You know the moment. Three weeks into a structured block, the form is building, the power numbers are ticking upward, the legs finally feel like they belong to you again. Then one morning you wake up and your throat is raw. By afternoon it's a full-blown head cold. By Friday you're lying on the couch wondering how long until you can ride again.
Two weeks later you get back on the bike, the fitness has evaporated, and the entire block is wasted.
Here's the thing nobody tells you: most of what you learned about training and immunity is wrong. The textbook model — that heavy training suppresses your immune system and leaves you vulnerable — was built on incomplete data and has been substantially rewritten in the last decade. The reality is more nuanced, more interesting, and considerably more useful.
I've spent years on the podcast talking to researchers who study this exact question. David Nieman at Appalachian State, who ran the foundational work on exercise and immune function. Neil Walsh at Liverpool, whose mucosal immunity research has reshaped how we think about upper respiratory infections in athletes. John Campbell at Bath, whose 2018 paper essentially dismantled the old model. The picture they paint is different from the one most coaches and cyclists still operate with.
And the good news is that when you understand what actually drives illness risk during training, the fixes are mostly simple.
The J-Curve: What You Were Taught and Why It's Outdated
For decades, the dominant model of exercise and immunity was the J-curve. The concept was clean: sedentary people have moderate infection risk, moderate exercisers have lower risk (exercise boosts immunity), and heavy exercisers have higher risk again (too much exercise suppresses immunity). The shape of the graph looks like a J. Neat. Intuitive. And mostly wrong.
David Nieman's early work in the 1990s provided much of the original evidence for this model. He tracked marathon runners and found that those running more than 96 kilometres per week had significantly higher rates of upper respiratory tract infections than those running less. His studies on the Los Angeles Marathon showed that runners who completed the race were six times more likely to report an illness in the following week compared to matched controls who trained for the race but did not run it.
The data was real. The interpretation was the problem.
What Nieman observed was correlation, not necessarily causation in the way the J-curve implied. The marathon runners who got sick were also the ones sleeping less, eating poorly around race day, experiencing high psychological stress, and training through fatigue. The exercise itself was one variable among many, and it may not have been the most important one.
Here's where it gets really interesting. John Campbell's group at the University of Bath published a landmark paper in 2018 that systematically challenged the J-curve model. Their argument was not that elite athletes never get sick — clearly some do. The argument was that the mechanism is not what we thought.
The old model said: heavy exercise suppresses the immune system by depleting immune cells.
Campbell's reinterpretation: heavy exercise redistributes immune cells from the bloodstream to peripheral tissues — the lungs, the gut lining, the skin — where pathogens are actually most likely to enter the body.
The distinction matters enormously. When researchers drew blood from athletes in the hours after a hard session and found fewer circulating lymphocytes and natural killer cells, they concluded the immune system was suppressed. But those cells had not disappeared. They had deployed. They had moved to the front lines. The bloodstream is a transport system, not a battleground, and measuring immune cells in transit tells you about deployment, not capacity.
Campbell described the old interpretation as fundamentally flawed — measuring immune function by counting cells in the blood is like measuring a country's military strength by counting the number of soldiers on the bus between bases.
This does not mean you can train as hard as you want without consequences. What it means is that the exercise itself is less immunosuppressive than the behaviours that surround heavy training. The real culprits are the things that typically accompany hard training blocks: sleep restriction, energy deficiency, chronic psychological stress, and monotonous high-volume work without adequate recovery.
The Open Window: Smaller Than You Think
The other pillar of the old model was the open window theory. The idea was that after hard exercise, there is a window — typically described as 3 to 72 hours — during which your immune system is suppressed and you are especially vulnerable to infection. The advice that followed was predictable: avoid crowded places after hard sessions, wash your hands obsessively, stay away from anyone with a cold.
Neil Walsh's work at Liverpool has done more than anyone's to refine this picture. His research on mucosal immunity — specifically salivary immunoglobulin A (SIgA), the antibody that lines your mouth, throat, and respiratory passages — showed that SIgA levels do drop after prolonged intense exercise. This part of the old model holds. The mucosal barrier, which is your first line of defence against airborne pathogens, does become temporarily weaker after a long hard session.
But the window is narrower than the textbook version suggests, and the magnitude of suppression is smaller. Walsh's data indicates that the reduction in SIgA is most pronounced after sessions lasting over 90 minutes at high intensity, and it typically recovers within 24 hours in well-rested, well-fuelled athletes. The 72-hour window that got repeated in coaching manuals appears to apply mainly to athletes who are already chronically under-recovered.
Walsh has been clear on this: the single most important factor in mucosal immune recovery is not any supplement or protocol. It is sleep. Athletes who sleep adequately after hard sessions show faster SIgA recovery than those who do not, regardless of other interventions.
