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Baker, C 
ORCID: https://orcid.org/0009-0000-0057-4838,
2025.
Effects of intensified training on immune and endocrine biomarkers: identifying biomarkers to highlight the negative states of overtraining.
PhD, Nottingham Trent University.
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Abstract
Intensified training coupled with sufficient recovery is required to improve athletic performance; however, a stress-recovery imbalance can lead to the negative states of overtraining (non-functional overreaching (NFOR) or the overtraining syndrome (OTS)). Blunted cortisol and testosterone responses to a 30-minute, high intensity cycling test consisting of alternating blocks of 1 minute at 55% maximum workload (Wmax) and 4 minutes at 80% Wmax (known as the 55/80) has previously been identified suggesting that these exercise induced hormones are possible biomarkers of NFOR/OTS in males. However, it remains unknown whether the same phenomenon occurs in females. These hormonal alterations associated with overtraining may impair the immune response. Cortisol, for example, promotes the maturation and migration of dendritic cells (DCs); key antigen presenting cells which subsequently stimulate a T cell response, yet very little is currently known about their response to intensified training. Specifically, there are currently no clear reliable immune biomarkers to highlight when athletes are entering states of NFOR/OTS. Therefore, the aims of this thesis were:
1. To examine the salivary and plasma cortisol and testosterone, and plasma progesterone responses in females to the previously developed 30-minute cycling stress test (55/80).
2. To assess the current literature examining the effects of intensified training periods on lymphocyte and DC number and function.
3. To establish the reproducibility of T cell and DC count responses to a newly developed 30-minute stress test utilising submaximal physiological thresholds to prescribe intensity (20/50).
4. To investigate the endocrine (salivary and plasma cortisol and testosterone) and immune (DC toll-like receptor (TLR) and cytokine) alterations that may occur with a 9-day intensified training period.
The first experimental Chapter, Chapter 4, investigated the plasma and salivary cortisol, testosterone and plasma progesterone response to the 55/80 cycling test in physically fit, healthy females. The effects of oral contraceptives on the hormonal responses to the 55/80 were also examined. Chapter 4 concluded that the 55/80 induced hormonal elevations in females, similar in magnitude to males, however, these exercise induced elevations were attenuated in oral contraceptive users. These findings indicate that the 55/80 is a valuable tool to highlight exercise-induced hormone alterations associated with NFOR/OTS in naturally menstruating females, but not females prescribed oral contraceptive pills. Given the known interaction between the endocrine and immune systems, Chapter 5 assessed the existing literature examining the effects of intensified exercise training on DC and lymphocyte counts and function, to identify potential gaps for investigation. The systematic review and meta-analysis concluded that although some immune biomarkers alter after a period of intensified training, definitive immune biomarkers are limited. Specifically, due to low study numbers, further investigation into the DC response in human models was required. Given the importance that DCs play in orchestrating the immune response, yet the limited literature investigating their response to heavy training periods, Chapter 6 developed an exercise stress test capable of eliciting robust and reproducible elevations in DC and T cell counts (known as the 20/50). The 30-minute 20/50 was developed to address weaknesses with the design of the 55/80 such that exercise intensity was prescribed utilising submaximal anchors (ventilatory threshold; VT1), rather Wmax to ensure physiological consistency. Chapter 6 concluded that the 20/50 induced robust and reproducible elevations in DC and T cell counts, assessed via coefficient of variations (CV), Bland-Altman plots, smallest real difference (SRD) and intraclass correlation coefficients (ICC). The 20/50 can therefore be used before and after a period of intensified training to highlight any exercise-induced DC and T cell alterations that may occur. Chapter 7 draws together the work of all previous Chapters to examine the resting and exercise-induced immune (DC and T cell count and function) and hormonal (plasma and salivary cortisol and testosterone) responses before and after a 9-day period of intensified training. Although the 20/50 led to elevated immune cell counts, stimulated DC TLR 7 and 9 expression and the percentage of interleukin (IL)-10 producing T cells, and reductions in stimulated plasmacytoid DC Interferon (IFN)-α secretion, no alterations were seen after the intensified training period compared with before the training period. Blunted exercise-induced plasma and salivary testosterone and plasma cortisol were observed in response to the 20/50 after the 9-day intensified training period, highlighting their usefulness as biomarkers of NFOR/OTS. The results indicate that the participants were able to withstand an ~80% intensification of their habitual training load without experiencing maladaptive immune responses, indicating that the immune system may be more robust than the endocrine system during periods of heavy training.
In conclusion, the studies in this thesis demonstrate that salivary testosterone, and plasma cortisol and testosterone, with the addition of plasma progesterone in females, are potential diagnostic biomarkers for detecting overreaching and possible OTS (Chapters 4 and 7). In contrast, immune biomarkers related to DC and T cell count and function appear less sensitive indicators of these states. However, the immune system in healthy, recreationally active males and females proved robust, tolerating ~80% increases in habitual training load over 9-days without significant declines in T cell or DC immunity. These findings support the view that heavy exercise may not be inherently immunosuppressive (Chapter 7), with the mobilisation and redistribution of effector cells potentially enhancing immunosurveillance at tissue sites (Chapter 6). Chapters 6 and 7 also contribute valuable insight into DC responses following intensified training; an area of limited research despite DCs’ central role in immune regulation (Chapter 5). Finally, the 20/50 stress test effectively detects endocrine alterations associated with intensified training, supporting its practical use in monitoring athlete training loads during periods of heavy training, such as training camps
| Item Type: | Thesis |
|---|---|
| Creators: | Baker, C. |
| Contributors: | Name Role NTU ID ORCID |
| Date: | July 2025 |
| 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: | Jeremy Silvester |
| Date Added: | 12 Feb 2026 16:14 |
| Last Modified: | 12 Feb 2026 16:14 |
| URI: | https://irep.ntu.ac.uk/id/eprint/55258 |
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