The Paradox of Elevated HbA1c in Elite Endurance Athletes with Optimal Metabolic Health

High HbA1c and Glucose Variability in Metabolically Healthy Athletes

Hemoglobin A1c (HbA1c) is one of the most widely used tools in modern medicine to assess long-term blood sugar regulation. In clinical settings, it’s routinely used to screen for prediabetes, monitor diabetes progression, and guide treatment decisions. The appeal of HbA1c lies in its simplicity: a single number that reflects average glucose levels over the previous two to three months. However, in specific populations with unique physiological adaptations, relying solely on this marker can lead to misinterpretation.

Among endurance-trained athletes and highly active individuals, HbA1c can behave unexpectedly. These are people who typically present with excellent metabolic health (low fasting glucose, strong insulin sensitivity, and high cardiorespiratory fitness), yet in some cases, their HbA1c falls into a range that would traditionally raise concern. The mismatch is subtle but significant: a marker suggesting elevated average glucose in a body that otherwise checks every box for optimal regulation.

Adding further complexity is the issue of glucose variability. Continuous glucose monitoring (CGM) reveals a day-to-day reality in athletes that often deviates sharply from clinical expectations: short bursts of hyperglycemia during prolonged training, unexpected lows during recovery, and wide oscillations that don’t match standard glycemic curves. These fluctuations often occur in individuals with high insulin sensitivity and excellent glycemic control by every other measure.

This research review examines this phenomenon and explores how adaptations in red blood cell turnover, stress physiology, and glucose dynamics may contribute to elevated HbA1c and glycemic variability in otherwise metabolically healthy athletes. By investigating the physiological underpinnings of this mismatch, we aim to better understand when elevated glycemic markers reflect true metabolic dysfunction and when they reflect a body that’s simply adapted for performance.

Longer-Lived Cells, Higher HbA1c: A Hidden Variable in Athletic Physiology

One of the most overlooked factors influencing HbA1c is the biology of red blood cells themselves. Hemoglobin A1c reflects a process called glycation, where glucose binds to hemoglobin, the protein in red blood cells that carries oxygen. This glycation is non-enzymatic and irreversible, meaning that once glucose attaches to hemoglobin, it remains there for the life of the red blood cell. Because the average red blood cell lives around 120 days, HbA1c is interpreted as a reflection of average glucose exposure over the previous two to three months.

But that 120-day assumption isn’t universal. The lifespan of red blood cells can vary depending on several physiological factors, and when they circulate for longer than average, they have more time to accumulate glycation. Think of each red blood cell as a sponge moving through sugar water: the longer it stays in that environment, the more sugar it picks up. Even if the sugar concentration in the water doesn’t change, a sponge that’s been there longer will end up holding more. Similarly, red blood cells exposed to normal glucose levels for an extended period can accumulate more glycated hemoglobin, causing the HbA1c level to appear elevated, even when metabolic health is excellent.

This becomes particularly relevant in the context of endurance-trained athletes. Chronic aerobic training leads to subtle yet significant adaptations in red blood cell physiology. For one, as the body becomes more efficient at delivering oxygen, thanks to increased capillary density, mitochondrial function, and improved oxygen extraction, there is less mechanical and oxidative stress on red blood cells. This allows them to circulate longer before being broken down and replaced. At the same time, consistent training tends to reduce systemic inflammation and upregulate antioxidant defenses, both of which help preserve red blood cell integrity and delay cell turnover. Over time, these adaptations can lead to a population of red blood cells that survives slightly longer than the standard 120 days. [2]

This longer lifespan doesn’t require the presence of iron deficiency or anemia. It’s part of a broader adaptation that includes more efficient erythropoiesis, the process by which the body produces new red blood cells. Well-trained individuals often have tightly regulated red blood cell production, with fewer immature cells being rushed into circulation, resulting in a more stable and longer-lived red blood cell population overall. Additionally, many athletes experience an expansion in plasma volume, something that can make red blood cell counts appear slightly lower on paper but, in reality, reflects a healthy adaptation. To maintain equilibrium between oxygen delivery and blood thickness, the body may slightly slow the turnover of red blood cells, indirectly extending their lifespan even further. [2] 

