Endurance athletes with a high training load experience respiratory tract infections, like the common cold, at a high rate.
Fascinatingly, exercise can have a positive or negative effect on the risk of a respiratory tract infection (RTI), like the common cold and sinusitis. As a general rule, people engaging in regular bouts of moderate exercise are at a lower risk of RTI compared to sedentary individuals, reaffirming what we all already know: exercise is freaking great for you.
In this article I’m going to discuss some epidemiological data that suggests athletes with a higher training load experience illness at a higher frequency than those with a moderate training load.
But before I do, I want to belabor the point that exercise and training are indisputably good for human health. Lifelong physical activity reduces the risk of non communicable diseases, like cancer, cardiovascular disease, and chronic inflammatory diseases.
Exercise even reduces the risk of infectious diseases, including viral and bacterial infections. Both epidemiological and randomized studies show that regular exercise results in an 18% - 67% reduction in the risk of upper respiratory tract infection. (Gleeson) (Campell)
Here’s where it gets...weird:
Those engaging in regular, moderate exercise experience lower RTIs than those participating in prolonged and/or intense exercise.
Studies show that stress, exhaustion, and fatigue from prolonged exercise and insufficient recovery time can depress a healthy immune response. (Nieman)
Dr. David Nieman pioneered a lot of the research investigating the relationship between illness and intense training.
In one study, Nieman et al. monitored the health of 2311 applicants of the 1987 Los Angeles Marathon. A whopping 42 percent of the runners reported at least one infection during the two months before the race.
Runners who trained more than 60 miles per week had roughly double the risk of infection of those who ran less than 20 miles per week.
Not only did training volume impact the rate of illness, but so did the event itself. Thirteen percent of runners who completed the marathon became ill shortly after the race compared to 2 percent of runners who trained for the marathon but didn’t participate. (Nieman)
More recent data further supports Nieman’s findings.
Spence et al examined the rate of upper respiratory infections (URIs) in 32 elite athletes.
- Thirty-seven URIs were reported in 28 subjects.
- Spence found that the incidence rate ratios for illness were higher in both the control subjects (1.93, 95% CI: 0.72-5.18) and elite athletes (4.50, 1.91-10.59) than in the recreationally competitive athletes.
- Researchers concluded that the results confirm a higher rate of URI among elite athletes than recreationally competitive athletes during the training season. (Spence)
This data suggests that athletes who train the hardest, both in duration and intensity, experience a greater incidence of sickness.
So why are trained athletes--arguably some of the healthiest people on the planet--experiencing illness at a higher rate than middle-of-the-pack enthusiasts?
Why trained athletes might be more susceptible to illness
There are a few possible reasons why elite athletes tend to get sick.
Intense and/or prolonged exercise can increase the stress hormone cortisol. High levels of cortisol can have all sorts of negative effects on immune function, including inhibiting white blood cell function, cytokine function (proteins related to immune response), and the production of T-cells (lymphocytes related to immune function). (Nieman)
And while the total number of immune cells (like the ones listed above) in the blood increases during exercise, they tend to fall rapidly and dramatically after exercise.
The idea that exercise in of itself can depress immune function is widely debated (Campbell), and it’s possible that these findings have less to do with exercise and more to do with lifestyle and the nature of endurance events.
For example, an inadequate diet (how many of us get enough nutrients in our day-to-day lives?), mental stress (like the kind that comes from balancing work, training, and life), and a lack of sleep (such as the sleepless nights before a big event or during a multi day event) can reduce immune response.
Criticism over the J-shaped curve
The relationship between training volume and intensity and rate of illness has come under debate.
In the 1990s, Dr. Nieman formulated the controversial “J-shaped hypothesis” to describe the relationship between exercise intensity and the risk of acquiring upper respiratory tract infections (URTI).
This hypothesis suggests a steep increase in the rate of infections due to an increased training volume and intensity, as you can see below.
The J-shaped curve has informed our understanding of training and illness for the last two decades. But in contrast, Martensson, et al performed an observational study that examined the relationship between self-reported, exercise-constraining days of sickness (days when the athlete decided not to train due to symptoms of disease, either self-reported or by a physician) and the volume of exercise training in elite endurance athletes.
As you can see in Martensson’s chart below, there is a clear, dramatic reduction in reported illness that correlates with an increase in training volume, with the reduction in illness leveling off between 400 and 700 hours of training and decreasing again toward the higher end of the spectrum.
