I love watching television and loathe going to the gym. I’m sure I’m not alone. I try to justify this preference by saying that I’m trying to help my microbes stay calm and avoid the perils that come with human physical exertion. But no one ever seems to buy it.
It’s true, though, that some microbes don’t much like it when we exercise. And if we exercise regularly, those species will jump ship and go in search of a lazier environment. This shift in microbial population occurs as a result of physiological stress. During exercise, we are wilfully depleting the body of oxygen and energy. The response to this is inflammation in the physical context—think “feel the burn”—and also the immunological one. When this happens, any unfriendly bacterium may end up having to deal with an unexpected attack by a group of immune cells called neutrophils.
Neutrophils eat and destroy uninvited visitors. When they come across a bacterium or virus that is unknown, or at least appears to be unhelpful, they ingest it and then break it down once inside the cell. Then they release toxic chemicals into the area to weaken any threatening entities that remain. These changes make the gut lining more permeable, allowing water to escape into the intestinal cavity. Our bowels become loose, and we may experience diarrhea, cramping, and nausea.
Our gut microbes are wary of neutrophils and take their own defensive actions when they encounter them. For some foes, the evasive manoeuvre is simple—catch a ride with the diarrhea and get out of town. For others, it means sending toxins into the body, which worsens the overall effect of the inflammation. The entire gastrointestinal tract can become a war zone in which the mightiest survive and the weakest fail.
Within a few hours after exercising, everything returns to normal. For our bodies, this type of short-term cycle is actually good, as it keeps our immune systems active and ready for any real battles to come. For the bacteria, continued exercise can lead to a more harmonious balance of friends, bystanders, and of course, foes. This in turn leads to a change in our dietary cravings, such that we lose that addiction to sugar and fats and instead seek out more nutritious sources of carbohydrates, as well as proteins for the energy they provide.
There is one caveat to the effects of exercise: if you choose not to change your diet and continue to seek out foods rich in simple sugars and high in fat, the shift in your gut population will be minimal at best and the lost benefits will discourage you from attending the next gym session. With each increase in unhealthy foods, the couch looks just that more inviting