You run the best track workout of your life. Four repeat miles, and you feel like Moses Kiptanui. You hang around in your sweat-drenched clothes, talking splits with the other runners, and savoring the atmosphere. The next morning you wake up with the Russian Army marching down your throat. You have the flu.
Did the track workout suppress your immune system and allow you to get sick?
The answer is not clear-cut. The immune system is a complex blend of lymphocytes, leukocytes, immunoglobulins, eosinophils, natural killer cells, and other beasts, each with its own unique role in protecting our bodies from disease. Recent research from McMaster University in Ontario, however, provides some interesting insights into running and your immune system.
In a study published in the August, 1995 issue of Medicine and Science in Sports and Exercise, Dr. J. Duncan MacDougall and colleagues investigated the effects of training on the immune systems of distance runners. Unlike previous studies, MacDougall's group looked at the effects on the immune system of increasing training volume and/or intensity, and at both acute (immediate), and chronic (longer-term) effects. I contacted Dr. MacDougall to find out more about his results and their implications for runners.
In this study, two groups of six runners each, trained for 40 days, consisting of four 10-day training phases. The volume and intensity of training differed between phases. Group 1 ran at low volume/low intensity during the 1st phase, followed by high volume/low intensity during the 2nd phase, then low volume/low intensity again during the 3rd phase, and high volume/high intensity during the final phase. Group 2 followed the same protocol, but switched phases 2 and 4.
"Low intensity" meant running at 60-70% of VO2 max, while "high intensity" involved running 1,000 meter reps at 95-100% of VO2 max every other day. "Low volume" represented each runner's typical training distance, while during the "high volume" phases, the runners completed twice their normal mileage.
What did the study find?
MacDougall et al. found reductions in the ratio of "immune helper" cells to "immune suppressor" cells with increases in either the volume or intensity of training. Dr. MacDougall relates, "This ratio is an accepted marker of immune function, and a reduction indicates an increased susceptibility to infection." The runners' immune systems were depressed more by increasing the intensity of training than by increasing the volume.
Dr. MacDougall also found that the runners' immune systems adapted to the increased training intensity and volume during the 10 day training phases. This suggests that you may be most at risk of getting sick following that first hard training session, but that the immune system adjusts relatively quickly to the increased stress.
How long does it take for your immune system to recover?
In Dr. MacDougall's study, the subjects' immune helper/immune suppressor ratios were found to return to normal by the following day after a workout. This study did not focus on how many hours it takes for the immune system to recover, but Dr. MacDougall states, "Other studies have found this ratio to return to normal within 30-90 minutes after exercise."
There is evidence, however, that other elements of the immune system may take somewhat longer to recover. In a recent study conducted by Dr. David Nieman and colleagues at Loma Linda University, the immune systems of ten experienced marathoners were analyzed after they ran for 3 hours to exhaustion. This study found alterations in several types of immune cells during recovery. All of these changes, except for one, however, returned to normal within 21 hours post-run. These results suggest that changes to the immune system, even after an extremely strenuous 3-hour run, return to normal in less than a day.
What are the implications for your training?
The McMaster University 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. You should, therefore, take extra precautions when adding speedwork, races, or extra miles to your running program.
Jason Kajiura, the graduate student who worked with Dr. MacDougall, and who also coaches runners at the Hamilton Olympic Club, adds "You are most susceptible to infection at the end of a workout and for the first couple of hours afterwards, so you really shouldn't run intervals with a training partner who is sick. Don't go out for a beer after the workout with him or her either."
What are your chances of getting sick?
An earlier study by Dr. Nieman, involving 2,000 runners who competed in the Los Angeles marathon, indicates that our immune systems may indeed be suppressed enough by hard running to increase the risk of illness. This study found that runners who trained more than 60 miles per week were twice as likely to get sick in the last 2 months before the marathon as those who trained 20 miles per week.
This study also compared the rates of illness during the week after the marathon of individuals who trained for the marathon but did not run (let's call them wimps) versus those who ran the marathon. Two percent of the "wimps" got sick during the post-marathon week, while 13% of those who ran the marathon got sick. It certainly looks as though high mileage training or running the marathon can increase our chances of getting sick.
We should be cautious, however, in reading too much into the findings of these studies. Dr. E. Randy Eichner, Professor of Medicine at the University of Oklahoma, asserts that psychological factors often determine the impact of exercise on the immune system. In The Physician and Sportsmedicine, Dr. Eichner writes, "Whether exercise enhances immunity or impairs it may, in fact, depend on whether the exercise is a joy or a stress." A positive attitude towards your running may help keep you healthy.
Whether or not your immune system will be suppressed enough by a hard run or race to allow you to get sick depends on the idiosyncracies of your individual immune system, and your overall stress level. The McMaster University results suggest, however, that immediately after increasing the intensity or quantity of running is when we are most at risk.
(This column originally appeared in Running Times Magazine.)