Date of Award

2021

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Integrative Physiology (PhD)

Administrative Home Department

Department of Kinesiology and Integrative Physiology

Advisor 1

William Cooke

Committee Member 1

Jason Carter

Committee Member 2

John Durocher

Committee Member 3

Kui Zhang

Abstract

Obesity is a chronic metabolic disorder associated with increased risk of cardiovascular disease. Evidence suggests that chronic intermittent fasting improves cardiometabolic health and reduces arterial blood pressure. However, the mechanisms underlying the reductions in blood pressure and improved cardiovascular health observed from chronic fasting studies remain unclear. The autonomic nervous system has a central role in the regulation of blood pressure and is essential for cardiovascular homeostasis. We conducted a study to investigate how acute fasting influences autonomic control of blood pressure at rest and during stress. Twenty-five young, healthy, normal weight, normotensive participants were tested twice, once in the fed state (3 hours postprandial) and again in the fasted state (24 hours postprandial). Aim 1 of the study was to determine the influence of an acute fast on hemodynamics, peripheral neural activity, and cardiovascular control at rest. To fulfill this aim we measured 24-hour ambulatory blood pressure for both conditions leading up to an autonomic function test. During the autonomic function test, we controlled breathing at 0.25 Hz and measured blood pressure, heart rate, muscle sympathetic nerve activity, and forearm blood flow for 10 minutes. Fasting reduced overall ambulatory blood pressure and heart rate compared to the fed condition. From the autonomic test we measured enhanced vagal modulation of the heart through 1) increased R-R interval and heart rate variability measured via spectral analysis; 2) Increased spontaneous (rest) and dynamic (Valsalva Maneuver) cardiovagal baroreflex sensitivity indicating enhanced reflexive vagal activation. Fasting did not alter peripheral sympathetic activity or blood pressure during the autonomic test. However, forearm vascular resistance and stroke volume were increased during the fasting condition. Aim 2 investigated if fasting influenced cardiovascular and neural reactivity to a mental stressor (5 min mental arithmetic). Fasting did not augment neural or cardiovascular reactivity to a mental stress challenge. Aim 3 investigated if fasting reduced orthostatic tolerance to intense lower body negative pressure (LBNP). LBNP was applied in a stepwise manner until participants became presyncopal. Fasting reduced the duration of negative pressure participants could tolerate before presyncope occurred. The reduced tolerance to central hypovolemia seems to have been caused by an impaired ability to increase peripheral resistance as measured from the forearm. This dissertation provides novel insight into how systemic energy balance influences autonomic regulation of blood pressure. Specifically, that fasting reduces 24-hour ambulatory blood pressure, increases vagal modulation of the heart, and enhances cardiovagal baroreflex sensitivity.

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