The practical takeaway is that the open window exists, but it is not the gaping vulnerability the old literature implied. It is more like a slightly ajar door — and whether anything gets through depends far more on your overall recovery state than on the exercise session itself.
What Actually Makes Endurance Athletes Sick
If the exercise itself is less immunosuppressive than we thought, why do endurance athletes undeniably get sick more often during heavy training blocks? The answer is not one thing. It is the accumulation of several.
Chronic Energy Deficiency
Under-fuelling is probably the single biggest immune risk factor for endurance athletes that most coaches underestimate. When you consistently burn more than you consume — whether deliberately through calorie restriction or accidentally through poor planning — the immune system is among the first systems to suffer.
Nieman's later work demonstrated this clearly. Athletes in energy deficit show elevated cortisol, reduced lymphocyte proliferation, and impaired mucosal immunity compared to energy-matched controls doing the same training. The mechanism is simple: the immune system is metabolically expensive to run, and when energy is scarce, the body downregulates it.
This is particularly relevant for cyclists trying to lean out during a training block. The impulse to drop weight while building fitness is common and understandable. It is also immunologically dangerous. You can lose weight or train hard. Trying to do both simultaneously is where illness risk spikes.
Sleep Restriction
The data on sleep and immunity is unambiguous and frankly alarming. Sheldon Cohen's research at Carnegie Mellon showed that people sleeping less than 7 hours per night were 2.9 times more likely to develop a cold when exposed to rhinovirus compared to those sleeping 8 or more hours. Not slightly more likely. Nearly three times.
For athletes, the effect compounds. Sleep is when the majority of tissue repair, hormonal recovery, and immune reconstitution occurs. The cyclist sleeping 6 hours, training 10 hours per week, and wondering why they keep getting sick has answered their own question.
Walsh ranks sleep as the most powerful immune modulator available to athletes — more effective than any supplement, protocol, or recovery gadget. The cyclist who adds one hour of sleep per night will likely see more immune benefit than the one who adds three supplements.
Psychological Stress
This one gets overlooked because it does not fit neatly into a training plan. But the evidence is clear: chronic psychological stress — work pressure, financial worry, relationship strain, family demands — independently suppresses immune function through sustained cortisol elevation.
For masters cyclists, this is particularly relevant. The 45-year-old training 10 hours a week is also managing a career, possibly raising children, dealing with ageing parents, and running a household. The total stress load — training plus life — is what determines immune vulnerability, not training alone.
A week with a big project deadline at work is not the week to run your hardest interval sessions. The training plan cannot be separated from the life that surrounds it.
Monotonous High-Volume Training
There is evidence that long blocks of unvaried training — same intensity, same duration, same stimulus, week after week — carry higher immune risk than periodised programmes with planned variation. The mechanism is not fully understood, but it likely relates to chronic cortisol elevation from repetitive stress without adequate recovery signalling.
This is an argument for structured periodisation with deliberate recovery weeks, not just for performance, but for staying healthy.
Practical Immune Protection: What Actually Works
The current evidence points to a set of interventions that are mostly unglamorous and mostly free. No magic supplement. No secret protocol. Just consistent execution of the basics.
Fuel the Work
Nieman's most practically useful finding is that carbohydrate intake during prolonged exercise substantially reduces the cortisol and inflammatory cytokine response to that exercise. The effect is dose-dependent and well-replicated.
The recommendation: consume 30-60g of carbohydrate per hour during any session lasting over 90 minutes. This is not primarily about performance fuelling (though it helps). It is specifically about reducing the immune cost of the session.
A gel every 30 minutes, a bottle of sports drink per hour, or a combination of both. The form does not matter much. The consistency does. The cyclist who rides three hours fuelled versus three hours fasted is doing the same mechanical work with substantially different immune outcomes.
This also means the fasted training trend is immunologically questionable for long sessions. Short sessions under an hour, fine. But long fasted rides during a heavy training block are an invitation for trouble.
Sleep Like It Matters
Eight hours minimum. Consistent timing. Cool, dark room. No screens before bed. You have heard all of this before. The difference is understanding that for immune function, this is not optional self-improvement advice. It is the foundation.
Walsh's position on this is unequivocal: if an athlete told him they could only change one thing to reduce illness, he would say sleep more. Not better supplements. Not hand sanitiser. Not avoiding gyms. Sleep.
For masters cyclists already managing recovery challenges, the sleep imperative is even stronger. The hormonal and immunological recovery that happens during deep sleep becomes harder to achieve with age, which means the total sleep requirement arguably increases rather than decreases as you get older.
