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Suppose you feel yourself coming down with a cold. Should you go running anyway?
As it turns out, this is a pretty complicated question. The reason is because running - and exercising in general - has both positive and negative effects on the immune system. It's probably not possible to predict how a single run will impact your body, but here's what we know:
First of all, regular exercise over the long run will tend to make your immune system stronger. The exact reason for this is not perfectly understood, but research suggests that exercising gets rid of senescent (old and ineffective) T-cells and stimulates the production of new T-cells:
The researchers first took blood samples from each of the volunteers to examine how many senescent and naïve T cells each had. Then, these study subjects were all enrolled into 12-week exercise programs at the Rocky Mountain Cancer Rehabilitation Institute. All programs were individualized for the study participants, incorporating elements of cardiovascular exercise, strength and endurance training, and exercises for flexibility, posture, and balance, with extra emphasis in areas where participants were weak.
After the 12-week program, the researchers drew a second blood sample from each volunteer and ran the same T cell analysis....
Results showed that the ratio of senescent to naïve T cells changed favorably in the majority of participants, with most of the study subjects regaining greater numbers of the naïve variety.Having an increased number of naive T-cells means that your body will be more effective at fighting infection. That's the immune system boost we're likely experiencing when we get regular exercise.
On the other hand, exercise does two very bad things for your immune system: it causes inflammation and agonizes the production of cortisol. These effects are acute, of course, not permanent, which is why exercising is felicitous in the long run. We just can't forget that in the short run, a session of exercise makes our immune systems weaker.
There are other, more indirect factors, too. For example, regular exercise helps you sleep better, relieves stress, and increases insulin sensitivity. All of those things will tend to reduce levels of both cortisol and inflammation. But, once again, these are long-run effects not attributable to any one instance of running you might do.
In a perfect world, we'd be able to measure our various hormone, inflammation, and T-cell levels, and then conduct personal tests to determine how much, on average, going running adversely impacts the immune system in the immediate term, and to what extent that leaves us vulnerable to either new infections or the worsening of an existing one. That might sound like science fiction right now, but as recently as the 1970s, no one dreamed that we'd be able to accurately measure blood glucose levels and determine how they respond to all the same kinds of things.
At any rate, here we are in an imperfect world, and the best advice I can give you is to use your judgment.
If you have a cold, you probably shouldn't run. Just wait it out and his the pavement when you're feeling better.
If you have something minor and nagging, like the sniffles, then you ought to take a careful look at things. If this has only recently happened, then maybe your immune system is doing what it needs to do, and exercising would only make things worse. If it's been a few days, and you mostly feel fine, you might want to go for a run to make sure the exercise you're getting is still "regular."
The kind of running you do is also relevant here. It is almost certainly a bad idea to do a big session of HIIT when you're trying to fight an infection. This kind of exercise produces even more inflammation and causes even more stress than more restrained forms of exercise.
Also, consider other recent factors. Have you lost a lot of sleep lately? Has your diet been particularly bad of late? Have you been under a lot of pressure at work?
Unfortunately, there isn't one, simple rule of thumb that can be applied to your decision about whether to run when you're not feeling 100% well. But familiarity with your body, combined with a little bit of knowledge and an analytical approach, should help you avoid bad colds for the most part, and keep you running regularly.