Interleukin-6 in Athletes: What Your Blood Work Is Actually Telling You
Table of Contents
Introduction: Interleukin-6 in Athletes
In my own clinical work, interleukin-6 is one of those immune-system markers that I think many clinicians know about, but often more on a theoretical than a practical level. It is not something I see measured routinely in everyday general practice. More often, IL-6 appears in selected immune-mediated conditions, infectious disease contexts, or specialist immunology workups.
That said, I do think IL-6 can sometimes add useful context, in a similar way to high-sensitivity CRP. For example, if I am trying to understand whether low-grade inflammation may be part of a broader clinical picture — such as lifestyle-related disease, chronic stress, depression, burnout, or poor recovery — IL-6 can occasionally be one extra piece of information. It does not diagnose any of these conditions on its own. But when interpreted carefully, it may help support the overall clinical impression.
This is exactly why interleukin-6 in athletes is one of the most misread markers in sports medicine blood work. A clinician seeing an IL-6 result of several hundred pg/mL in a hospital patient would rightly be alarmed. The same result drawn two hours after a marathon finish may be entirely physiological — and reflects a fundamentally different biological process. Understanding this distinction is not a technical detail. It determines whether an elevated interleukin-6 in athletes is treated as a warning sign, a training artefact, or evidence of beneficial adaptation.
For me, the key point is that IL-6 should never be interpreted as a simple “inflammation number.” It is less familiar than CRP, ferritin, or white blood cell count, but when understood properly, it can be a useful marker in selected situations — especially when the real question is not simply “is there inflammation?” but “what kind of inflammatory, metabolic, or stress-related signal are we actually looking at?”
IL-6 is part of a broader panel of inflammation markers in athletes that each behave in distinct, exercise-specific ways. Unlike procalcitonin (PCT) — which tends to remain low after exercise and rises more specifically in bacterial infection — IL-6 can spike dramatically after prolonged training sessions even when no infection is present. Understanding where each marker fits in the picture is essential for avoiding both false alarms and missed diagnoses.
What Is Interleukin-6?
Interleukin-6 (IL-6) is a cytokine that is produced at the site of inflammation and plays a key role in the acute phase response. It is one of the chief stimulators of the production of most acute-phase proteins, such as C-reactive protein (CRP), from the liver. The most important cellular sources of IL-6 during inflammatory processes are macrophages and monocytes [22].
The discovery that changed everything for sport science came in the year 2000, when Steensberg and colleagues demonstrated that contracting skeletal muscle produces and releases IL-6 directly into the circulation. This led Pedersen and Fischer to propose the term myokine — a cytokine produced by muscle fibres and exerting effects in an endocrine manner — and to describe interleukin-6 as the prototypical exercise factor [2].
This distinction between muscle-derived IL-6 and immune cell-derived IL-6 is the conceptual foundation for interpreting interleukin-6 in athletes.
In general, most clinicians recognise interleukin-6 primarily as an immune-system cytokine. Its secondary role as a myokine — a signalling molecule released by contracting skeletal muscle — is much less widely understood, even among clinicians. Among athletes and patients, this distinction is usually even less familiar.
That gap matters. If IL-6 is seen only through the lens of infection and inflammation, a post-exercise rise can easily look pathological. But in an athlete, the same marker may partly reflect skeletal muscle activity, energy stress, glycogen availability, and normal exercise adaptation.
The Dual Identity: Pro-Inflammatory vs Myokine
The same cytokine behaves differently depending on where it originates and what the surrounding inflammatory environment looks like.
When IL-6 is released by infiltrating leukocytes — typically alongside tumour necrosis factor-alpha (TNF-α) — it amplifies the classical pro-inflammatory cascade operative in sepsis, rheumatoid arthritis, and cytokine release syndrome [3].
When skeletal muscle contracts and releases IL-6 in the absence of TNF-α elevation, the effects are largely anti-inflammatory. Muscle-derived IL-6 promotes the release of the anti-inflammatory cytokines IL-10 and IL-1 receptor antagonist (IL-1ra), suppresses TNF-α production, and contributes to a regulatory rather than destructive immune environment [3]. As Severinsen and Pedersen summarised in their 2020 Endocrine Reviews article, muscle-derived IL-6 mediates exercise-associated anti-inflammatory effects both acutely with each bout and as a consequence of longer-term training adaptation, including reductions in abdominal adiposity [4].
The practical implication: interleukin-6 in athletes post-exercise is not the same biological signal as interleukin-6 in a patient with active infection. The tissue of origin and the co-cytokine environment determine the downstream effect.
