High-intensity physical activity for improving glucose regulation: can science justify IT?

Background: Interest in high-intensity interval training (HIIT) has recently resurfaced as a way to improve adherence to physical activity by addressing the common barrier of a “lack of time”. The benefit HIIT and vigorous-intensity physical activity (VPA) have on cardiorespiratory fitness is well established however, the effects on glucose regulation and insulin sensitivity in individuals at high risk of type 2 diabetes (T2DM) are less clear. Aims: The aims of this thesis were: to pool the available evidence regarding the effects of HIIT on markers of glycaemic control and insulin sensitivity, conduct observational analyses investigating the relationship between A) continuously and intermittently (≥10 minutes and <10 minutes, respectively) accumulated physical activity and B) increasing exercise intensity with markers of insulin sensitivity, and finally, to design and undertake an acute experimental study comparing the effect of high-intensity interval and moderate-intensity continuous exercise with sitting on post-challenge glucose and insulin responses. Key findings: In individuals at high risk of T2DM, HIIT improves HbA1c and insulin sensitivity to a similar extent as moderate-intensity continuous training, despite a lower overall workload. Continuously, but not sporadically, accumulated physical activity is positively associated with insulin sensitivity, and increasing physical activity intensity is associated with increasingly greater benefits in glucose regulation and insulin sensitivity. High-intensity interval exercise significantly improves the postprandial insulin response to a greater extent than moderate-intensity continuous exercise and sitting. Both forms of exercise are effective at reducing glycaemic variability. Conclusions: This thesis demonstrates that VPA is more effective than moderate-intensity physical activity at improving insulin sensitivity for a given unit of time. It shows that these benefits of vigorous-intensity physical activity can be achieved by performing HIIT, which may be a viable option for individuals at high risk of T2DM. HIIT could therefore be incorporated into the physical activity guidelines as a recommended health tool for improving diabetes outcomes.