Cycling and immune aging: the King's College study that changed everything

In 2018, studies from King’s College London gave researchers a clearer view of how lifelong exercise may shape immune aging. The team examined 125 amateur cyclists aged 55 to 79 - people who had ridden regularly for decades, not professional athletes - and compared them with inactive older adults and younger controls.

The most interesting signal was thymic output. The cyclists had higher markers of fresh T-cell production than inactive older adults, and in some measures looked closer to younger controls. That does not mean exercise makes the immune system biologically 20 again, but it strongly challenges the idea that immunosenescence is entirely fixed.

This was not a magic-cycling result or proof that one sport reverses aging. It was a strong observational signal that long-term aerobic activity is linked with more resilient immune aging.

What you’ll learn:

What the King’s College study actually found
How the thymus gland ages — and how exercise preserves it
The T-cell revolution: young immune cells in old bodies
Why cycling is uniquely protective
Zone 2 training and the immune system sweet spot
Cardiovascular benefits that compound immune health
How to track your immune resilience with SuperAge
Practical guidelines: how to ride for longevity
Frequently asked questions

Quick answer

The King’s College cyclist studies did not prove cycling makes the immune system young again. They showed that highly active older cyclists had several immune features closer to younger adults than inactive peers, including better thymic output and more naive T cells.

The practical takeaway is consistency. Decades of moderate-to-vigorous aerobic activity appear to preserve immune, cardiovascular, and muscle function better than sedentary aging, but the study was observational and cannot prove cycling alone caused every difference.

For most people, the goal is not extreme mileage. A sustainable mix of zone 2 riding or similar aerobic work, strength training, recovery, and trend tracking is the safer longevity strategy.

Key facts

Cycling -> aerobic consistency: lifelong riders provided a model of sustained activity, not a prescription for one sport only.
Thymic output -> naive T cells: the Duggal study found higher TREC levels and naive T-cell markers in active older cyclists.
IL-6 and IL-7 -> immune context: cyclists showed lower IL-6 and higher IL-7 than inactive older adults.
Study design -> association: the findings compare active cyclists with controls; they do not prove that starting cycling reverses decades of immune aging.
Dose -> moderate consistency: repeated aerobic work appears more protective than sporadic extreme training.
Tracking -> recovery matters: HRV, resting heart rate, training load, sleep, and inflammation markers help keep exercise adaptive rather than excessive.

What the King’s College study actually found

The research that changed the field was published across two landmark papers in Aging Cell in 2018. The first, led by Ross Pollock and colleagues, examined the physiological profiles of 125 male and female amateur cyclists aged 55 to 79. The second, led by Niharika Arora Duggal, focused specifically on immune function — and it was this paper that sent shockwaves through immunology.

The study design

The inclusion criteria were deliberate and precise. The cyclists had to be able to cycle 100 km in under 6.5 hours (men) or 60 km in under 5.5 hours (women). These aren’t competitive targets — they’re well within the reach of dedicated recreational riders. The point was to study people who had maintained a consistent aerobic habit for most of their adult lives, not elite performers who might represent genetic outliers.

The study included two control groups: healthy but inactive older adults (aged 57–80) and a group of younger adults (aged 20–36) who were not competitive athletes. This three-way comparison — active old, inactive old, young — was critical to the findings.

Pollock et al. (2018): the physiological findings

The first paper documented that the cyclists showed no age-related loss of muscle mass or strength when compared to population norms. Their body fat percentages, cholesterol profiles, and testosterone levels (in men) didn’t follow the usual age-related trajectory. In many physiological parameters, the active older adults more closely resembled the young control group than the inactive people their own age.

But the most striking finding came from Duggal’s immunological analysis.

Duggal et al. (2018): the immune system revelation

Duggal’s team measured thymic output by quantifying T-cell receptor excision circles (TRECs) — molecular markers that indicate freshly produced naive T-cells. In the general population, TREC levels decline steeply after age 40 as the thymus atrophies. By 70, most people have vanishingly low thymic output, relying instead on a shrinking pool of memory T-cells to fight infections.

The cyclists were different. Their TREC levels were comparable to those of the young control group. Their thymuses were producing new T-cells at rates that defied their chronological age. Additionally, their naive T-cell populations were significantly higher than those of inactive older adults, while their senescent T-cell accumulation was significantly lower.

Key finding: Amateur cyclists aged 55-79 had higher thymic output and naive T-cell measures than inactive older adults, with some markers closer to adults aged 20-36. The finding supports immune preservation with lifelong activity, not a proven reversal of immune aging.

