Let's move on and discuss exercise.
Exercise also affects immune function.
Moderate exercise of one hour per day for five days a week has been shown to improve
the immune system.
But intense exercise for two hours or more has been shown to increase the risk of illness.
This can be described as a J-Curve.
Another example of this J-curve response is a study, including elite cyclists, recreational
cyclists, and sedentary controls.
Recreational cyclists had the fewest number of sick days.
Sedentary controls had twice as many sick days as recreational cyclists, but elite cyclists
had four times as many sick days as recreational cyclists.
A 2019 review outline 9 studies where endurance exercise was shown to decrease the number
of respiratory illnesses.
The workouts in these studies range from 30 to 60 minutes and were moderate in intensity,
such as jogging.
Antibody response to vaccination has been used as a proxy for immune response to exercise.
A comprehensive review investigated the relationship between exercise and the immune response to
a vaccine and found that in eight studies, frequent moderate to vigorous exercise increases
the body's immune response to a vaccine.
However, strenuous prolonged exercise did not show as strong of an immune response.
For example, prolonged intense exercise is unable to boost antibody responses to vaccines
like moderate exercise was able to.
For example, prolonged intense exercise is unable to boost antibody responses to vaccines
like moderate exercise was able to.
One study vaccinated 22 male triathletes, 30 minutes after completing a triathlon.
There was no difference in antibody levels in the male triathletes compared to controls.
There are several studies that demonstrate that extreme endurance exercise does not benefit
the immune system and can increase the chances of getting respiratory illness.
For example, the J-Curve study we mentioned earlier found that elite cyclists had four
times as many sick days as the recreational cyclist and twice as many as the sedentary
group.
Other studies have found an increased rate of respiratory illnesses after intense endurance
exercises that lasts at least 90 minutes in length.
Shorter duration running events, such as 5, 10 kilometers and half marathons did not appear
to elicit and increase incidents of self-reported upper respiratory tract infections.
Elite endurance athletes are exposed to many factors that could contribute to increase
viral susceptibility.
For example, they often attend events with large attendance, frequent traveling, psychological
stress, potential nutritional deficiencies, lack of sleep, and more.
However, when compared to other athletes that attend the same events and have similar risk
factors, the endurance athletes still have a significant increased risk of illness compared
to, for example, power or speed athletes.
There are a couple of proposed mechanisms that might explain why extreme exercise may
depress immune function.
But before talking about the proposed mechanisms, I think that it goes without saying that extreme
exercise like that at the elite level is very energetically-demanding.
Immune cell activation, such as the case when fighting off a pathogen, is also very energetically-demanding.
It seems very logical that there may be a triaging of the available energy to perform
the exercise since that is being forced, thus leaving the immune system temporarily unable
to function optimally without the necessary precursors and energy it requires.
All right, with that said, let's discuss the proposed mechanism one.
The Open Window Theory is characterized by short-term suppression of the immune system
following an exercise.
During exercise stress hormones, like cortisol increase in order to maintain adequate blood
glucose.
These stress hormones could transiently suppress immune function.
Shorter exercises may not allow enough time for the pathogen to replicate, but a two-hour
plus exercise may allow enough time for it to replicate to a critical threshold that
the innate immune system cannot resolve.
Some argue that the decrease in immune cells after exercise supports this theory.
But as we will later discuss, the decrease in immune cells may actually be due to a beneficial
relocation of immune cells from the blood to tissues.
So let's talk about mechanism number two.
The immune system cannot handle the pathogen burden associated with a two plus hour.
While there may be a boost to the immune system in an intense 2-hour plus exercise as there
is in a moderate 45-minute exercise, the burden on the immune system hits a tipping point
that leaves it depleted.
In support of this theory is the observation that vitamin C demand is increased in long
endurance exercises.
For example, 6 trials involving more than 600 marathon runners, skiers and soldiers
reported 50% fewer cold when supplementing with vitamin C. In addition, a randomized
controlled study in 20 athletes in their early 20s who participated in doula online competitions
found that antioxidant supplementation increased the antioxidant defenses of neutrophils.
There has been some debate about whether very strenuous exercise suppresses immune function
as a consequence of decreased immune cell numbers.
White blood cell numbers have been shown to increase during acute bouts of exercise followed
by rapid reduction in immune cell numbers, 30 to 60 minutes post exercise.
The drop in immune cell number can be 30% to 50% less than pre-exercise levels and can
last up to 6 hours.
This increase in sequential, decreased sparked the theory of the J-shaped curve.
Although many studies have validated the J-shaped curve, the mechanism is no longer believed
to be due to the rise and fall and immune cells.
This is because the number of immune cells in the blood is not a good indicator of immune
response.
The increase of immune cells is due to the immune cells being pulled off of the edges
of blood vessels by the increased blood flow and pressure.
Afterwards, the decrease is due to redistribution of immune cells to various tissues and out
of the bloodstream.
This has been shown in mice where fluorescent cell tracking showed immune T-cell numbers
rise during bouts of exercise, but then were redistributed primarily to the lung and other
areas like the small intestine.
If anything, the immune surveillance is actually heightened during the period where a few immune
cells are measured in the blood.
As a result, the mechanism behind the J-shaped curve of respiratory infection and exercise
duration is not due to the number of immune cells in the blood.