The Impact of Intermittent Fasting on Energy Balance and Associated Health Outcomes

Abstract

Intermittent fasting may be an effective strategy for managing obesity and the associated dysfunction. It involves punctuating typical patterns of nutritional intake with scheduled periods of abstinence from all energy-providing nutrients. The few prior studies of this approach have yielded promising insights; yet there remains a dearth of knowledge regarding how intermittent fasting affects energy metabolism and health in humans, which this research sought to address. Initially, diurnal variations in subjective appetite ratings were established as a robust foundation upon which to design temporal nutritional interventions (Chapter 4). Specifically, appetite increased throughout the day to a peak in the evening, despite the apparent inversion of the accompanying rhythm in key regulatory peptides. This led to the development of a novel intermittent fasting intervention in which a complete fast was applied in alternating 24-hour periods for 20 days, with transitions from fasting to feeding occurring at 15:00 each day. In separate lean and overweight/obese cohorts, the impact of this diet, both eucaloric (i.e. complete refeeding in fed periods) and with a 25% calorie restriction (i.e. 50% refeeding during fed periods), was contrasted against a standard 25% calorie restriction. The experiments featured measures of postprandial metabolic responses and free-living physical activity (combined heart rate/accelerometry). In lean adults (Chapter 5), combining intermittent fasting with calorie restriction decreased physical activity thermogenesis relative to intermittent fasting or calorie restriction in isolation, largely due to reduced spontaneous activity during fasting. However, there were no improvements in metabolic health, whilst intermittent fasting also resulted in smaller declines in fat mass than daily calorie restriction. A similar pattern was seen for body composition in overweight/obese individuals (Chapter 6) but none of the interventions caused adaptive changes in energy expenditure. Instead, combining intermittent fasting with calorie restriction reduced postprandial insulinaemia and improved fasted and postprandial plasma lipid concentrations. Collectively, this suggests that the amount and timing of energy intake exert interactive effects on metabolism and health, with baseline adiposity being an important determinant of responses.

Date of Award	19 May 2019
Original language	English
Awarding Institution	University of Bath
Supervisor	James Betts (Supervisor), Dylan Thompson (Supervisor) & Javier Gonzalez (Supervisor)