IL-6 should always be interpreted in relation to the patient’s clinical situation and the rest of the laboratory picture. As I mentioned earlier, IL-6 does not really tell us much on its own. Its interpretation is complex because the same elevated value may mean very different things depending on the context: are we looking at an athlete shortly after exercise, or are we looking at a patient with a genuine inflammatory disease process?
In practice, the first question should always be: why was IL-6 measured in the first place? It is not a routine marker that I would normally follow in athletes. For an athlete, IL-6 does not directly tell us anything specific by itself. The decision to order it should be guided by the clinical picture, symptoms, and suspected diagnosis — not simply by the fact that the person is an athlete.
In everyday clinical work, CRP and procalcitonin are usually much more useful inflammatory markers. They give us more immediately actionable clinical information. For example, if IL-6 is elevated on its own, that finding rarely creates much concern without a supporting clinical picture. But if CRP or procalcitonin is clearly elevated, even as an isolated finding, it usually carries more clinical weight and often gives a stronger reason to look further for infection, inflammation, or another underlying disease process.
The fact that IL-6 rises in athletes for different reasons, and may have different downstream effects in exercise compared with disease-related inflammation, is scientifically fascinating. But clinically, it is more of a physiological context than a routine monitoring tool. I would not measure IL-6 in athletes as part of a standard follow-up panel or periodic health check. I would order it only when there is a genuine clinical reason to investigate inflammation, infection, immune activation, or another suspected disease process — whether the patient is an athlete or not.
How Much Does Interleukin-6 Rise with Exercise in Athletes?
The magnitude of exercise-induced IL-6 elevation depends on three primary factors: exercise duration, intensity, and the muscle mass recruited.
Following exercise, plasma interleukin-6 in athletes may increase up to 100-fold relative to pre-exercise baseline, though less dramatic rises are more common in shorter or lower-intensity efforts [5]. The increase follows an exponential trajectory and peaks at or shortly after the end of exercise.
Some illustrative data from the primary literature:
- In the 246-km Spartathlon ultramarathon, IL-6 concentrations rose approximately 8,000-fold above baseline in 15 male athletes who completed the race in under 36 hours. Notably, interleukin-6 returned to normal within 48 hours, while other inflammatory markers such as CRP and serum amyloid A remained elevated [6].
- In a four-day long-distance walking event, IL-6 showed a tenfold increase on the first day, with levels not returning to baseline across the multi-day event [7].
- In five hours of single-leg knee-extensor exercise at 40% peak power output, arterial plasma IL-6 rose 19-fold compared to rest, attributable to release from the contracting leg [8].
Duration is one of the major determinants of post-exercise IL-6 amplitude — together with intensity, muscle mass recruited, and pre-exercise glycogen status. Mode also matters: a limited muscle mass working alone may be insufficient to elevate plasma interleukin-6 in athletes appreciably [5].
This is why IL-6 in athletes can occasionally create confusion in clinical practice. Exercise-induced IL-6 elevations may sometimes reach levels that overlap with values seen in infectious or inflammatory states. Seeing unexpectedly high IL-6 levels can therefore be surprising if the recent training context is not known.
For me, the practical point is straightforward: if IL-6 happens to be measured in an athlete as part of a broader immunological workup or inflammatory panel, athletic status itself matters — and so does the timing of recent exercise. A result cannot be interpreted in isolation from what the athlete has been doing in the previous hours or days. Knowing whether the person completed a hard endurance session yesterday may be just as important as the number itself.
The Glycogen Connection: Why Nutrition Affects Your IL-6 Result
One of the most clinically relevant mechanistic findings for interleukin-6 in athletes concerns the relationship between pre-exercise glycogen stores and IL-6 release.
Research by Steensberg et al. demonstrated that IL-6 release from contracting skeletal muscle is linked to pre-exercise glycogen availability: starting exercise in a glycogen-depleted state augments IL-6 release from working muscles. Conversely, carbohydrate ingestion during prolonged exercise attenuates the plasma IL-6 response [10]. The authors proposed that, beyond its immune roles, IL-6 may function as a metabolic energy sensor — signalling energy deficit and prompting compensatory responses in liver glucose output and peripheral lipid mobilisation.
This has direct practical implications: an athlete training fasted or with deliberately depleted glycogen will generate a larger IL-6 spike than one who is well-fuelled. A blood sample drawn after a fasted morning run cannot be directly compared to one drawn after a post-meal workout. This is one reason why standardising the conditions around blood collection matters enormously when tracking interleukin-6 in athletes over time.
The effect of fasting on IL-6 is clinically interesting because many broader laboratory panels include at least some fasting blood tests. As a result, IL-6 is often measured in a fasting state, even though the test itself is usually not an emergency investigation. It is more commonly ordered in a calm, non-urgent setting, often as part of a wider panel where fasting samples are already being collected.