The result of these changes is that red blood cells in athletes may live just 10 or 20 days longer than average. That may not seem like much, but for a lab test like HbA1c, it can be enough to tip the results upward. A red blood cell that circulates longer accumulates more glycation simply because it’s been exposed to glucose for a greater amount of time, not because glucose levels are abnormal. This means that in highly trained individuals, an HbA1c reading of 5.6 to 5.8 percent may not indicate prediabetes or even impaired glucose metabolism at all. Instead, it may reflect an efficient, well-adapted physiology in which red blood cells are healthier, more stable, and longer-lived than those of the average person. [2]

In these cases, HbA1c becomes a marker not of metabolic dysfunction but of a body operating under a different physiological baseline. Without understanding the impact of red blood cell lifespan, it’s easy to misinterpret an elevated HbA1c as a problem when in reality, it’s just a sign that the body is functioning efficiently and has shifted the underlying dynamics of red blood cell turnover.

When Training Stress Looks Like Dysregulation: The Hormonal Distortion of HbA1c

Just as the lifespan of red blood cells can distort the meaning of HbA1c in athletes, so too can the body’s hormonal adaptations to chronic training stress. Intense physical training not only challenges the cardiovascular and musculoskeletal systems, but it also activates the body’s stress response, particularly the hypothalamic-pituitary-adrenal (HPA) axis. In endurance athletes training at high volumes or intensities, this activation can become a near-daily feature of physiology, subtly influencing glucose metabolism in ways that may raise HbA1c without indicating pathology.

Cortisol, the body’s primary glucocorticoid hormone, plays a central role in this process. Its release is part of a healthy, adaptive response to physical stress, helping mobilize energy by increasing blood glucose levels. It does so primarily by stimulating gluconeogenesis in the liver and reducing glucose uptake in peripheral tissues, ensuring that fuel is available for the muscles and brain during prolonged exertion. In short bursts, this response is beneficial. However, when cortisol levels remain elevated, particularly during protracted training blocks, insufficient recovery, or the accumulation of physiological load, the result can be a more sustained rise in baseline glucose levels. Over time, even mild elevations can contribute to higher HbA1c readings. [3]

Importantly, the relationship between chronic cortisol elevation and HbA1c is not entirely linear, nor is it the same in every individual. Athletes with robust recovery protocols may mitigate much of this hormonal stress, and their metabolic flexibility often shields them against more pronounced insulin resistance. Still, evidence suggests that endurance athletes do experience prolonged cortisol exposure compared to sedentary controls. A study by Skoluda and colleagues used hair samples to estimate long-term cortisol levels. They found that amateur distance runners, triathletes, and cyclists had significantly higher hair cortisol concentrations compared to non-athletes. These levels reflected exposure over several months, rather than just isolated training days, and they increased in proportion to the volume of training. While the study didn’t reveal clinical metabolic dysfunction, it did show a clear signal: athletes operate in a state of elevated physiological stress that could influence how glucose is handled and how HbA1c appears on a lab report. [3]

Cortisol’s influence doesn’t act in isolation. High-intensity training also triggers spikes in catecholamines, such as adrenaline and noradrenaline. These fast-acting hormones drive acute glucose elevations by breaking down glycogen stores in the liver and limiting insulin secretion, keeping glucose readily available for rapid energy demands. In training sessions with large muscle group engagement, short rest intervals, or competitive intensity, these hormonal surges can produce sharp but transient spikes in blood glucose. While these glucose peaks are brief, they can still contribute to the overall glycation process when averaged over time, nudging HbA1c upward even in athletes with excellent insulin sensitivity and metabolic health. [1, 3]

Taken together, the hormonal adaptations to training, particularly elevated cortisol and episodic catecholamine release, form a backdrop in which HbA1c may reflect something more complex than just average glucose levels. A slightly elevated HbA1c in an endurance athlete may reflect the body’s orchestrated attempt to support repeated physical exertion, not a failure in glucose control. Although cortisol promotes insulin resistance and gluconeogenesis, and catecholamines increase glucose availability during exercise, these responses are fundamentally adaptive. They are designed to prioritize performance and resilience, rather than to maintain metabolic readings within clinical thresholds. 

