficial compared to the other (Table 4-3). Similarly, when comparing the change in absolute VO_2max from pre to post between the same groups, it was still statistically ‘unclear’ whether or not one exercise training group elicited superior changes relative to its counterpart (Table 4-3).

Blood analysis

Mean values and standard deviations for fasting cholesterol, blood glucose, insulin concentrations and estimated insulin sensitivity are presented in Table 4-1. Total cholesterol decreased by 4.8% and 10.3% respectively in the HIIT and CMIET groups, whilst it increased by 1.8% in the control group. The CMIET group had the largest decrease in LDL cholesterol, with a similar decrease observed in the HIIT group and a slight increase in the control group. The CMIET and control groups experienced decreases in HDL cholesterol, whilst there was a slight increase in the HIIT group. Triglycerides increased in the HIIT and control groups, with no change observed in the CMIET group. The only group to alter their fasting blood glucose values after the intervention was the HIIT group, where it increased slightly (Table 4-1). Both the HIIT and CMIET groups experienced increases in their fasting insulin concentrations, whilst this decreased by 8% in the control group. Consequently, estimated insulin sensitivity, as estimated by HOMA2S, decreased in both the HIIT and CMIET groups and the only increase was observed in the control group. When analysing the mean differences for the fasting blood tests in the HIIT group, relative to the control group, there were mean differences of -0.5 and -0.6 mmol/L for total and LDL cholesterol respectively, and it was deemed to be ‘possibly beneficial’ for both (Table 4-4). With respect to fasting blood glucose and insulin, the true effect was deemed ‘unclear’. However, with a mean difference of -27.1% in insulin sensitivity, the HIIT group (relative to the control group) was deemed to be ‘possibly harmful’. When examining CMIET, relative to the control group, there were observed mean differences of -0.8 and -0.7 mmol/L for total and LDL cholesterols, with each deemed to be ‘very likely beneficial’ and ‘likely beneficial’ (Table 4-5). The effect of CMIET on insulin sensitivity still remains ‘unclear’. With respect to HDL cholesterol, CMIET was deemed to be ‘possibly harmful’. When analysing the mean differences for the fasting blood variables in the HIIT group, relative to the CMIET group, the only clinically significant difference was seen for HDL, where the HIIT group was deemed to be ‘possibly beneficial’ (Table 4-3). The true effect was said to be ‘unclear’ for total cholesterol, triglycerides and blood glucose, whilst LDL cholesterol, fasting insulin and insulin sensitivity had a ‘possibly trivial’ effect.

Anthropometric measures

The largest reduction in body mass was observed in the CMIET group, followed by the HIIT group; weight increased in the control group (Table 4-1). Despite the larger decrement of body mass in the CMIET group, the largest decreases in hip and waist circumference were in the HIIT group. Whilst there were still modest decreases in hip and waist circumference for the CMIET group, both variables increased in the control group. When comparing the effect of the HIIT group, relative to the control group, on body mass and BMI change it was deemed to be ‘possibly beneficial’ and hip and waist circumference changes ‘likely beneficial’ (Table 4-4). The greatest mean differences were seen for waist circumference followed by hip circumference and body mass. When comparing the effect of CMIET, relative to the control group, hip and waist circumferences had practical inferences of ‘likely beneficial’, whilst BMI was deemed to be ‘possibly beneficial’ (Table 4-5). Despite a mean difference of -1.9 kg for body weight, the true effect sill remained ‘unclear’. When comparing the effect of the HIIT group, relative to CMIET, on hip circumference, it was deemed ‘possibly beneficial’ with a mean difference of -0.9 cm (Table 4-3). There were ean differences of 0.8 kg and 0.1 kg·m² for body mass and BMI, with both variables having ‘unclear’ practical inferences. The true effect on waist circumference was deemed ‘possibly trivial’, with a -0.4 cm mean difference.