The study also reported lower IL-6 and higher IL-7 in the cyclists than in inactive older adults. That supports a plausible mechanism for preserved thymic function through cytokine signaling, but it remains an association, not proof that cycling alone reverses thymic aging.

How the thymus gland ages — and how exercise preserves it

To understand why the King’s College findings were so significant, you need to understand the thymus and its central role in immune defense.

The thymus: your immune system’s training academy

The thymus is a small, bilobed gland located behind the sternum. Its function is arguably the most important in the entire immune system: it’s where immature T-cells (thymocytes) undergo selection and maturation, learning to distinguish between self and non-self. Without a functioning thymus, the body cannot produce new naive T-cells — the versatile immune cells capable of recognizing novel threats like new viruses, bacteria, and cancer cells.

Thymic involution: the immune system’s ticking clock

Starting after puberty, the thymus undergoes a process called involution — it gradually shrinks and its functional tissue (the epithelial space where T-cells are trained) is progressively replaced by fatty tissue. By age 40, the thymus has lost roughly 80% of its functional capacity. By 70, it’s largely adipose tissue with minimal immune output.

This is one of the primary drivers of immunosenescence — the broad decline in immune function that makes older adults more susceptible to infections, less responsive to vaccines, and more prone to cancer. It’s also why older adults experienced disproportionately severe outcomes from COVID-19.

The exercise-thymus connection

The safer reading is that sustained aerobic activity may help preserve some features of thymic function. IL-7 is a plausible pathway because it supports thymic epithelial cells and T-cell development, but the study does not show complete thymus regeneration.

The T-cell revolution: young immune cells in old bodies

The implications of maintaining thymic output extend far beyond a single laboratory measurement. T-cell diversity is the foundation of adaptive immunity, and its loss is one of the most consequential aspects of aging.

Naive T-cells vs. memory T-cells

Your immune system relies on two broad populations of T-cells:

Naive T-cells: freshly produced cells with wide-ranging receptor diversity, capable of recognizing and responding to novel threats. These are the cells your thymus produces.
Memory T-cells: cells that have already encountered a specific pathogen and are “programmed” to respond to it quickly upon re-exposure. These accumulate over a lifetime of infections and vaccinations.

In a young immune system, the balance favors naive T-cells — a large, diverse repertoire ready for anything. As the thymus involutes, the balance shifts dramatically toward memory T-cells, and the pool of naive cells shrinks. By old age, the T-cell repertoire becomes dangerously narrow, dominated by expanded clones of memory cells targeting pathogens the body has already encountered.

Why T-cell diversity matters for longevity

A narrow T-cell repertoire creates several compounding vulnerabilities:

Reduced vaccine efficacy: flu and pneumonia vaccines are less effective in older adults precisely because their immune systems lack the naive T-cells needed to mount a novel response.
Cancer surveillance failure: T-cells are responsible for identifying and destroying aberrant cells before they become tumors. A restricted repertoire means more cancer cells slip through.
Chronic inflammation: senescent T-cells — old cells that have stopped dividing but refuse to die — secrete pro-inflammatory cytokines, contributing to the chronic low-grade inflammation known as “inflammaging.”
Vulnerability to novel pathogens: without a diverse naive T-cell pool, the immune system struggles to respond to pathogens it hasn’t encountered before.

The King’s College cyclists had maintained diverse naive T-cell populations and accumulated fewer senescent T-cells. This is the immunological equivalent of having a well-stocked armory rather than a collection of rusty, single-purpose weapons.

The connection to telomere health

There’s a downstream connection here worth noting. Senescent T-cells are characterized by critically short telomeres — the protective caps on chromosome ends that shorten with each cell division. When T-cells exhaust their replicative capacity, they enter senescence and begin producing inflammatory signals. By maintaining thymic output and thus a fresh supply of naive T-cells, exercise reduces the immune system’s reliance on over-proliferated, telomere-depleted memory cells.

Why cycling is uniquely protective

While the King’s College study focused on cyclists, the question naturally arises: is there something special about cycling, or would any form of sustained exercise produce the same benefits?

The honest answer is that most forms of regular aerobic exercise likely confer significant immune benefits. However, cycling has several characteristics that make it particularly well-suited for lifelong practice — which is the critical variable.