That said, these hormonal patterns have consequences. Chronic cortisol elevation, in particular, can impact recovery, sleep, immune function, and mood. And while it may not always result in overt metabolic dysfunction, its effects on glucose dynamics are real. Some athletes may experience subtle impairments in glucose disposal, especially if compounded by life stress, poor sleep, or underfueling. Yet even in the absence of these complicating factors, elevated cortisol and training-induced glucose variability can make HbA1c appear deceptively high. [3]

What emerges is a nuanced picture: the endurance athlete’s physiology is tuned for performance, not for meeting conventional biomarkers of metabolic health. Hemoglobin A1c, though valuable, was never designed to account for this complexity. It reads the results of glycation, but not the reasons behind it. For athletes, elevated HbA1c may reflect the intersecting effects of longer-lived red blood cells and a stress-responsive endocrine system, a body finely calibrated for output, recovery, and resilience. Interpreting that number requires an understanding not just of glucose, but of everything the body does to make energy available when it’s needed most. 

Glucose in Motion: Why Exercise-Induced Spikes Don’t Always Spell Trouble

Another important layer to consider involves real-time glucose dynamics. Unlike HbA1c, which reflects average glycemic exposure over weeks, short-term fluctuations, captured through tools like continuous glucose monitoring (CGM), tell a very different story.

Paradoxically, one of the healthiest and most metabolically supportive behaviors—vigorous exercise—can cause brief but significant spikes in blood sugar. This isn’t a sign of dysfunction; instead, it’s an adaptive physiological response to the intense fuel demands of training. During prolonged or intense aerobic activity, working muscles require immediate access to glucose. To ensure that glucose remains available for muscle use, the body temporarily reduces insulin sensitivity in non-exercising tissues, such as the liver and adipose tissue. This adaptive shift, often referred to as transient insulin resistance, prevents glucose from being stored in the bloodstream, allowing it to be directed to the muscles as needed. As a result, blood sugar may rise during the workout and remain elevated for several hours afterward. [4]

This short-term glucose elevation is not a failure of metabolic control, but rather part of a finely tuned system designed to support optimal performance. However, it illustrates why real-time data can sometimes look concerning in athletes, even when the underlying health is excellent. A 2023 study captured this: after three hours of continuous cycling at 65% VO₂max, endurance-trained athletes exhibited significantly impaired glucose tolerance the next day compared to their baseline when rested. Their blood sugar levels rose higher and remained elevated for an extended period during an oral glucose tolerance test (OGTT), indicating temporary insulin resistance. Notably, this effect was specific to prolonged endurance activity; high-intensity interval training did not result in the same next-day impairment. [4]

CGM studies provide an even closer look. In elite endurance athletes, glucose levels during training sessions typically average around 108 mg/dL, with peaks reaching 140–150 mg/dL, particularly during longer efforts. These peaks are short-lived and usually resolve within hours, but they reflect the intensity of energy demand during training and recovery. Despite these fluctuations, most of these athletes demonstrate excellent fasting glucose, insulin sensitivity, and metabolic flexibility when tested outside of these acute windows. [4, 5]

While brief glucose surges like these don’t typically affect HbA1c in a meaningful way, since HbA1c reflects average exposure over weeks, not momentary highs, there’s growing interest in whether repeated spikes, especially in the context of longer red blood cell lifespan, might subtly nudge HbA1c upward in some individuals. It’s not a primary driver, but it may be a contributing factor in the larger picture. 

More importantly, this short-term glucose behavior serves as a reminder that blood sugar regulation in athletes operates under a distinct physiological rhythm. Transient insulin resistance during and after exercise is not a sign of dysfunction; it’s a deliberate reallocation of fuel. But if glucose is only ever measured in these recovery windows, or if lab values are interpreted without context, the result can be misleading. What may appear as poor glucose control is often just a reflection of a high-performing body shifting into a different metabolic gear. [1]

This brings us to a key qualifier in interpreting all of these patterns: the volume and intensity of training. Not all “active” individuals experience these shifts. Much of the so-called HbA1c paradox appears only in individuals engaging in sustained, high-volume exercise, which exceeds public health guidelines for physical activity.