Practical steps that make a real difference:
- Set a fixed bedtime and wake time, even on weekends
- No caffeine after 2pm
- Last meal at least 2-3 hours before bed
- Room temperature 16-18 degrees Celsius
- 30g casein protein or Greek yogurt before bed (supports overnight muscle protein synthesis and does not disrupt sleep)
Vitamin D Through Winter
The evidence on vitamin D and immune function has moved from interesting to compelling over the last decade. Vitamin D is not just a bone mineral — it plays a direct role in both innate and adaptive immune responses, affecting antimicrobial peptide production, T-cell function, and inflammatory regulation.
The problem for cyclists in northern latitudes — the UK, Ireland, northern Europe, Canada, the northern US — is that from roughly October through March, skin synthesis of vitamin D from sunlight is negligible. If you are training through winter (and you are), your vitamin D levels are almost certainly declining.
Multiple studies show that athletes with serum vitamin D levels below 75 nmol/L have significantly higher rates of upper respiratory tract infections. Walsh's data from Liverpool athletes showed a clear inverse relationship between vitamin D status and illness episodes through the winter training period.
The fix is cheap and simple: 2,000-4,000 IU of vitamin D3 daily through the winter months, ideally taken with a fat-containing meal for absorption. This is well within safe supplementation levels and is supported by current sports science guidelines.
The better approach is to get tested. A simple blood test in October tells you your baseline, and you can supplement accordingly. If you are already below 50 nmol/L at the start of autumn, you may need a loading dose before settling into maintenance.
Manage Training Load Intelligently
The best immune protection from a training perspective is good periodisation. Planned recovery weeks, appropriate load progression, and deliberate variation in training stimulus all reduce the chronic stress that actually drives illness risk.
Specific guidelines:
- Build load progressively. Week-on-week training load increases of more than 10-15% are associated with higher illness rates. The body needs time to adapt not just muscularly and cardiovascularly, but immunologically.
- Take recovery weeks seriously. Every third or fourth week, reduce volume by 30-40% and intensity by 50%. These weeks are not lost training. They are when the immune system reconsolidates.
- Avoid the double hit. Do not combine a hard training week with high life stress, poor sleep, or calorie restriction. Any one of these is manageable. Two or three together is where the immune system breaks.
- Monitor for early warning signs. Persistent fatigue, elevated resting heart rate, disturbed sleep, loss of appetite, irritability — these are not just overtraining signals. They are immune vulnerability signals. Respond to them before you get sick, and you often avoid getting sick.
The Above-the-Neck Rule
When illness does arrive — and it will occasionally, regardless of how well you manage things — the above-the-neck rule remains the most practical clinical guideline.
Above the neck only — runny nose, sneezing, mild sore throat, no fever: light exercise is generally safe. An easy spin on the turbo, a short walk, gentle movement. Nothing intense. Nothing long.
Below the neck — chest congestion, productive cough, muscle aches, diarrhoea, fever of any degree: complete rest. No exceptions.
Training with a fever is not just unproductive. It is actively dangerous. Fever indicates systemic infection, and the combination of elevated body temperature from illness plus exercise-induced hyperthermia puts significant stress on the cardiovascular system. In rare but documented cases, training through viral illness with fever has triggered myocarditis — inflammation of the heart muscle. The risk is small but the consequence is severe.
The evidence on return-to-training timelines is consistent: athletes who rest at the first sign of illness recover in roughly half the time compared to those who try to train through it. A three-day cold treated with rest stays a three-day cold. The same cold trained through typically becomes a seven-to-ten-day illness with a longer fitness rebuild.
Take the rest day. Take two. The maths always favours early rest.
Why Masters Cyclists Get Hit Harder
Everything above applies to all endurance athletes, but masters cyclists face a compounded challenge that needs specific attention.
Immunosenescence
The immune system declines with age. This is not a theory — it is one of the most well-documented phenomena in human biology. From roughly age 40 onward, thymic output of new T-cells decreases, the diversity of the T-cell repertoire narrows, inflammatory baseline (sometimes called inflammaging) creeps upward, and the speed and magnitude of immune responses to new pathogens reduces.
The practical effect: the 50-year-old cyclist's immune system has a smaller margin of error than the 25-year-old's. The same training stress that leaves the younger rider slightly fatigued but immunologically intact may push the older rider into a zone of genuine vulnerability.
This does not mean you should train less as you age. Regular exercise is one of the most powerful countermeasures to immunosenescence — physically active older adults maintain immune function far better than sedentary ones. The evidence on this is overwhelming. Exercise is protective. The question is how much, and with what recovery.
The Narrower Margin
For the masters cyclist, the margin between productive training stress and immunological overreach is narrower. This means:
- Recovery weeks matter more and should be more frequent — every second or third week rather than every fourth.
- Sleep disruption hits harder. The 25-year-old can get away with a few bad nights. The 50-year-old cannot, because the hormonal and immune recovery processes are already less efficient.