Low impact, high sustainability

Running, the most common aerobic exercise, imposes significant musculoskeletal stress. Joint degeneration, stress fractures, and overuse injuries become increasingly common with age, and many lifelong runners are forced to reduce or abandon the activity. Cycling, by contrast, is non-weight-bearing — the bike supports your body weight, eliminating impact forces on knees, hips, and ankles. This allows people to maintain high training volumes well into their 70s and 80s without the accumulated joint damage that sidelines runners.

Controllable intensity

Cycling allows precise intensity control through gearing, cadence, terrain selection, and—when available—FTP-based cycling power zones. This makes it straightforward to train consistently in specific physiological zones — particularly zone 2, which emerging evidence suggests is the sweet spot for immune and metabolic benefits. Runners can modulate pace, but the relationship between effort and impact stress is less favorable.

The King’s College cyclists were not solitary riders. Many were members of cycling clubs and had been riding in groups for decades. The social component of group cycling addresses another dimension of longevity: social engagement, which is independently associated with reduced inflammation and better immune outcomes. The joy and community around cycling likely contributed to the consistency that made the immune findings possible.

Where cycling fits among longevity sports

Cycling ranks highly in population studies of exercise and lifespan. The Copenhagen City Heart Study (Schnohr et al., 2018), which followed over 8,500 people for 25 years, found that cycling, tennis, and swimming were among the sports most strongly associated with longevity. Cycling was associated with 3.7 additional years of life expectancy compared to sedentary individuals.

Zone 2 training and the immune system sweet spot

Not all exercise intensities affect the immune system equally. Understanding the dose-response relationship between exercise intensity and immune function is critical for designing a longevity-oriented training plan.

The J-curve of exercise and immunity

Exercise immunology has long recognized a J-shaped relationship between training load and immune function. Moderate, consistent exercise enhances immune surveillance and reduces infection risk. But excessive high-intensity training — the kind common in elite endurance athletes — can temporarily suppress immune function, creating an “open window” for infection in the hours following exhaustive exercise.

The King’s College cyclists occupied the sweet spot on this curve: consistent, moderate-to-vigorous activity sustained over decades, without the extreme training loads of competitive athletes.

Why zone 2 matters

Zone 2 training — exercise at an intensity where you can still hold a conversation, typically 60–70% of maximum heart rate — has emerged as the cornerstone of longevity-focused exercise. Its benefits for the immune system specifically include:

Mitochondrial biogenesis: zone 2 is the most effective intensity for stimulating the creation of new mitochondria in both muscle cells and immune cells. T-cells and macrophages with more mitochondria function more effectively.
Anti-inflammatory myokine release: moderate-intensity exercise triggers the release of IL-6 from contracting muscles in a pattern that is anti-inflammatory (acute, pulsatile release), as opposed to the chronic, low-grade IL-6 elevation seen in inflammatory conditions. This exercise-induced IL-6 stimulates the release of IL-10 and IL-1ra, both potent anti-inflammatory cytokines.
Cortisol regulation: zone 2 training produces a moderate cortisol response that supports immune function. Excessive high-intensity training chronically elevates cortisol, which suppresses lymphocyte proliferation and natural killer cell activity.
Improved VO2 max: sustained zone 2 training builds the aerobic base that supports higher VO2 max — itself one of the strongest predictors of all-cause mortality and a marker of cardiovascular health that directly impacts immune cell trafficking.

The ideal dose

Based on the King’s College participants and corroborating research, the immune-protective dose of aerobic exercise appears to be:

3 to 5 sessions per week of sustained aerobic activity
Duration: 45 to 90 minutes per session
Intensity: primarily zone 2, with occasional higher-intensity efforts
Consistency: measured in years and decades, not weeks

This aligns closely with the training patterns of the King’s College cyclists, who averaged 100–150 km per week — substantial but not extreme volume.

Cardiovascular benefits that compound immune health

The immune benefits of cycling don’t exist in isolation. They’re amplified by the cardiovascular improvements that come with sustained aerobic exercise, creating a reinforcing loop between heart health and immune function.

Reduced arterial stiffness

Arterial stiffness increases progressively with age and is a primary driver of hypertension, organ damage, and cardiovascular events. The King’s College cyclists showed significantly lower arterial stiffness compared to inactive peers. Flexible arteries improve blood flow to every organ, including the thymus and bone marrow — the two primary sites of immune cell production.