This matters because fasting itself may influence IL-6 levels. If an athlete comes in fasted, especially after recent training or with low glycogen availability, the IL-6 result may not reflect a neutral baseline. For clinicians, this is an important pre-analytical detail: when interpreting IL-6, it is worth knowing not only when the athlete last trained, but also whether the sample was taken after fasting and what the nutritional context looked like.
Interleukin-6 in Athletes: Metabolic Roles Beyond Inflammation
Beyond immune signalling, muscle-derived IL-6 participates in energy homeostasis during prolonged effort.
IL-6 has been shown to increase hepatic glucose output, promote fatty acid oxidation, and enhance glucose uptake by contracting muscle [11]. In mouse models where IL-6 knockout was applied, animals deficient in IL-6 consistently showed impaired endurance exercise capacity compared to wild-type controls, supporting a metabolic role for IL-6 in sustaining prolonged effort [12].
More recently, a randomised double-blind human trial using the IL-6 receptor blocker tocilizumab found that IL-6 blockade during exercise reduced post-exercise appearance of oral glucose and lowered the insulin response — though it did not significantly alter glycogen resynthesis rates in the immediate recovery window, and had no effect on exercise performance or substrate utilisation during exercise itself [13]. These findings indicate a role for IL-6 in post-exercise metabolic regulation, while suggesting that not all proposed substrate effects are obligatory during the exercise bout itself.
From a practical clinical perspective, this is mostly an intellectual curiosity rather than something that directly changes day-to-day decision-making. I would not order IL-6 in an athlete to assess fuel availability or metabolic readiness. But it does highlight an important physiological point: IL-6 is not only an inflammation signal. In exercising muscle, it is also connected to the body’s metabolic energy state, glycogen availability, and the way the body coordinates fuel use during and after prolonged effort.
Trained vs Untrained Athletes: A Blunted Interleukin-6 Response
Well-trained athletes show a characteristically attenuated acute IL-6 response to a given exercise load compared to untrained individuals. Research by Fischer and colleagues noted that blood lactate is lower at a given power output in trained athletes, and the same pattern holds for IL-6 — likely because higher glycogen reserves reduce the metabolic stress signal driving IL-6 release [14].
A direct comparison study found that athletes showed a lesser magnitude of cytokine change following a longer duration of exercise than non-athletes, suggesting an attenuated cytokine response with training adaptation [15].
This fits a broader clinical pattern I often think about when interpreting athletes’ blood work: trained bodies are generally better adapted to the physiological stress of exercise. Recovery systems are more efficient, glycogen handling is improved, and a given submaximal workload usually causes less metabolic disruption than it would in an untrained person. In that sense, the body may simply have less need for a large IL-6 signal after a familiar training load.
The implication is important: a plasma interleukin-6 reading in a highly trained endurance athlete after submaximal effort may be unremarkable, while the same session in a sedentary individual would produce a considerably larger spike. Population reference ranges do not account for this training-related attenuation.
Resting Interleukin-6 in Athletes: Reference Ranges and Their Limits
Standard laboratory reference ranges for resting IL-6 in healthy adults are typically cited as below 5–7 pg/mL. A large meta-analysis of 57 studies including 3,166 healthy adult donors reported a pooled mean IL-6 concentration of 5.19 pg/mL (95% CI 4.63–5.74 pg/mL), although individual values ranged from 0 to 43.5 pg/mL depending on assay and population [16]. These reference ranges were established in general healthy adult populations and were not derived from athletic cohorts.
In well-trained athletes at rest, interleukin-6 levels are generally expected to be at the lower end of the normal range. Regular training appears to reduce resting IL-6 in metabolically compromised populations compared to sedentary counterparts [17], though evidence in already-healthy athletes is more limited.
A single resting measurement provides limited diagnostic information about chronic inflammation in an overtrained athlete. As summarised in a 2017 biomarker review in Journal of Strength and Conditioning Research, IL-6 provides little diagnostic information about chronic inflammation during overtraining in athletes because it has both pro-inflammatory and anti-inflammatory roles and responds to many stimuli acutely and chronically [1].
The picture from overtraining research is similarly uncertain: despite plausible mechanistic links between cytokine dysregulation and overtraining symptoms, evidence for persistently elevated resting interleukin-6 in clinically overtrained athletes is inconsistent. One study specifically examining overtrained cyclists found no changes in IL-6 or TNF-α concentrations [18]. For a deeper look at how to distinguish overtraining from normal training adaptation using blood markers — including cortisol and the testosterone-to-cortisol ratio — see the dedicated guide on HRV and blood work.
