COLD SWEAT

An intense workout may make you more susceptible to flu

An intense workout may make you more susceptible to flu
Two recent experiments hit rather close to home. In the first,published in the journal Brain,Behavior,and Immunity,researchers divided mice into two groups. One rested comfortably in their cages. The other ran on little treadmills until they were exhausted. This continued for three days. The mice were then exposed to an influenza virus. After a few days,more of the mice whod exhausted themselves running came down with the flu than the control mice. They also had more severe symptoms.

In the second experiment,published in the same journal,scientists from the University of Illinois and other schools first infected laboratory mice with flu. One group then rested; a second group ran for a leisurely 20 or 30 minutes,an easy jog for a mouse; the third group ran for a taxing two and a half hours. Each group repeated this routine for three days,until they began to show flu symptoms. The flu bug used in this experiment is devastating to rodents,and more than half of the sedentary mice died. But only 12 per cent of the gently jogging mice passed away. Seventy per cent of the mice in the group that had run for hours died; those that survived were more debilitated than the control group.

The bulk of the new research reinforces what physiologists advanced some years agothat the risk both of catching a cold or the flu and of having a particularly severe form of the infection drop if you exercise moderately, says Mary P. Miles,PhD,an associate professor of exercise sciences at Montana State University and the author of an editorial about exercise and immunity published in the journal Exercise and Sport Sciences Review. But the risk both of catching an illness and of becoming especially sick when you do jump right back up if you exercise intensely or for a prolonged period of time,surpassing the risks among the sedentary. Researchers define intense workout as a workout or race of an hour or more during which your heart rate and respiration soar

Why exercise should affect either your susceptibility to catching an illness or how badly a particular bug affects you is still unclear. But it does appear that intense workouts and racing suppress the bodys immune response for a period of time immediately after youve finished exercising and that the longer the duration and the more intense the exercise,the longer the temporary period of immunosuppression lastsanything from a few hours to a few days has been suggested, says Nicolette Bishop,an associate professor of sport and exercise sciences at Loughborough University.

A telling new study,published in August in the Journal of Strength and Conditioning Research,looked at cellular markers of immune system activity in the saliva of 24 soccer players,before and after a strenuous,70-minute match. Before play,the saliva of most of the players showed normal levels of immunoglobulins,substances that help to fight off infection. Afterward,concentrations of saliva immunoglobulins in many of them had fallen dramatically.

If scientists arent sure yet why intense exercise temporarily depresses the immune system,however,they seem to be closer to understanding why,once youve caught a bug,intense exercise can make the symptoms and severity worse. In work at the University of Illinois,reported last month in the journal Exercise and Sport Sciences Review,some of the same scientists whod studied mice and flu looked at just what was going on inside the cells of the affected animals. They found that the leisurely jogging rodents showed signs of a very particular immune response to the flu. In general,and this is true in both mice and men,says Jeffrey A. Woods,a professor of kinesiology and community health at the University of Illinois and one of the scientists involved,viruses evoke an increase in what are called T1-type helper immune cells. These T1-helper cells induce inflammation and other changes in the body that represent a first line of defence against an invading virus. But if the inflammation continues for too long,it becomes counterproductive. The immune system needs,then,at some point to lessen the amount of T1-mediated inflammatory response,so that,in fighting the virus,it doesnt accidentally harm its own host. The immune system does this by gradually increasing the amount of another kind of immune cell,T2-helper cells,which produce mostly an anti-inflammatory immune response. Theyre water to the T1 fire. But the balance between the T1- and T2-helper cells must be exquisitely calibrated.

In the mice at the University of Illinois,moderate exercise subtly hastened the shift from a T1 response to a T2-style immune responsenot by much,but by just enough,apparently,to have a positive impact against the flu. Moderate exercise appears to suppress TH1 a little,increase TH2 a little, Woods says.
On the other hand,intense or prolonged exercise may suppress TH1 too much, he says. Long,hard runs may shut down that first line of defence before it has completed its work,which could lead,Woods says to increased susceptibility to viral infection.

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Moderate exercise,such as a leisurely jog may prop up your immune response and lessen the duration and severity of a mild infection,but be honest about your condition. If you have fever or body aches,stop daily exercise until you have recovered, Woods says. It is okay to exercise if you have a simple cold or congestionit may actually improve the way you feel. I would avoid heavy,prolonged exercise with a head cold,though, since it can unbalance that important T1 and T2-helper cell response.
Gretchen Reynolds,NYT