You've just run the best track workout of your life. A set of killer repetitions at 5k race pace and you feel like Olympic champion Hicham El Guerrouj.
Of course, you hang around in damp clothes, the sweat steaming off you in the cool night air and pore over the splits with other runners. The next morning you waken up and it feels like you gargled with battery acid the night before. You have the flu.
Did the track workout suppress your immune system and allow you to get sick?
It is not possible to give a definite yes-or-no answer. The immune system is a complex blend of lymphocytes, leukocytes, immunoglobulins, eosinophils and natural killer cells, each with an important role in protecting our bodies from disease. A brief look at some sport science research might provide some insight into how your running affects your immune system.
One study investigated the effects of training on the immune systems of distance runners. It looked at the effects on the immune system of increasing either the training volume or the intensity, and at both immediate, and longer-term effects.
The researchers found reductions in the ratio of "immune helper" cells to "immune suppressor" cells with increases in either the volume or intensity of training.
This reduction indicates an increased susceptibility to infection. It was also found that the runners' immune systems were depressed more by increasing the intensity of training than by increasing the volume.
So, a session of hard intervals on the track will suppress your immune system more than simply adding some easy miles onto your long Sunday run.
Over time the immune systems of runners will adapt to the increased training intensity and volume. It seems that you may be most at risk of getting sick following an increase in training, but that the immune system adjusts relatively quickly to the increased stress.
In the study, the subjects' immune helper/immune suppressor ratios were found to return to normal by the following day after a workout. Other studies have found this ratio to return to normal as quickly as 30-90 minutes after exercise.
Another study analysed the immune systems of 10 experienced marathoners after they ran for three hours to exhaustion.
Cell alterations
This study found alterations in several types of immune cells during recovery. Most of these changes returned to normal within 21 hours post-run. These results suggest that changes to the immune system, even after an extremely strenuous three-hour run, return to normal in less than a day.
Results indicate that you shouldn't suddenly increase the intensity of your training or your mileage because it can overwhelm your immune system. The subsequent suppression of your immune system, although short-lived, can open the door to illness. Take extra precautions immediately after hard speed-work, or races, or extra-long runs.
The temporary suppression of your immune system that such efforts can cause, may leave you vulnerable (for a short time) to infection. You are most susceptible to infection at the end of a workout or race and for the first few hours afterwards.
You should not go for a run with a training partner who is sick. And be extremely careful going back home from hard training if you know someone in your family is sick.
Do not forget that other, perhaps work-related, stresses could be adding to your total stress level. One way to judge might be that if you are genuinely looking forward to a particular training session then your overall stress level is low.
If an upcoming track workout seems like a chore, then it might be better to postpone it for another day and either go for a short easy run or have a day off.
Forcing yourself to complete a training session might be one stress too many and leave you vulnerable to illness. Don't forget, immediately after increasing the intensity or quantity of running is when we are most at risk.
Next time you run some killer intervals, don't hang around but jog a cool-down and head for a warm shower and get some carbohydrate-rich food in you as soon as possible.