This fits the broader inflammatory profile I discussed in my article on inflammation markers in athletes. Regular endurance training is associated with long-term anti-inflammatory effects, largely mediated by muscle-derived IL-6. While an acute training session temporarily increases inflammatory markers, habitual exercise leads to a net reduction in chronic low-grade inflammation over time, including reductions in abdominal adiposity [4].
This is one of the reasons I find athlete blood work so interesting clinically. Exercise can create inflammatory-looking signals in the short term, while at the same time contributing to a lower baseline inflammatory tone in the long term. IL-6 sits at the centre of this apparent paradox: it may rise sharply after exercise, yet regular training appears to support a broader anti-inflammatory physiological environment.
Exercise Mode, Muscle Damage, and IL-6 in Athletes
The type of exercise matters when interpreting interleukin-6 in athletes. A reasonable assumption would be that eccentric exercise — which causes greater muscle fibre disruption — produces larger IL-6 spikes than concentric exercise. The evidence does not fully support this. Studies have shown that eccentric contractions are not necessarily associated with a larger acute IL-6 increase than concentric exercise. In one crossover trial, acute IL-6 concentration rose significantly after concentric exercise but not after non-muscle-damaging eccentric exercise at the same workload [19]. This suggests that the primary driver of acute exercise-induced IL-6 is not muscle damage itself, but rather the energy demands of contraction, the muscle mass recruited, and the associated metabolic signalling.
This distinction is useful when IL-6 is interpreted alongside more traditional muscle damage markers. Creatine kinase and myoglobin tend to track eccentric load and muscle fibre disruption more closely than IL-6. IL-6, by contrast, is more strongly shaped by the overall metabolic demand of the exercise bout.
The same principle applies when comparing aerobic dynamic exercise with resistance training. Cycling and running tend to produce clearer systemic IL-6 responses because they recruit large muscle groups for sustained periods. In one comparative study, pedalling on a bicycle ergometer raised plasma IL-6 in trained athletes by approximately fourfold, while weightlifting had negligible impact on plasma IL-6 in strength-trained athletes [20]. This means that a modest IL-6 response after resistance training does not necessarily indicate an inadequate inflammatory or adaptive response. It may simply reflect the shorter-duration, lower-systemic-metabolic nature of the session compared with prolonged endurance exercise.
Interleukin-6 in Athletes and Iron Metabolism: The Hepcidin Connection
An often-overlooked consequence of IL-6 physiology is its direct impact on iron regulation. IL-6 is a major inflammatory stimulus for hepatic hepcidin synthesis. Hepcidin is a peptide hormone produced in the liver that functions as the master regulator of iron homeostasis: when hepcidin is elevated, it binds to ferroportin — the only known cellular iron exporter — triggering its degradation and thereby blocking iron release from intestinal cells, macrophages, and hepatocytes into the circulation.
When exercise drives acute IL-6 spikes, hepcidin follows 3–6 hours later, and during that window iron absorption from the gut is measurably reduced [21][23]. Research with trained collegiate cross-country runners has confirmed that a single prolonged run produces a significant post-exercise hepcidin rise that reduces fractional dietary iron absorption — an effect that compounds over high training volumes and contributes to the high prevalence of iron deficiency in endurance athletes. The full mechanism and practical timing implications — including the evidence for alternate-day iron supplementation — are covered in the dedicated guide on ferritin levels for athletes.
This interaction also complicates blood work interpretation in a second way. Ferritin is itself an acute-phase reactant that rises alongside IL-6, potentially masking underlying iron depletion. An athlete with genuinely low iron stores but a post-training blood draw may show an artificially elevated ferritin alongside elevated IL-6 — a combination that could mislead clinical assessment. For a deeper discussion of when ferritin is and is not a reliable reflection of iron stores, see the guide on serum iron vs ferritin.
The key practical point: iron-rich meals and supplements are often best timed away from the post-exercise hepcidin peak — for example earlier in the day before hard training or on rest days, depending on the athlete’s schedule — specifically because of the post-exercise IL-6 → hepcidin axis.
In my own clinical work, I often see iron supplementation discussed in terms of dose, formulation, side effects, and ferritin targets — but much less often in terms of timing. This is especially relevant in athletes. Many patients feel that oral iron “does not absorb well,” yet we do not always ask the practical question: when are they taking it, and how close is that timing to training?
For an athlete with iron deficiency, I think this can be a genuinely useful point. If the athlete is taking iron during the post-exercise hepcidin window, absorption may be lower than expected. By moving the supplement away from hard training sessions, oral iron may work better in some cases. Clinically, that matters. It can reduce frustration for active patients, improve adherence, and sometimes help avoid unnecessary escalation to intravenous iron when oral replacement might still succeed with better timing.
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