Active Voice: Can HIIT Training Improve Function of the Diabetic Heart?
By J. Chris Baldi, Ph.D., FACSM

J. Chris Baldi, Ph.D., FACSM
The incidence of type 2 diabetes continues to increase. Moreover, the prolonged management of the disease is crippling health care systems worldwide. Large prospective trials show that improved glycemic control and lifestyle changes result in reduced symptoms of neuropathy, retinopathy and nephropathy, thus improving outcomes for people with diabetes. Unfortunately, reductions in cardiovascular morbidity/mortality have not been realized in these patients, and their cardiovascular disease risk factors remain elevated.

Increasing aerobic capacity by chronic exercise training is, arguably, the best prevention for heart disease. Therefore, exercise is a cornerstone of diabetic treatment. Yet, the function of the diabetic heart is often impaired, making it harder for people with diabetes to exercise effectively. Prior to any clinical evidence of heart disease, people with type 2 diabetes may have impaired left ventricular filling, blunted left ventricular contractility during exercise and smaller heart rate reserve. These characteristics combine to dramatically reduce cardiac reserve. It is unclear whether exercise training will increase the performance of the diabetic heart as much as it does in non-diabetic cohorts. Also unresolved is the question of whether people with diabetes will adhere to vigorous exercise training.

Our study, as presented in the June 2019 issue of Medicine & Science in Sports & Exercise® (MSSE), determined whether a three-month program of high intensity interval training (HIIT) would improve left ventricular performance during exercise in adults with type 2 diabetes. To achieve this objective, semi-recumbent resting and exercising (40% and 60% VO2peak) left ventricular volumes were compared in healthy adults with uncomplicated type 2 diabetes before and after random allocation to a three-month HIIT training program (n=11) or a non-exercising control group (n=5). The effects of the HIIT program on aerobic capacity (V?O2peak) and total blood volume also were compared.

We found that participants randomized to the HIIT group attended nearly 80% of scheduled exercise sessions and increased VO2peak by 15%. The HIIT group also reduced their resting heart rate and increased their resting left ventricular stroke volume as compared to non-exercising controls. Most importantly, participants in the HIIT group had increased left ventricular stroke volume responses to submaximal exercise and were capable of significantly higher relative workloads after training—in comparison with controls. The increase in stroke volume response was caused primarily by an increase in the left ventricular end-diastolic volume (filling) response but was not associated with any change in total blood volume.

There are two important clinical implications of this work. First, adults with type 2 diabetes will adhere to HIIT and are as capable of comparable increases in aerobic capacity and left ventricular exercise response as those reported in other studies of non-diabetic adults. Second, the larger left ventricular end-diastolic volumes attained during exercise suggest that diastolic dysfunction, which is thought to be a precursor to diabetic heart disease, can be reversed—at least partially—with effective exercise training programs.

About the author:
J. Chris Baldi, FACSM, received his Ph.D. in exercise physiology from The Ohio State University in Columbus, Ohio. Currently, he is a senior research fellow at the Dunedin School of Medicine at the University of Otago, New Zealand. Dr. Baldi’s research broadly focuses on cardiovascular responses to exercise in clinical populations, with a specific emphasis on left ventricular responses to acute exercise and physical training in people with diabetes.