Improved heart rate variability

The cyclists’ cardiovascular fitness was reflected in higher resting HRV (heart rate variability), indicating strong parasympathetic tone and autonomic flexibility. HRV is not just a cardiovascular marker — it’s an immune one. The vagus nerve, which mediates parasympathetic activity, has a direct anti-inflammatory effect through the cholinergic anti-inflammatory pathway. Higher HRV is associated with lower levels of C-reactive protein, TNF-alpha, and IL-6.

The cardiovascular-immune feedback loop

Better cardiovascular fitness → improved blood flow → more efficient immune cell trafficking → better surveillance → lower chronic inflammation → less vascular damage → better cardiovascular fitness. This positive feedback loop helps explain why the benefits of sustained exercise seem to compound over time, and why the longest-term exercisers show the most dramatic differences from their sedentary peers.

How to track your immune resilience with SuperAge

The King’s College study revealed that the body’s response to lifelong exercise can be measured. But you don’t need to visit a research lab to track the biomarkers that matter most for immune aging. SuperAge integrates the key metrics that reflect your immune and cardiovascular resilience into a single, actionable picture.

What SuperAge tracks

Heart rate variability (HRV): your daily HRV trends reveal autonomic balance and parasympathetic tone — both directly linked to anti-inflammatory signaling and immune regulation. SuperAge shows your HRV trends over time, helping you identify whether your training is enhancing or depleting your recovery capacity.
Training load curve: maintaining the right training balance is essential for staying in the immune-protective zone rather than the immunosuppressive one. SuperAge’s training load tracking helps you build volume progressively without overreaching.
Resting heart rate trends: a declining resting heart rate over months of consistent training reflects improving cardiovascular fitness — the same adaptations that supported the King’s College cyclists’ immune resilience.
Recovery metrics: sleep quality, daily readiness, and recovery scores help you ensure that your training stimulus is being matched by adequate rest — the period when immune repair and adaptation actually occur.

Why tracking matters

The King’s College study didn’t just show that exercise preserves immune function — it showed that consistency over years and decades is the key variable. SuperAge helps you maintain that consistency by making the invisible visible: tracking the trends that confirm your training is working, flagging periods of overtraining before they compromise your immune function, and showing the long-term trajectory of your cardiovascular fitness.

You can’t directly measure your thymic output from a wrist-worn device. But you can track the cardiovascular and autonomic markers that correlate with the immune benefits observed in the study — and you can use that data to keep yourself in the training zone where those benefits accumulate.

Use SuperAge: Download SuperAge to track HRV, resting heart rate, training load, recovery, and biological-age signals alongside your cycling routine.

Practical guidelines: how to ride for longevity

Based on the King’s College findings and the broader exercise immunology literature, here is a practical framework for cycling — or any sustained aerobic exercise — to maximize immune and longevity benefits.

Weekly structure

Day	Session type	Duration	Intensity
Mon	Zone 2 ride	60–90 min	Conversational pace
Tue	Rest or light activity	—	—
Wed	Zone 2 ride	45–60 min	Conversational pace
Thu	Rest or strength training	30–45 min	Moderate
Fri	Zone 2 ride	60–90 min	Conversational pace
Sat	Longer endurance ride	90–150 min	Mostly zone 2, some tempo
Sun	Active recovery or rest	—	Easy walk/spin

Cyclists who want to turn this longevity framework into a sport-specific week can use our cycling training plan after 50 to combine endurance rides, intervals, strength, and recovery.

Key principles

Prioritize consistency over intensity. The King’s College cyclists weren’t doing interval sprints every day. They were riding steadily, frequently, and for decades. Three to five sessions per week of moderate effort is the target.
Keep most training in zone 2. The 80/20 rule — 80% of training time at low intensity, 20% at moderate-to-high — is well-supported for both performance and immune health. Use a heart rate monitor to stay honest about intensity.
Build volume gradually. The 10% rule (increase weekly volume by no more than 10%) exists for a reason. Rapid volume increases spike cortisol and can temporarily suppress immune function.
Don’t skip recovery. Immune repair and adaptation happen during rest, not during exercise. Sleep, nutrition, and recovery days are not optional — they’re part of the training stimulus. The science of recovery between workouts explains the supercompensation cycle and how to time rest days for maximum adaptation.
Include strength training. While the King’s College study focused on aerobic exercise, maintaining muscle mass is independently important for immune function. Myokines — the immune-supporting signaling molecules — are produced by skeletal muscle during contraction. More muscle mass means a larger “pharmacy” of anti-inflammatory signals.
Track your trends. Use HRV, resting heart rate, and training load metrics to ensure you’re in the productive zone. A persistently declining HRV or rising resting heart rate may indicate overtraining — the point where exercise starts suppressing rather than supporting immune function. Learn to spot the 7 signs of overtraining before they derail your consistency.

