With the peak of Flu Season right around the corner and so much media attention being paid to whether or not the flu shot is for everyone or if there is even enough, it is a prudent time to consider prevention techniques and examine potential risk. In the fit population, exercise is one factor (with varying reports) that may impact one’s risk of acquiring the seasonal flu. A number of studies have been conducted on both mice and humans that examine the effects of moderate and high intensity exercise on the susceptibility of catching the flu as well as the severity of the flu-like symptoms one experiences once they have the flu.

A study conducted by scientists from the University of Illinois published in the journal Brain, Behavior, and Immunity examined mice that were injected with a very harmful strain of the flu virus. The mice were divided into 3 groups: one group remained sedentary, one group ran leisurely for 20-30 minutes (very easy for a mouse), and one group ran for a strenuous two and half hours. This routine was repeated for three days until flu-like symptoms developed. Due to the devastating nature of this flu strain, over 50% of the mice in the sedentary group died. Interestingly, of the mice who exercised at a low intensity (leisurely run for 20-30 minutes) only 12% died, while the high intensity group had a 70% mortality rate. This research seems to support a theory from physiologists that has developed over the past couple of years that a “J-shaped curve” exists between exercise and immunity. According to Mary P. Miles, PhD, an associate professor of exercise science at Montana State University and lead author of a study examining exercise and immunity published in Exercise and Sport Sciences Review, the risk of catching a cold or the flu and the potential severity of the illness “drop if you exercise moderately.” In addition, the risk of catching a cold or flu and the severity of the illness “jump right back up” when exercise is performed at a high intensity.

Another study from the University of Illinois was published in last month’s Exercise and Sport Science Review examined what occurred at the cellular level in both mice and men during exercise as it relates to immunity. According to one of the scientists involved with the study, Jeffrey A. Woods, a professor of Kinesiology and Community Health at the University of Illinois, “viruses in the body result in an increase of T1-type helper immune cells which induce inflammation and therefore are the body’s first line of defense against illness.” As the amount of T1-type helper immune cells increase and the inflammation response increases, the body must do something to counter-balance the response to avoid inflammatory detriment. Therefore the body increases production of T2-type helper cells which dampen “or balance” the inflammatory response. Professor Woods explains that “moderate exercise appears to suppress TH1 a little, increase TH2 a little.” However, Woods also points out that higher intensity or prolonged exercise “may suppress TH1 too much” thereby shutting down the body’s first line of defense. This helps explain why the mice that exercised at a moderate intensity had a lower mortality rate than both the sedentary mice and the mice that exercised at a high intensity for a longer duration. It has long been known that high intensity training leads to physiological strain that can surpass recovery capabilities. This is likely and potentially worsened when additive to an invasive species.

The take home message from this research for Personal Trainers during this time of year is to be prudent with exercise prescriptions for clients who may be feeling the first symptoms of the flu or a cold. In general, moderate exercise such as a leisurely jog or walk may boost the immune system’s response and help to minimize risk of contracting the flu as well as the severity of the symptoms if the flu or a cold is contracted. However, if a client has acquired the flu, it is advisable to cease daily exercise until the individual is fully recovered. Premature return or “sweating out” the cold through training can prolong the illness and reduce immune function leading to increased risk for more significant symptoms or worse.