Effects of exercise training on cardiac function and cardiometabolic risk factors in type 2 diabetes

Abstract

EFFECTS OF ENDURANCE TRAINING ON CARDIAC FUNCTION AND CARDIOMETABOLIC RISK FACTORS IN TYPE 2 DIABETES In 2014, 422 million people around the world had diabetes (global prevalence: 8.5%). Approximately 90–95% of the population with diabetes have type 2 diabetes (T2D). The prevalence is expected to increase by 55% worldwide in the next 20 years. The countries with the most rapid diabetes increase are middle- and low-income countries (i.e., South-East Asia and Western Pacific Regions). In Norway, 300,000 to 400,000 people have T2D, including up to 50% who are undiagnosed. Inactivity, being overweight, and obesity are important contributing factors to the development of T2D. Many T2D risk factors are the same as those that increase the risk of cardiovascular disease. Nearly 80% of the T2D population die from cardiovascular disease; the risk of dying from cardiovascular disease is at least doubled and the risk of developing heart failure is up to five times greater than in non-diabetes individuals. The prevalence of heart failure is increasing in concordance with the increasing diabetes prevalence. About every second person with T2D without traditional symptoms of cardiovascular disease has reduced cardiac function in the phase of the cardiac cycle when the heart is filled with blood (diastolic dysfunction). Diastolic dysfunction can ultimately lead to heart failure. Exercise training improves risk factors related to T2D. Research suggests that high-intensity exercise training ismore effective than moderate-intensity exercise training when it comes to reducing risk for cardiovascular disease. However, it is not established which exercise training intensity is the most effective in reducing risk factors for cardiovascular disease in general and in improving diastolic cardiac function in particular in T2D. Diastolic function is determined by a number of factors; among others, the wringing and unwringing motion that happens when the heart ejects blood (systole) and fills with blood (diastole), respectively. The unwringing of the heart is important for diastolic function as it contributes to the suction of blood into the heart in early diastole. However, even though diastolic dysfunction is common in T2D, it is uncertain whether people with T2D and diastolic dysfunction have reduced wringing and/or unwringing properties and whether exercise (high and/or moderate-intensity exercise) can influence these properties. Fast food, high in energy, refined carbohydrates and saturated fat can contribute to excessive increase in circulating glucose and lipids, which is common in people with T2D. High blood sugar and/or lipid levels in general and/or great fluctuations in blood sugar throughout the day represents an increased risk for cardiovascular disease in the long run. However, it is uncertain how fast food affects cardiac workload in the acute phase after the meal in people with T2D. Furthermore, we do not know whether a single exercise bout prior to food ingestion can reduce putative acute negative effects to the heart after a meal, similar to what is previously observed in arterial function in healthy individuals. The aim of this PhD-thesis was to investigate whether endurance training (and if so, at which exercise intensity) could improve cardiac function and cardiovascular risk factors in T2D individuals, without signs of cardiovascular disease, but with diastolic dysfunction when assessed by echocardiography. Furthermore, the aim was to investigate whether the wringing properties of the heart are different in people with T2D compared to healthy individuals, and if so whether endurance exercise (at high or moderate-intensity) can modify the difference. In addition, we wanted to investigate whether fast food affects cardiac workload in individuals with T2D versus overweight healthy counterparts, and if so, whether acute exercise can contribute to modulating these effects on cardiac function. Results from the studies carried out in this thesis show that endurance exercise training can improve cardiac function in T2D individuals with diastolic dysfunction and indicate that intensity is important to achieve improvements. Furthermore, this thesis shows that patients with T2D have similar wringing velocities as their healthy counterparts, but that they have delayed time to peak unwringing velocity. Endurance exercise, independent of intensity, normalized the timing of peak unwringing velocity. Moreover, we found that fast food induces increased cardiac workload in both T2D and healthy overweight individuals. Our findings also indicate that fast food induces compensations in diastolic function, blood pressure, and heart rate to a larger extent in T2D individuals compared to healthy overweight controls. A single prior exercise bout did not modulate the observed alterations in cardiac function or other variables measured after the meal.