Starting later in life

An important note: the King’s College participants had been cycling for most of their adult lives, but that doesn’t mean starting at 50 or 60 is pointless. Multiple studies have shown that previously sedentary older adults who begin regular aerobic exercise show measurable improvements in immune markers within months. The thymus may not fully regenerate, but even partial restoration of thymic output and reductions in inflammatory markers deliver meaningful clinical benefits.

Starting now is always better than not starting.

Frequently asked questions

How much cycling do you need to see immune benefits?

Based on the King’s College study and supporting literature, 3 to 5 sessions per week of 45 to 90 minutes at moderate intensity (zone 2) appears to be the effective range. The cyclists in the study averaged 100 to 150 km per week. However, even lower volumes of consistent aerobic exercise have been shown to reduce inflammatory markers and improve immune function.

Does the type of cycling matter — road, indoor, mountain bike?

The study participants were primarily road cyclists, but the immune benefits are driven by sustained aerobic effort, not the specific form of cycling. Indoor cycling (trainer or spin bike), gravel riding, and mountain biking all provide the cardiovascular stimulus that supports immune health. The key is sustaining the right intensity for the right duration, consistently.

Can other forms of exercise produce the same results?

Almost certainly, though cycling’s low-impact nature makes lifelong practice more feasible than high-impact sports. Swimming, rowing, brisk walking, and cross-country skiing are all strong candidates. The critical factor is decades of consistent aerobic activity at moderate intensity. The King’s College study chose cyclists specifically because cycling culture includes many lifelong practitioners who could be studied in their 60s, 70s, and 80s. Swimming has its own compelling longevity data — the Cooper Institute found swimmers age 50% slower in all-cause mortality than sedentary controls, driven by unique cardiovascular and respiratory adaptations that land-based exercise cannot replicate.

Is there a point where too much cycling hurts the immune system?

Yes. The J-curve of exercise immunology shows that extreme training volumes — the kind seen in professional cyclists doing 30,000+ km per year — can temporarily suppress immune function and increase upper respiratory infection risk. For longevity purposes, moderate and consistent is superior to extreme and episodic. Monitor your training load and recovery metrics to stay in the productive zone.

I’m 60 and have never exercised regularly. Is it too late?

No. While the King’s College cyclists had decades of training history, studies on previously sedentary older adults show that beginning regular aerobic exercise at any age produces measurable immune improvements within 8 to 12 weeks. Thymic output may not match that of a lifelong cyclist, but reductions in inflammatory markers, improvements in vaccine responses, and enhanced natural killer cell activity have been documented in late starters. Consult your physician, start conservatively, and build gradually.

How does this connect to biological age?

The immune system is one input into biological aging because immunosenescence and chronic inflammation affect many organ systems. Sustained exercise may support a more favorable biological-age trajectory by preserving immune and cardiovascular function, but it does not directly or automatically reduce biological age. Tracking cardiovascular fitness, recovery, and inflammation signals with tools like SuperAge helps you monitor whether training is adaptive over time.

References

Pollock RD, Carter S, Velloso CP, et al. “An investigation into the relationship between age and physiological function in highly active older adults.” The Journal of Physiology. 2015;593(3):657-680. DOI
Duggal NA, Pollock RD, Lazarus NR, Harridge S, Lord JM. “Major features of immunesenescence, including reduced thymic output, are ameliorated by high levels of physical activity in adulthood.” Aging Cell. 2018;17(2):e12750. DOI
Pollock RD, O’Brien KA, Daniels LJ, et al. “Properties of the vastus lateralis muscle in relation to age and physiological function in master cyclists aged 55-79 years.” Aging Cell. 2018;17(2):e12735. DOI
Schnohr P, O’Keefe JH, Holtermann A, et al. “Various leisure-time physical activities associated with widely divergent life expectancies: The Copenhagen City Heart Study.” Mayo Clinic Proceedings. 2018;93(12):1775-1785. DOI
Simpson RJ, Kunz H, Agha N, Graff R. “Exercise and the regulation of immune functions.” Progress in Molecular Biology and Translational Science. 2015;135:355-380. DOI
Nieman DC, Wentz LM. “The compelling link between physical activity and the body’s defense system.” Journal of Sport and Health Science. 2019;8(3):201-217. DOI