A third-degree polynomial bivariate fit (solid line) of Sick days per year (y-axis) and Training hours per year (x-axis) shows a significant (R2 = 0.48, F Ratio = 18, p < 0.0001, N = 61 training years from 11 subjects) decrease in the number of sick days reported as the number of hours of training increases. The shaded area indicates the 95% CI of the model and the dashed lines indicate the 95% CI for the individual data points.
Source: Mårtensson, et al
So, what’s going on? Are highly trained athletes more likely to get sick, or not?
Part of the problem is that many of these studies are self reported and are not clinically confirmed.
To address this gap, researchers used throat swabs to confirm self-reported illness in people who reported URTI symptoms over a 5-month period (including during competition).
Participants included 32 elite athletes, 31 recreationally competitive athletes, and 20 sedentary controls. Twenty-eight subjects reported 37 URIs. Incidence rate ratios for illness were higher in both the control subjects (1.93, 95% CI: 0.72–5.18) and elite athletes (4.50, 1.91–10.59) than in the recreationally competitive athletes. (Spence)
Yet, researchers identified illness in only 11 (two control, three recreationally competitive, and six elite) out of 37 reported cases.
Researchers conclude that the results confirm a higher rate of URI among elite athletes than recreationally competitive athletes, but suggest that “despite the common perception that all URI are infections, physicians should consider both infectious and noninfectious causes when athletes present with symptoms.” (Spence)
Overtraining as a possible explanation for illness
Unfortunately, the small sample size of the study from Spence et. al., paired with the mostly self-reported, epidemiological work by others, suggest that we don’t have enough information to say with certainty whether this is true.
For example, in the case of Martensson’s chart (shown above), researchers note that the results show that the number of training days missed by an athlete due to illness is negatively correlated to the volume of training.
As the researchers point out, an athlete can’t be elite if they are missing a considerable amount of training days due to illness. However, it should be noted that these findings were self-reported. It’s possible that elite athletes, due to their intense training programs, simply end up training when they have a cold.
Or it could be that elite athletes also have elite immune systems. Perhaps someone’s ability to fight infection is what allows them to participate in the training volume it takes to truly be an elite athlete, making these findings biased from positive selection.
Consider two athletes, one who has five colds throughout the year, with each episode causing them two missed days of training and one week of poor performance, and the other who has one cold, causing one missed day and three days of poor performance.
The first athlete will end up missing or performing poorly for 45 days a year, while the latter will experience poor performance for three. It is easy to see how the second athlete will, over time, be able to outperform the other. As Martensson points out, an 800 hour/year training schedule demands few sick days.
Another explanation for conflicting data is that the rate of illness only increases when an athlete overtrains, or when an individual exceeds the body’s ability to recover from exercise.
As Martensson, et. al. discusses, the J-shaped curve proposed by Nieman could be true for less trained individuals, whereas the curve flattens as fitness increases. In other words, the farther an athlete reaches past their fitness level, the greater the likelihood of infection.
Overtraining is especially common in competitive athletes. Some just started a new sport, signed up for a big event they aren’t quite ready for, or are just increasing their training volume and duration too quickly and without guidance. Either way, overtraining is both debilitating and frustrating.
How can competitive athletes stay healthy?
If you are a competitive athlete with a high training volume, consider keeping an illness log with your training log.
At the end of the month or year, compare days ill to hours trained and the intensity of those hours. Make sure to record races and events, too.
If you notice you are often sick after a hard week of training or a big event, you may have linked your exercise load to your rate of infection.
The best way of mitigating the negative effects of overtraining is to consult an expert and consider temporarily decreasing either the intensity or duration of exercise through periodized training.
Periodization allows variation and includes phases of high training and planned periods for recovery.
As you evaluate your training schedule, also evaluate your diet. Nutrition is vital to recovery. Make sure you get the right amount of protein, carbs, and fat at the right time to promote critical adaptations, like glycogen storage.
ImmuneElite is clinically proven to reduce upper respiratory tract infections and symptoms, as well as decrease the total sick days (compared to placebo).
About The Author:
Matt Mosman (MS, CISSN, CSCS) is a research scientist, endurance athlete, and the founder and Chief Endurance Officer at EndurElite. Matt holds his B.S. in Exercise Science from Creighton University and his M.S. in Exercise Physiology from the University of California. Matt and his family reside in Spearfish South Dakota, where they enjoy running, mountain biking, camping, and all the outdoor adventures Spearfish has to offer.
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