This thesis provides insight into metabolic and substrate utilisation profiles for a range of high intensity intermittent exercise (HIIE) models performed by healthy untrained males. It investigates the optimal HIIE protocol that may potentially enhance fat metabolism and induce an energy deficit. The ideal method found for the healthy, untrained male gender is also compared to healthy, untrained females to see if beneficial metabolic changes observed in males are transferrable across the genders.
1.1 Thesis scope
In this obesogenic environment, maintaining physical health and body weight is a primary concern for many individuals. Diet pills, weight loss shakes and fad diets are commonly promoted and employed to maintain healthy weight. However the most economical and flexible method of controlling body fat or adiposity is altering exercise and the amount of physical activity.
Skeletal muscle comprises approximately 40% of an individual’s body weight (normal healthy range) and is the major site of energy production and expenditure (Gropper et al.,
2009). Hence it is advantageous to manipulate exercise protocols which have a significant impact on enhancing skeletal muscle metabolism.
Constant, moderate intensity exercise elicits optimal fat oxidation rates (Romijn et al., 1993), exercising (walking/jogging) at 60-65% of VO2max, and known as the ‘fatmax’ zone
(Achten et al., 2002). Recently it has been suggested that high intensity intermittent exercise (HIIE) leads to reductions in adiposity at a faster rate than submaximal continuous exercise (CON) (Tremblay et al., 1994; Trapp et al., 2008; Macpherson et al., 2011; Gremeaux et al., 2012). This seems counter-intuitive as HIIE is short, rapid bouts of high
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intensity exercise, followed by a rest period, with this combination repeated for the duration of exercise protocol. In training studies comparing fat loss between HIIE and CON, HIIE has been shown to utilise fat sources despite high intensity exercise being a glycolytic process, utilising glycogen, resulting in lactate accumulation. This has been attributed to the work to rest nature of HIIE. Using single bouts of workload matched HIIE and CON, similar plasma FFA and glycerol concentrations were observed, indirect plasma markers reflective of fat oxidation. There are limitations of using plasma FFA and glycerol to describe fat oxidation, as plasma FFA can be re-esterified back into adipose tissue. This study also showed that HIIE induces greater ATP turnover and degradation compared to CON as measured by plasma purine accumulation and enhanced urinary purine loss (Borg et al, 2008), elucidating that HIIE creates metabolic disturbances that elevate subsequent energy loss with the requirement for de novo synthesis of ATP post exercise (Newsholme and Leech, 1983).
Much of the research pertaining to HIIE focuses on muscle adaptations associated with training (HIIT), specifically the mitochondrial enzyme protein changes, in particular the up regulation of oxidative enzyme activity and improvement in respiratory health and metabolic processes such insulin sensitivity. Without producing any succinct metabolic reasons, this collection of studies have depicted HIIT as beneficial for many population groups (Wisloff et al., 2007; Tjonna et al., 2009; Little et al., 2011). However, without knowing metabolic mechanisms behind this exercise type, whether these beneficial changes can be further promoted or optimised remains unknown. Obvious metabolic disorders that may benefit from this exercise model are obesity and diabetes, due to proven decreases in fat mass (Tremblay et al., 1994; Trapp et al., 2008; Macpherson et al., 2011; Gremeaux et
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al., 2012) and improvements in insulin sensitivity with HIIT (Little et al., 2011; Gillen et al.,
2012). In addition to metabolic illness, healthy individuals looking to maintain healthy weight or those whom have lifestyles that are too ‘time poor’ to partake in longer, less intense physical exercise, may also benefit from an exercise type that will yield physical changes in a timely manner, encouraging the continuation of exercise (Gibala 2007; Gibala and Little 2010). Therefore the main focus of this thesis explores possible metabolic explanations behind greater fat loss after HIIT compared to CON training, as well as optimising HIIE to further exacerbate these changes.
1.2 General aim of the thesis
The primary aim of this thesis was to determine metabolic profiles (urine, blood and muscle) of various HIIE models and to investigate the nature of HIIE (duration and intensity) that optimise the metabolic processes that elevate energy deficit for potential increase energy expenditure and influence energy balance and subsequent fat utilisation.
1.3 Significance
High intensity intermittent training (HIIT) accelerates decreases in adiposity and improves insulin sensitivity, resulting in HIIE becoming the preferred exercise model to induce favourable changes in those afflicted with weight challenges and diabetes.
The metabolic profile of HIIE, including respiratory, plasma, urinary and muscular markers, is yet to be comprehensively determined, therefore the significance of this project was to produce blood and urinary profiles of important metabolites that may help elucidate and understand the augmented fat loss associated with HIIE. The metabolic mechanisms
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behind this exercise type must first be elucidated in the healthy, untrained population, thus this thesis primarily focuses on employing untrained males, and with one study also including females for comparisons across the sexes.
Results from this thesis may be applied to disease population cohorts. Numerous diseases already benefit from employing HIIT, but the improvements may be quicker, favourable and encouraging if the optimal and easiest model of HIIE is found for each specific disease state. HIIE has already been shown to improve insulin sensitivity, but there may be another protocol that can further enhance these improvements whilst decreasing fat mass at an accelerated rate, decreasing the progression of obesity and diabetes, ultimately preventing damage to the endocrine and metabolic systems. HIIE is also known to improve cardiorespiratory health in individuals suffering cardiovascular disease. It also has been shown to decrease blood lipid profile as well as decreasing blood pressure, which are two risk factors for coronary heart disease. Other risk factors associated with cardiovascular disease are obesity, type II diabetes and poor attitude to exercise, hence if each of these factors can be addressed in a single exercise model, individuals may be more likely to persevere with the exercise to obtain benefits.
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