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Balancing carbohydrates and fats: a dual approach to fuelling endurance exercise
Endurance running requires precise nutritional strategies to meet energy requirements and optimize performance. Both carbohydrates (CHO) and fats play crucial roles in fuelling prolonged exercise, with their interaction influencing energy availability and efficiency during exercise. Recognizing thes...
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| Format: | Thesis |
| Language: | English English |
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Department of Human Biology
2026
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| _version_ | 1869483672517738496 |
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| access_status_str | Open Access |
| author | Byrnes, Devon |
| author2 | Bosch, Andrew |
| author_browse | Bosch, Andrew Byrnes, Devon |
| author_facet | Bosch, Andrew Byrnes, Devon |
| author_sort | Byrnes, Devon |
| collection | Thesis |
| description | Endurance running requires precise nutritional strategies to meet energy requirements and optimize performance. Both carbohydrates (CHO) and fats play crucial roles in fuelling prolonged exercise, with their interaction influencing energy availability and efficiency during exercise. Recognizing these dynamics is essential for improving athletic outcomes. This PhD explores the roles of CHO and fats, focusing on their individual and combined utilization to optimize running performance. The thesis includes a scoping review, an experimental trial, and an observational, descriptive study to provide a holistic view of endurance nutrition. Methods: The scoping review systematically analysed medium-chain triglycerides (MCT) and their impact on endurance performance across studies, identifying critical gaps in methodologies and limited application in runners. The experimental trial employed a randomized crossover design to compare the effects of CHO (10%), MCT (4.3%), or MCT+CHO (4.3% + 10%) supplements in trained endurance runners during a 90-minute steady-state run at 75% Peak Treadmill Speed (PTS) followed by a 5km time-trial (TT). Outcomes measured included substrate utilization, oxygen consumption (VO2), heart rate (HR), rate of perceived exertion (RPE), gut discomfort, glucose, beta-hydroxybutyrate (BHB), glycerol, free-fatty acids (FFA) and lactate concentrations, and TT finishing times. In the observational, descriptive study, South African marathon runners from two cohorts (2014 and 2024) provided information on dietary habits and training logs to evaluate macronutrient and energy intake, energy expenditure, energy balance, and macronutrient distribution in the month before a marathon, as well as CHO and fat-loading strategies used on the days immediately before a marathon. Results: The scoping review emphasized limited research on the effects of MCT in runners and females, identifying only two studies conducted on runners and none on female athletes. The review highlights the need for future runner-specific research on the effects of MCT on running performance. Additionally, the review underpinned the potential benefit of combining MCT with CHO to improve performance. The experimental trial revealed that the use of CHO reduced GI discomfort compared to the MCT group (0.4 ± 0.9 and 1.6 ± 1.9, respectively) (p = 0.0001), and resulted in the lowest HR (151 ± 11, 154 ± 11 and 155 ± 11 bpm for MCT+CHO, MCT, and CHO groups, respectively) and RPE (5.9 ± 1.3, 6.1 ± 1.1 and 6.5 ± 1.1 for MCT+CHO, CHO, and MCT, respectively) during the 90-minute steady run. The observational study found that a balanced diet of 46-52% CHO, 16-17% protein, and 29-34% fat best described the macronutrient intake of marathon runners. Additionally, an equal energy balance was noted between daily energy intake and expenditure. Daily intake of CHO relative to body weight was substantially low compared to recommended levels for endurance athletes (< 5 g.kg-1.day-1 compared to recommendations of 8-12 g.kg-1.day-1). When comparing the data from 2014-2024, there was a notable increase in daily fat intake (p < 0.001). Additionally, the study found 59-63% of runners CHO-load on the days before an event, whilst only 3-6% chose to fat-load. Finally, the study found that marathon runners were less likely to purposely restrict their fat intake in 2024 than in 2014. Conclusion: Although the combination of MCT and CHO shows potential to improve endurance performance, this was not the case in the experimental study. Combining MCT+CHO did not affect TT performance; however, it did result in reduced HR, and RPE. The addition of MCT to CHO aligns with the current dietary habits of marathon runners, who now habitually consume more fat compared to a decade ago. Finally, the low CHO intake observed in marathon runners warrants further investigation to identify the reasons for not meeting the recommended guidelines. Educating runners on the importance of adequate CHO intake for optimizing training and performance is essential |
| format | Thesis |
| id | oai:open.uct.ac.za:11427/43396 |
| institution | University of Cape Town (South Africa) |
| language | English eng |
| last_indexed | 2026-07-01T04:02:43.421Z |
| license_str | Not specified — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from UCTD — University of Cape Town Open Access Repository |
| publishDate | 2026 |
| publishDateRange | 2026 |
| publishDateSort | 2026 |
| publisher | Department of Human Biology |
| publisherStr | Department of Human Biology |
