The impact of time of the day on muscle and metabolic responses to resistance exercise in healthy adults

Dighriri, Anas (2025) The impact of time of the day on muscle and metabolic responses to resistance exercise in healthy adults. PhD thesis, University of Glasgow.

Abstract

Metabolic disorders is a term used for a group of diseases including obesity, insulin resistance and type 2 diabetes. Today more than 537 million adults (age: 20–79 years)worldwide are living with diabetes, with a prevalence of 10%. This number is projected by international diabetes federation to reach 643 million by 2030 and 784 million by 2045(Al Ozairi et al., 2023, Federation, 2021). Regular exercise is a powerful strategy for the treatment and prevention of metabolic disease with previous studies demonstrating that this improves insulin sensitivity over time. Concurrently there are data indicating there are circadian rhythms in metabolism and that these might interact with exercise performance. However, data remains unclear on the impact of time of the day at which exercise is performed and how this influences metabolic responses.

Therefore, the aim of Chapter 2 is to systematically review and carry out a meta-analysis on the impact of exercise timing on metabolic responses to exercise in adults, considering both acute and more chronic studies and those with and without disease. The narrative analysis showed that the time of the day at which the exercise is performed does not have a clear impact on metabolic responses. Furthermore, the quantitative analysis found no significant difference in 24-hour CGM measured glucose levels, on the day of exercise or the following day regardless of whether the exercise was performed either in the morning or afternoon/evening.

Although aerobic exercise can improve insulin sensitivity acutely, resistance exercise has been less well studied in that regard. Therefore, the aim of Chapter 3 is to determine whether the time of the day has an effect on insulin sensitivity, measured during an oral glucose tolerance test, in response to a single resistance exercise session in healthy adultsin a cross over study. The two-way repeated measures ANOVA revealed no significant effect of time (p=0.586), group (p=0.720) or time*group interactions (p=0.511) for glucose area under the curve. Similarly, insulin area under the curve revealed no significant effect of time (p=0.735), group (p=0.663) or time*group interaction (p=0.973). Finally, insulin sensitivity revealed no significant effect of time (p=0.134), group (p=0.780) or time*group interactions (p=0.250). This finding indicate that a single session of resistance exercise has little effect on insulin sensitivity in healthy young adults, with no differences if this was performed in the morning or afternoon.

One of the main benefits of regular resistance exercise is that it increases muscle strength and mass which are important for maintenance of physical function, glucose control, and morbidity/mortality risk. However, the time at which resistance exercise is performed may be related to in improvements in muscle strength, muscle mass and metabolic responses. Therefore, the aim of Chapter 4 is to determine the effect of time of day at which resistance exercise is performed on the muscle and metabolic responses in healthy adults. We recruited 38 participants (30±7 years old; and 28±4 kg/m2) who were randomised to either a control, exercise in the morning (6:00-10am) or exercise in the evening (4:00-8:00pm) group. Those in the exercise groups performed 8 resistance exercises 3 times a week for 6 weeks, at their allocated time of day. The findings demonstrated that over the 6-week intervention period there were effects of time on insulin sensitivity (p<0.001), muscle thickness (p=0.008) and knee extensor maximal torque (p<0.001) but no interactions with time of day. Post-hoc tests revealed increases in insulin sensitivity and knee extensor maximal torque in the morning group (+7.55 [3.33 to 11.77] mg l2 mmol−1 mU−1 min−1p=0.003), with increases in muscle thickness (+1.17 [0.37 to 1.97] mm p=0.009) and knee extensor maximal torque in the evening group (+5.68 [2.36 to 8.99] Nm p=0.003). However, there was no clear effect of time of day. These results suggest that the benefits of resistance exercise may be achieved regardless of the time of day for healthy young adults and this should be the focus of public health strategies.

Overall, the current thesis demonstrated that there was no clear effect of the time of day on metabolic responses with exercise, the benefits of resistance exercise may be achieved regardless of the time of day for healthy young adults, and this should be the focus of public health strategies. Limitations, such as, a small sample sizes mean that further work is needed to corroborate these findings.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QP Physiology; R Medicine > RA Public aspects of medicine
• Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Cardiovascular & Metabolic Health
• Supervisor's Name: Gray, Professor Stuart
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85575
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 06 Nov 2025 16:26
• Last Modified: 06 Nov 2025 16:27
• Thesis DOI: 10.5525/gla.thesis.85575
• URI: https://theses.gla.ac.uk/id/eprint/85575

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