Endurance Training and HbA1c: A Nuanced Relationship

While short-term glucose spikes during and after intense exercise may contribute to the complexity of interpreting glycemic markers in athletes, it's essential to take a broader view and consider the context. The idea that intensive endurance training consistently elevates HbA1c would be an oversimplification. In reality, long-term aerobic training is widely associated with improvements in metabolic health, including enhanced insulin sensitivity, improved glucose disposal, and a reduction in HbA1c over time. What we’re observing in athletes is not a failure of exercise to regulate glucose, but a mismatch between conventional clinical markers and the dynamic physiology of high-performing individuals. [1]

Hemoglobin A1c reflects long-term glucose exposure, not momentary excursions. In most cases, transient spikes that occur during training or recovery are too brief to meaningfully influence this marker. Over time, endurance training enhances the body’s ability to regulate glucose more efficiently, typically resulting in improved glycemic control. However, as with the lifespan of red blood cells and hormonal stress, the reality is more nuanced. Repeated exposure to training-induced glucose variability, particularly in individuals with prolonged red blood cell lifespan or elevated cortisol levels, may exert a subtle upward pressure on HbA1c in certain athletes, even in the absence of metabolic dysfunction. Further research is needed to fully understand these interactions. [1]

Insights from continuous glucose monitoring (CGM) add critical depth to this discussion. A case series published by the Gatorade Sports Science Exchange (SSE) synthesized data from non-diabetic endurance athletes. They revealed a surprising pattern: even among lean, highly trained individuals, average glucose values during heavy training blocks often crept into the prediabetic range. Fasting glucose levels between 100–110 mg/dL were common, despite regular oral glucose tolerance tests (OGTTs) and high insulin sensitivity when measured at rest. [6]

The review emphasized significant individual variability. Glucose responses varied significantly based on diet composition, exercise modality and intensity, psychological stress, and individual physiology. Even athletes with similar training volumes showed markedly different CGM patterns under identical conditions. Despite this variation, some common trends emerged. During prolonged or high-stakes training sessions, CGM readings frequently exceeded 140 mg/dL, driven by surges in adrenaline, glucagon, or the intake of high-glycemic carbohydrates. In contrast, overnight or post-exercise lows, sometimes dipping below 70 mg/dL, were not uncommon and often occurred without symptoms. [6]

These fluctuations are far more pronounced than those observed in sedentary individuals. While healthy non-athletes typically remain within a narrow glycemic range, endurance athletes often spend 10–20% of the day above 140 mg/dL and 5–7% of the time below 70 mg/dL. For example, professional soccer players have been shown to exceed 180 mg/dL during matches, and elite cyclists training at altitude demonstrated average glucose levels around 108 mg/dL, with peaks near 144 mg/dL and dips in the low 70s, all within the same session. [6]

These CGM profiles help redefine what "normal" looks like in highly active individuals. Rather than indicating dysfunction, this wider glycemic range may reflect a metabolically flexible system tuned to mobilize and clear energy rapidly. The takeaway is that athletes operate within a different metabolic rhythm, one where the tools used to assess health in sedentary populations may not always apply. When interpreting HbA1c or CGM data in this context, it is essential to account for physiological variability, rather than relying on standard reference ranges.

This distinction is especially relevant in longevity and biohacking communities, where many individuals pursue high-volume training as a strategy to extend healthspan. Without recognizing the unique metabolic signatures that accompany this level of activity, there is a real risk of misinterpreting lab results, mistaking adaptive physiology for early disease.

Significantly, not all exercise induces these paradoxical effects. Moderate-intensity movement, resistance training, and time-efficient workouts typically reduce HbA1c and support conventional glycemic improvements. The HbA1c paradox appears to be primarily restricted to sustained, high-dose endurance training, where repeated bouts of stress-hormone activation, cumulative metabolic load, and subtle shifts in red blood cell lifespan converge to create a physiology that sits just outside traditional diagnostic norms.