| record_format | dspace |
| source_str | UCTD — University of Cape Town Open Access Repository |
| spelling | oai:open.uct.ac.za:11427/43396 Balancing carbohydrates and fats: a dual approach to fuelling endurance exercise Byrnes, Devon Bosch, Andrew endurance exercise performance optimization Endurance running requires precise nutritional strategies to meet energy requirements and optimize performance. Both carbohydrates (CHO) and fats play crucial roles in fuelling prolonged exercise, with their interaction influencing energy availability and efficiency during exercise. Recognizing these dynamics is essential for improving athletic outcomes. This PhD explores the roles of CHO and fats, focusing on their individual and combined utilization to optimize running performance. The thesis includes a scoping review, an experimental trial, and an observational, descriptive study to provide a holistic view of endurance nutrition. Methods: The scoping review systematically analysed medium-chain triglycerides (MCT) and their impact on endurance performance across studies, identifying critical gaps in methodologies and limited application in runners. The experimental trial employed a randomized crossover design to compare the effects of CHO (10%), MCT (4.3%), or MCT+CHO (4.3% + 10%) supplements in trained endurance runners during a 90-minute steady-state run at 75% Peak Treadmill Speed (PTS) followed by a 5km time-trial (TT). Outcomes measured included substrate utilization, oxygen consumption (VO2), heart rate (HR), rate of perceived exertion (RPE), gut discomfort, glucose, beta-hydroxybutyrate (BHB), glycerol, free-fatty acids (FFA) and lactate concentrations, and TT finishing times. In the observational, descriptive study, South African marathon runners from two cohorts (2014 and 2024) provided information on dietary habits and training logs to evaluate macronutrient and energy intake, energy expenditure, energy balance, and macronutrient distribution in the month before a marathon, as well as CHO and fat-loading strategies used on the days immediately before a marathon. Results: The scoping review emphasized limited research on the effects of MCT in runners and females, identifying only two studies conducted on runners and none on female athletes. The review highlights the need for future runner-specific research on the effects of MCT on running performance. Additionally, the review underpinned the potential benefit of combining MCT with CHO to improve performance. The experimental trial revealed that the use of CHO reduced GI discomfort compared to the MCT group (0.4 ± 0.9 and 1.6 ± 1.9, respectively) (p = 0.0001), and resulted in the lowest HR (151 ± 11, 154 ± 11 and 155 ± 11 bpm for MCT+CHO, MCT, and CHO groups, respectively) and RPE (5.9 ± 1.3, 6.1 ± 1.1 and 6.5 ± 1.1 for MCT+CHO, CHO, and MCT, respectively) during the 90-minute steady run. The observational study found that a balanced diet of 46-52% CHO, 16-17% protein, and 29-34% fat best described the macronutrient intake of marathon runners. Additionally, an equal energy balance was noted between daily energy intake and expenditure. Daily intake of CHO relative to body weight was substantially low compared to recommended levels for endurance athletes (< 5 g.kg-1.day-1 compared to recommendations of 8-12 g.kg-1.day-1). When comparing the data from 2014-2024, there was a notable increase in daily fat intake (p < 0.001). Additionally, the study found 59-63% of runners CHO-load on the days before an event, whilst only 3-6% chose to fat-load. Finally, the study found that marathon runners were less likely to purposely restrict their fat intake in 2024 than in 2014. Conclusion: Although the combination of MCT and CHO shows potential to improve endurance performance, this was not the case in the experimental study. Combining MCT+CHO did not affect TT performance; however, it did result in reduced HR, and RPE. The addition of MCT to CHO aligns with the current dietary habits of marathon runners, who now habitually consume more fat compared to a decade ago. Finally, the low CHO intake observed in marathon runners warrants further investigation to identify the reasons for not meeting the recommended guidelines. Educating runners on the importance of adequate CHO intake for optimizing training and performance is essential 2026-06-26T08:03:00Z 2026-06-26T08:03:00Z 2026 2026-06-26T08:01:57Z Thesis / Dissertation Doctoral PhD http://hdl.handle.net/11427/43396 en eng application/pdf Department of Human Biology Faculty of Health Sciences University of Cape Town |
| spellingShingle | endurance exercise performance optimization Byrnes, Devon Balancing carbohydrates and fats: a dual approach to fuelling endurance exercise |
| thesis_degree_str | Doctoral |
| title | Balancing carbohydrates and fats: a dual approach to fuelling endurance exercise |
| title_full | Balancing carbohydrates and fats: a dual approach to fuelling endurance exercise |
| title_fullStr | Balancing carbohydrates and fats: a dual approach to fuelling endurance exercise |
| title_full_unstemmed | Balancing carbohydrates and fats: a dual approach to fuelling endurance exercise |
| title_short | Balancing carbohydrates and fats: a dual approach to fuelling endurance exercise |
| title_sort | balancing carbohydrates and fats a dual approach to fuelling endurance exercise |
| topic | endurance exercise performance optimization |
| url | http://hdl.handle.net/11427/43396 |
| work_keys_str_mv | AT byrnesdevon balancingcarbohydratesandfatsadualapproachtofuellingenduranceexercise |