Other physiological variables

Resting heart rate decreased by 10.8% and 2.3% in the HIIT and control groups, whilst it increased by 4.4% in the CMIET group (Table 4-1). Both exercising groups had decreases in systolic and diastolic blood pressures following the 12 week intervention. Systolic blood pressure increased in the control group and diastolic blood pressure remained unchanged. When comparing the mean difference in resting heart rate in the HIIT group, relative to the control group, it was deemed ‘possibly beneficial’ (Table 4-4). Following a large mean difference of -8.9 mmHg in systolic blood pressure, it was deemed that the true effect was ‘likely beneficial’. The effect of CMIET, relative to the control group, on systolic and diastolic blood pressure were mean differences of -6.1 mmHg and -7.3 mmHg and this was deemed to be ‘likely beneficial’ for both (Table 4-5). Resting heart rate had a mean difference of 4.9 beat·min^-1, however, the inference for this effect is ‘unclear’ When comparing the mean difference in resting heart rate in the HIIT group, relative to the CMIET group, it was deemed ‘very likely beneficial’ (Table 4-3). Although there were mean difference of -2.8 mmHg and 3.9 mmHg for systolic and diastolic blood pressures, these were said to be ‘possibly trivial’ and ‘unclear’ respectively.

Quality of life

All participants completed a SF-36 health survey to assess changes in perceived mental and physical health and mean scores and standard deviations are listed in Table 4-1. Perceived physical health increased across all the intervention groups, with the largest improvement seen in the HIIT group, followed by the CMIET and control groups. Perceived mental health increased by 29.7% and 19% in the HIIT and CMIET groups and decreased by 5.6% in the control group. Similar trends were observed for perceived total health, with a 22% and 13.2% increase in the HIIT and CMIET groups, whilst there was a slight 1.5% decrease in the control group. When comparing the true effect in the HIIT group, relative to the control group for perceived mental and total health, it is deemed to be ‘likely beneficial’, whilst the practical inference for perceived physical health remains ‘unclear’ (Table 4-4). The true effect of CMIET, relative to the control group on perceived mental health is deemed to be ‘possibly beneficial’. However, the true effect remains ‘unclear’ for perceived physical and total health (Table 4-5). The true effect of the HIIT group, relative to CMIET alone on perceived mental, physical and overall health remains ‘unclear’, despite mean differences of 6.2, 6.9 and 5.1 respectively (Table 4-3).

Chapter 1. The Problem
1.1. Introduction
1.2. Cardiorespiratory fitness as a risk factor for CVD
1.3. Pathophysiology of insulin resistance
1.4. Overview of exercise intensity on cardiorespiratory fitness
1.5. Overview of exercise intensity on insulin sensitivity
1.6. Research Aims
1.7. Experimental Hypotheses
1.8. Significance of this study
Chapter 2. Literature Review
2.1. Introduction
2.2. Overview – High Intensity Interval Training
2.3. Changes in cardiorespiratory fitness following HIIT in sedentary & recreationally trained individuals
2.4. Changes in cardiorespiratory fitness following HIIT in overweight/high-risk individuals
2.5. Changes in insulin sensitivity following HIIT
Chapter 3. Methods
3.1. Experimental Design: Overview
3.2. Sample Size Estimation
3.3. Participant Recruitment
3.4. Participant Requirements
3.5. Testing Procedures
3.6. Training Procedures
3.7. Measures.
3.8. Statistical Data Analysis
Chapter 4. Results
4.1. Participant Characteristics
4.2. Exercise Training
4.3. Maximal Oxygen Consumption
4.4. Blood analysis
4.5. Anthropometric measures
4.6. Other physiological variables
4.7. Quality of life
Chapter 5. Discussion
5.1. Overview of Hypotheses
5.2. Effects of the intervention on VO2max
5.3. Effects of the intervention on insulin sensitivity
5.4. Effects of the intervention on HDL and LDL
5.5. Effects of the intervention on body composition
5.6. Practical implications
5.7. Limitations
5.8. Future Research
5.9. Conclusion
Appendices

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The effect of continuous moderate intensity exercise training combined with high intensity interval training on cardiovascular disease risk factors