When "Healthy" Looks High: Reframing Elevated HbA1c in Endurance Athletes

Given the wide glycemic fluctuations observed in endurance athletes and the mounting evidence that traditional markers often fall short in this context, it’s increasingly clear that HbA1c cannot be interpreted the same way across all populations. For most people, a mildly elevated HbA1c signals a shift toward metabolic dysfunction and triggers appropriate concern. However, in highly active individuals, especially those engaged in prolonged or high-volume training, the same number may indicate adaptation rather than pathology. Clinical guidelines for HbA1c are based on population averages that tend to skew towards sedentary individuals. Applying them without nuance to athletic populations can lead to misclassification and unnecessary intervention.

How much activity is enough to produce this effect? The answer lies not in elite status but in cumulative training load. Public health guidelines define “active” as 150 minutes of moderate aerobic activity or 75 minutes of vigorous activity per week, thresholds designed for disease prevention, not for capturing the physiology of serious training. Many recreational athletes far exceed these baselines. Endurance runners, cyclists, triathletes, and even fitness enthusiasts who consistently and frequently train, especially when combining aerobic volume with strength work or high-intensity intervals, can develop the physiological patterns that contribute to HbA1c drift. 

You don’t need to be a professional Ironman competitor to experience these changes. Individuals engaging in regular long-distance runs, back-to-back training days, or demanding metabolic workouts may experience a mildly elevated HbA1c, not as a warning sign, but as a reflection of their body adapting to consistent, high physical demand. And importantly, what constitutes “high” activity varies; what pushes one person into this adaptive zone might be entirely manageable for another, depending on genetics, recovery capacity, training history, and hormonal sensitivity.

This is why context is essential. Viewed in isolation, HbA1c can mislead. But when interpreted alongside complementary markers, a different picture emerges. Many athletes with elevated HbA1c also have normal or low fasting glucose levels, low fasting insulin levels, and low HOMA-IR scores, which are strong indicators of excellent metabolic health. An athlete with an HbA1c of 5.8% and a fasting glucose of 85 mg/dL is likely not exhibiting early diabetes, but rather the byproduct of a longer-lived red blood cell population and post-exercise glucose excursions.

Dynamic testing helps clarify this distinction. Oral Glucose Tolerance Tests (OGTT) often reveal that these athletes clear glucose rapidly and return to baseline efficiently. One study of elite cyclists found that, despite significant glucose spikes during training, their OGTT responses at rest were entirely normal, further supporting the idea that HbA1c does not always track with underlying dysfunction.

Continuous Glucose Monitoring (CGM) offers the most precise window into these fluctuations. Rather than capturing a long-term average, CGM reveals the real-time highs and lows of daily life. In endurance athletes, glucose levels are typically stable but exhibit sharp, temporary elevations around workouts or high-carbohydrate meals. These spikes rarely last long enough to cause concern; time-in-range metrics and 24-hour averages often remain excellent. Still, these short bursts can tip the average glycation burden just sufficiently to raise HbA1c slightly, especially in combination with longer red blood cell lifespan.

Ultimately, interpreting HbA1c in athletic populations requires nuance. When elevated values coexist with low fasting insulin, normal glucose tolerance, and stable CGM profiles, they often reflect a well-trained physiologist, not creeping metabolic disease. For clinicians, recognizing this pattern is essential to avoid misdiagnosis and prevent unnecessary dietary restrictions or pharmacologic interventions.

Closing the Gap Between Physiology and Interpretation

Interpreting biomarkers, including HbA1c, through a one-size-fits-all lens is no longer sufficient, especially in individuals whose physiology sits outside conventional norms. Endurance athletes and highly active individuals embody this reality. Their elevated HbA1c or wide glycemic swings may look abnormal on paper, but in many cases, these markers are not red flags; they are reflections of a body finely tuned for performance, resilience, and metabolic efficiency under load. What the HbA1c paradox ultimately reveals is not a flaw in the biomarker itself, but a gap in how we contextualize it. By looking beyond static numbers, toward real-time glucose behavior, red blood cell dynamics, stress physiology, and cumulative training demands, we can shift from misinterpretation to informed insight.

For clinicians, this means asking not just what the number is, but why it might be elevated, and whether that elevation is adaptive or alarming. For athletes and high performers, this means recognizing that optimal health may not always align with textbook definitions, and that metrics must be viewed through the lens of function, rather than just form. Ultimately, HbA1c remains a valuable tool. But like any tool, its value depends on who, and in whom, it’s used.