- Energy availability matters more. Under-fuelling while training is risky at any age, but the consequence arrives faster for the older athlete.
- Total life stress must be factored into the training equation. The masters cyclist who ignores work stress, family demands, and sleep debt when planning training loads is working with incomplete information.
The 45-year-old running the same programme as a 25-year-old needs to be more careful about the recovery side. Not less training, but more deliberate recovery. The total training dose includes the recovery, and at 45 the recovery dose needs to be larger.
Monitoring That Matters
For masters cyclists specifically, a few monitoring practices make a real difference in catching immune vulnerability before it becomes illness:
- Morning resting heart rate. An elevation of 5+ beats above baseline for two or more consecutive days is a warning signal. Not definitive, but worth noting.
- Sleep quality tracking. Not the absolute numbers from your watch (those are approximate at best), but the trend. Declining sleep quality across a training block is an early warning sign.
- Subjective wellbeing. Rate your energy, mood, and motivation on a simple 1-5 scale each morning. Sustained decline across several days is more informative than any biomarker.
- SIgA testing. If you want objective data, some sports medicine clinics offer salivary SIgA testing. It is the closest thing to a direct measure of mucosal immune readiness. Expensive and impractical for most, but available for those who want it.
The Supplement Question
The temptation when reading about immune function is to look for a supplement that fixes everything. The evidence does not support that approach for most products.
What has evidence:
- Vitamin D (2,000-4,000 IU daily in winter): Strong evidence, discussed above.
- Vitamin C (200-500mg daily): Modest evidence for reducing cold duration by about half a day. Not dramatic, but cheap and low-risk.
- Zinc lozenges (at symptom onset): Some evidence for reducing cold duration if taken within 24 hours of symptom onset. The effect is modest.
- Probiotics (multi-strain, daily): Emerging evidence that specific strains reduce upper respiratory infection rates in athletes. Walsh's group has published data supporting this. The effect is moderate but consistent across several trials.
What does not have good evidence:
- Mega-dose vitamin C (above 1g daily) — no additional benefit over moderate doses.
- Echinacea — inconsistent results, no reliable effect.
- Most "immune boosting" proprietary blends — marketing, not science.
The supplement hierarchy for immune protection in order of evidence strength: vitamin D first, sleep second (not a supplement, but more effective than any supplement), probiotics third, and vitamin C as a cheap addition. Everything else is noise.
Putting It Together: A Seasonal Immune Strategy
Rather than reacting to illness when it arrives, the current evidence supports a proactive seasonal strategy.
Autumn (September-October):
- Get vitamin D levels tested
- Begin supplementation if below 75 nmol/L
- Review training periodisation for the winter block — ensure recovery weeks are scheduled
- Establish consistent sleep schedule before the clocks change
Winter (November-February):
- Maintain vitamin D supplementation (2,000-4,000 IU daily)
- Fuel all sessions over 90 minutes with 30-60g carbohydrate per hour
- Prioritise sleep — this is when infection pressure is highest and sunlight is lowest
- Reduce training load during high life-stress periods rather than pushing through
- Consider adding a multi-strain probiotic
Spring (March-April):
- Recheck vitamin D levels if you had a low baseline
- Be cautious with rapid load increases as the race season approaches — the combination of building fitness and emerging from winter fatigue is a common illness trigger
- Continue consistent fuelling and sleep practices
Year-round:
- Monitor morning resting heart rate and subjective wellbeing
- Respond to early warning signs with an extra rest day rather than training through
- Apply the above-the-neck rule without exception when illness does arrive
- Never combine heavy training with deliberate calorie restriction
The Real Takeaway
The science on exercise and immunity has moved significantly in the last decade. The J-curve is outdated. The open window is smaller than your coach probably told you. And the real drivers of illness in endurance athletes are not the training sessions themselves but the recovery behaviours that surround them.
Sleep, fuelling, vitamin D, intelligent load management, and honest self-monitoring. None of it is complicated. All of it is fixable. The cyclist who gets these basics right will spend more weeks training and fewer weeks on the couch, which compounds across a season into a meaningful fitness advantage.
For masters cyclists, the margin is narrower but the principles are the same. You cannot outrun immunosenescence, but you can manage it. The 50-year-old who trains smart, sleeps well, fuels properly, and respects early warning signs will stay healthier than the 35-year-old who ignores all of it.
The pattern of getting sick after every hard block is not inevitable. It is a signal that something in the recovery equation is off — and the science has finally caught up to tell you exactly where to look.
If you want to talk through how this applies to your own training, the Roadman Cycling community on Skool is where we dig into this stuff daily. Real conversations with riders working through the same challenges, plus direct access to me and the coaching team. Come in, ask questions, and stop accepting illness as the price of fitness.