The modification of heart rate variability in normal, overweight and type 2 diabetic individuals.

Abstract

The aim of this thesis was to improve our understanding of the effects that dietary therapies have on improving cardiac autonomic activity in healthy and diabetic people, particularly the effects of omega 3 polyunsaturated fatty acids (PUFA) on healthy people. Research conducted to date suggests that diet has specific effects on cardiac autonomic activity; however, much of this research has ignored the underlying influence of specific therapies and weight loss. In this thesis, heart rate variability (HRV) is used to assess cardiac autonomic activity. Cardiac autonomic activity is chiefly responsible for the beat to beat control of heart rate and has been implicated in sudden cardiac death and prognosis of an adverse cardiovacular event following myocardial infarct. The first experiment was designed to systematically examine the dose-response changes in cardiac ANS activity and vascular compliance after supplementation with omega 3 polyunsaturated fatty acids. In sixty seven overweight middle aged volunteers, HRV, cardiac sympathetic activity (assessed via low frequency component of HRV), parasympathetic activity (assessed by the high frequency component of HRV), Low Frequency/High Frequency (LF/HF) ratio (representing the balance of sympathetic/parasympathetic nervous activity on heart rate), heart rate (HR), arterial compliance, systolic and diastolic blood pressure were assessed during rest. All variables showed the greatest change in the highest dose group. Arterial compliance and the LF/HF ratio changed in a dose-dependent manner with the omega 3 PUFAs. These results suggest that the observed relationships between fish oil dose and changes in arterial compliance and LF/HF suggest that regular fish oil supplementation can improve the regulation of HR, HRV and consequently blood pressure by increasing parasympathetic regulation of cardiac autonomic tone in a dose-dependent manner. In the second experiment twenty healthy, young male subjects were subjected to graded lower body negative pressure (LBNP) before and after a 6 week dietary supplement intervention of omega 3 PUFAs. Both periods of LBNP were immediately followed by venepuncture to assess lipid and omega 3 content of the blood cells. After the intervention of omega 3 PUFAs an improvement in cardiac autonomic activity (HRV frequency measures) together with a reduction in HR demonstrated that cardiac autonomic activity was improved during rest. Graded LBNP significantly reduced overall HRV and increased the LF/HF ratio of the frequency domain. After the 6 week intervention of omega 3 PUFAs, the autonomic control of heart rate was improved at the highest level of LBNP. Omega 3 PUFAs were significantly increased in the treatment group. In conclusion, the changes in HR and HRV measures during orthostatic stress demonstrated a cardiovascular response likely to be caused by increasing parasympathetic regulation of cardiac autonomic tone in young active males. These mutual changes may reduce CVD risk from an early age and provide further justification for increased intakes of fish oil. In the third experiment forty nine type 2 diabetic middle aged subjects undertook a 16 week dietary weight loss intervention. Before and after the trial, HRV measures were recorded for 10 minutes while the patients were supine and at rest for 10 minutes followed by venepuncture for metabolic and lipids markers. HRV frequency and time domain data indicated that weight loss produced an improvement in cardiac autonomic activity and the mean level of cardiac PNS activity (assessed via the root mean square of the successive differences in R-R intervals, RMSSD) during rest. The observed changes in cardiac ANS activity were attributed to weight loss only, despite similar reductions in several metabolic and cardiovascular blood markers. The results of this study suggest that a calorically restricted diet has favourable effects on cardiac ANS activity and implicate weight loss as a mediator of these effects. The results of this thesis indicate that dietary intervention in people with and without disease, particularly type 2 diabetes, may specifically influence cardiac autonomic activity, which may improve cardiovascular health outcomes. Moreover, the observed effects of diet on cardiac autonomic activity support the notion that weight loss and omega 3 PUFAs have positive cardiovascular health outcomes. The results of the thesis demonstrate that in order to comprehensively understand the effects of dietary therapeutics on cardiac autonomic activity, it is essential that concomitant changes in HRV are considered.