Effects of fasting and meal composition on appetite and the metabolic responses to exercise

Slater, T. ORCID: 0000-0003-2764-3148, 2022. Effects of fasting and meal composition on appetite and the metabolic responses to exercise. PhD, Nottingham Trent University.

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Dietary restriction and regular exercise are traditional strategies which independently demonstrate success in managing body weight and improving markers of metabolic health in the short term, but often lack long-term success. As a result, even 'best-case scenario' predictions suggest that most of the English population will be at increased risk of disease because of excess body weight until at least the year 2035. Incorporating specified periods of fasting within a dietary regime, commonly referred to as 'intermittent fasting', has gained popularity as an alternative method of dietary restriction. Interestingly, fasting may also augment some of the benefits that are attained from exercise, possibly due to mechanisms related to increased fat oxidation. This thesis investigated the acute effects of two methods of increasing fat oxidation during exercise: fasting and carbohydrate restriction, before examining the utility of a novel meal containing virtually no energy, as an alternative method to mitigate against some of the challenges associated with fasting-based regimes. To maximise adherence and long-term success, it is crucial that exercise and nutrition interventions can be conveniently embedded into lifestyles. Therefore, this programme of work considered exercise timing as an important factor in study design and implementation.

Firstly, common exercise timing behaviours, opportunities, and preferences were surveyed (Chapter 4). Results of this survey showed that, despite most people preferring to exercise in the morning (08:00–11:59), there was a lack of opportunity to engage in morning exercise during the week, likely determined by a working lifestyle. As a result, the early evening (16:00–19:59) was the most common time for exercise during the week.

Informed by these findings, the acute metabolic, appetite, energy intake, and performance responses to a bout of fasted evening exercise were examined (Chapter 5). Fasting for 7 h before evening cycling exercise (18:30) increased fat oxidation and reduced net energy intake compared to exercising 2 h after a meal. Fasted evening exercise, however, was associated with increased appetite and reduced motivation to exercise, exercise enjoyment, and voluntary exercise performance, highlighting potential difficulties with adopting a fasted evening exercise regime.

Chapter 6 examined evening exercise after a low-carbohydrate, high-protein lunch, based on findings that carbohydrate consumption suppresses fat oxidation, but protein consumption may not. Consuming a low-carbohydrate, high-protein meal 3 h before evening cycling exercise (16:15) increased fat oxidation compared to a high-carbohydrate meal, but by a lesser extent than an 8 h fast. Importantly, the low-carbohydrate, high-protein meal also suppressed subjective and hormonal markers (PYY and GLP-1) of appetite and reduced ad-libitum energy intake in the evening compared to the high-carbohydrate meal and fasting, thus potentially offering an alternative method of increasing fat oxidation whilst also mitigating the challenges associated with fasted evening exercise.

An alternative strategy to manage fasting-induced elevations in appetite without providing energy was examined in Chapter 7. In this study, a very low-energy, viscous 'placebo' meal reduced subjective appetite compared to fasting, although this response was shorter-lived (~1 h) compared to that following a more typically consumed, whole-food meal (~2 h). The transient suppression of subjective appetite following a very low-energy, placebo meal could increase the efficacy of fasting-based interventions, without providing calorie-containing nutrients which would interrupt the fasted metabolic state and offset the energy deficit created by fasting.

Overall, the findings within this thesis suggest that the metabolic benefits of overnight-fasted morning exercise might also be attained during fasted evening exercise, but challenges such as elevated appetite, reduced voluntary exercise performance, and reduced motivation to exercise and exercise enjoyment may preclude its success in the long term. A low-carbohydrate, high-protein meal and a very low-energy, viscous 'placebo' meal may offer alternative strategies to offset the challenges associated with fasting, although longer-term studies are required to assess the chronic effects of these interventions on indices of body weight/composition and metabolic health.

Item Type: Thesis
Creators: Slater, T.
Date: October 2022
Rights: The copyright in this work is held by the author. You may copy up to 5% of this work for private study, or personal, non-commercial research. Any re-use of the information contained within this document should be fully referenced, quoting the author, title, university, degree level and pagination. Queries or requests for any other use, or if a more substantial copy is required, should be directed to the author.
Divisions: Schools > School of Science and Technology
Record created by: Linda Sullivan
Date Added: 21 Jun 2023 13:51
Last Modified: 21 Jun 2023 13:51
URI: https://irep.ntu.ac.uk/id/eprint/49245

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