Exercise boosts your immune system, but too much exercise suppresses it. Everyone agrees on that.
But with peak cold and flu season barrelling toward us, the more pressing issue is how to find the sweet spot. A new technique that tests immune function in humans rather than in Petri dishes suggests that the duration of your workout may have a greater effect on your immune system than how hard you push – yet another argument in favour of including some high-intensity exercise in your workout routine.
"Our data challenge the concept that short, high intensity exercise decreases immunity," says Dr. Neil Walsh of Bangor University in Britain. Instead, "it appears that exercise lasting two hours or more decreases the immune response."
The basic relationship between physical activity and immune function is illustrated by two oft-repeated epidemiological observations. First is the fact that, in the general population, the more active you are, the less likely you are to get sick. For example, a 2011 study of 1,002 adults by Dr. David Nieman at Appalachian State University found that those who exercised five or more days a week were 43 per cent less likely to develop upper respiratory tract infections during the fall and winter than those who exercised once a week or less.
At the other extreme, elite athletes – and particularly endurance athletes – are more likely to get sick during periods of intense training or immediately after arduous competitions such as marathons. A forthcoming study of French elite swimmers in the journal Medicine & Science in Sports & Exercise, for example, found that the odds of illness rose by 50 to 70 per cent during periods of intense training. And, like the rest of us, the swimmers were significantly more likely to get sick during the winter months.
The more difficult question to answer is: What types of workouts are hardest on the immune system? While hundreds of studies have explored the links between exercise and immune function, nearly all use rodents or approximate immune function by measuring the abundance of certain markers in the blood or saliva. But the immune system is a co-ordinated network throughout the body that includes neural and hormonal responses, so testing the response of a few cells in a test tube doesn't give a reliable prediction of immune behaviour, Walsh says.
In a study published this month in Medicine & Science in Sports & Exercise, Walsh and his team administered a chemical called diphenylcyclopropenone (DPCP) through a patch on the lower back, 20 minutes after exercise bouts of various lengths and intensities. DPCP is an antigen that triggers the development of a brand new immune response; the strength of the response can be assessed by applying more DPCP to the skin four weeks later and measuring the resulting redness and skin thickening.
The skin test is a good proxy for overall immune response, Walsh explains: "We would expect those with a very low response to our skin patch test to be more susceptible to opportunistic infections and the like."
The researchers divided their subjects into four groups. One group did 30 minutes of treadmill running at a moderate pace; another did 30 minutes of more intense running at 80 per cent of peak oxygen uptake, "where you would struggle to hold a conversation," Walsh says; a third did 120 minutes of moderate running; and the fourth was a control group that did no exercise.
Contrary to earlier test-tube studies, 30 minutes of intense running had no effect on immune response. In contrast, 120 minutes of slower running did spark a significant reduction in immune response. One theory is that prolonged exercise triggers a gradual rise in blood stress hormones such as cortisol, which in turn temporarily suppresses immune function. That dip explains why runners are more likely to catch an infection in the week immediately following a marathon. The results are consistent with previous studies on exercise and immune function, according to Dr. Peter Darlington, a professor in Concordia University's department of exercise science who studies immunology.
"The way they measured the immune system in vivo was quite informative, as other studies have used less specific tests," he said in an e-mail.
If you do have a multihour workout planned, the best way to protect your immune system probably won't come out of a bottle, according to Dr. Michael Gleeson of Loughborough University in Britain: "There are so many of those things in the health-food shops that are claimed to boost immunity," he noted at a recent conference, "but most of the studies that have been done in athletes have shown that they don't really work."
Instead, studies have found that a more effective approach is to take in carbohydrate to minimize your energy deficit during intense exercise lasting longer than 90 minutes, which will reduce the immune-suppressing rise in stress hormones.
For most of us, the risks of overdoing it at the gym are dwarfed by the likelihood that our workouts will actually bolster immune function. But it you still have doubts, take a cue from Walsh's study – and push harder instead of longer.
Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com.