It’s the time of year when goals are made and determination is renewed. We all hit the gym excited and overzealous to make this the best fitness year ever. It’s also wintertime, though; the time of year when people spread germs, get sick, run fevers, and lose progress or motivation to achieve their fitness goals. There are numerous factors that go into how likely you are to get sick, your fitness and exercise being a couple of them, and a bunch of super sciencey physiological things, too. In an effort to help you achieve your goals with fewer bumps along the way, it helps to have some understanding of how training affects your immune system and what you can do to avoid getting sick this winter.
The immune system is a very complicated matter and studying the effects of exercise with regards to infection has proven to be a difficult task. Many of the studies to date present contradictory results, and changes in particular measures don’t always correlate with risk of infections as nicely as we might like. But we can start by looking at some of the markers of immune stress that are commonly measured and their roles in resisting infections and illness. There are several types of immunological agents or markers that are commonly investigated with regards to exercise, but we’ll focus on leukocytes, cytokines, and immunoglobulins.
White Blood Cells
Leukocytes actually refer to white blood cells, which are then differentiated into a variety of subcategories based on the function of the cell in defending the body from outside pathogens and germs. Natural killer cells are one such subcategory of leukocytes, which act as one of the first lines of defense for the body. They destroy other infected cells without the need for specific antibodies; sort of like a rogue agent that gets to shoot first and ask questions later. Strenuous resistance training has been shown to result in an immediate increase in the number of white blood cells circulating in the blood stream (including NK cells), but during recovery, these white blood cell counts drop below baseline levels measured prior to training. [3,5,7]. This effect has been shown to last for hours after exercise. The resulting decrease in Dirty Harry-esque immune cells could potentially be an increased risk factor for infection after demanding training days.
Cytokines are a type of proteins that act as signaling molecules in the body. There are many types and these proteins play a big role in signaling both pro-inflammatory and anti-inflammatory responses. Recovery and adaptations from resistance training as well as general immune function both rely on these types of signaling molecules to mediate the response from various types of cells and organs within our body. Whether its a response to mediate a cleanup of muscle damage and activate protein synthesis pathways or a signal for your immune system to destroy cells infected by virus, cytokines work to flip the switch for these types of cell functions and pathways. After resistance training, though, circulation of cytokines signaling both types of responses are increased . Whether or not this interferes or enhances the immune response following exercise remains to be seen, because it is unclear if the increased levels of cytokines due to exercise would enhance a response by the immune system or compete with it to enable certain signaling pathways and actions.
Finally, immunoglobulins are antibodies that label foreign particles and germs by binding with them. This allows for certain types of leukocytes to identify foreign and potentially infectious particles within the body and destroy them appropriately. Immunoglobulin A or IgA is an immunoglobulin found in saliva that is commonly measured to assess immune function and stress. It has been shown that salivary IgA levels are also reduced after prolonged bouts of resistance training lasting two hours.  This reduced level of salivary IgA potentially makes it more difficult for the body to label and identify foreign pathogens that enter the body, which could be yet another increased risk for infection.
Effects Of Training On The Immune System
In addition to studies investigating the effects of training on various components of the immune system, epidemiological studies have shown that runners engaging in high mileage training or races are at higher risk for upper respiratory infections.  A more recent study however, has shown that increased rates of illness could potentially be resultant of a lack of adequate recovery from prior illnesses before engaging in high intensity and duration running again, i.e. competing in a marathon.  No studies have looked at incidence of infection among predominantly strength-based athletes. However, studies examining the effects of aerobic exercise and the effects of resistance training have resulted in similar changes to leukocytes and immunoglobulins. It is reasonable to expect similar risks to infection so long as training is sufficiently strenuous in terms of volume and intensity.
Training status also plays a role in how exercise affects the immune system. As with DOMS or muscle damage, it seems that immune system factors are also subject to the repeated bout effect. When researchers have subjects repeat the same resistance training protocols and leukocytes are examined a second time, the effects of exercise on leukocyte concentrations are attenuated.  The body adapts so that the stress of exercise is reduced after repeated bouts. Seasoned lifters may not experience as much increase in risk of infection as noobs unless they are training to greater absolute volumes and intensities than they are accustom to.
Avoiding Sickness When Training
All of this information ultimately leads to some very common sense interventions to be taken during this season of resolution chasing and germ swapping. If you are new to resistance training and exercise, by all means start easy and try to focus on learning based strength gains. Most initial adaptations will be neural in nature, so there is no need to risk illness by crushing the body. If you are an experienced lifter, be sure to take precautions after strenuous training days. Limit your exposure to the elements when you are still sweaty, and make sure to get plenty of sleep, food and recovery time in between training sessions. And remember, gyms are veritable petri dishes for the recent edition of the cold or flu. Wash your hands.
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