Carbs During Workouts May Fend Off Colds

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Until recently, many scientists believed that exercise reduces the body’s ability to fight off infections. Past studies had found, for example, that after workouts, especially strenuous ones, people had fewer infection-fighting white blood cells in their bloodstreams than before working out, suggesting that their immune response had been weakened.

But a timely new review of studies about exercise and immunity indicates that the interactions between exercise and immunity are far more intricate than scientists once suspected. Some types of workouts may hinder the immune response, according to the review, while others bolster it. Encouragingly, it also seems that a few simple precautions, including consuming carbohydrates during exhausting workouts, might help to keep our immune systems robust.

To learn more about the latest science concerning exercise and immunity, I contacted two of the authors of the new review, which was published last week in the Journal of Applied Physiology. Jonathan M. Peake is a lecturer in sports science at Queensland University of Technology in Australia, while his co-author Oliver Neubauer is a senior research fellow at the same university. What follows are edited excerpts from our conversation.

Q. Why would exercise affect the immune system in the first place?

Jonathan Peake: Exercise is a form of stress. The immune system responds to stress.

Q. What actually happens to the immune system during a workout?

Dr. Peake: White blood cell numbers typically increase in the blood during exercise, much as they would during an infection. Body temperature rises, and immune cells move from the lymph nodes, spleen, the walls of blood vessels, and the bone marrow into the bloodstream. The stimulus here is obviously not infectious, but occurs because of a rise in stress hormones such as cortisol, epinephrine, norepinephrine and growth hormone, which all can increase during exercise.

Q. And after exercise?

Dr. Peake: The number of white blood cells in the bloodstream, especially a type of cell that is particularly good at fighting infections known as natural killer cells, rapidly falls. People often have fewer natural killer cells in their blood after exercise than before they started. For many years, it was thought that exercise was destroying these cells and causing immunodepression.

Q. But it isn’t?

Dr. Peake: We now believe that the cells are not destroyed. Instead, it’s more likely they move out of the bloodstream and to other regions of the body, such as the lungs, gut, skin and mucous membranes of the respiratory system. This occurs in part because we can breathe in or ingest microorganisms that wind up in these tissues and require an immune response, but also because the stress of exercise causes physiological and biochemical changes within these tissues. As a result, signals go out from the tissues to the immune cells in the blood telling them that there is a potential threat to the body there, and the immune cells move to those tissues.

Q. So although we get more infection-fighting cells in our bloodstream during a workout, many of them wind up diverted afterward into other parts of the body? Can this development leave us particularly vulnerable to infections after a stressful workout?

Dr. Peake: Yes.

Q. What if the exercise is relatively moderate, like a brisk walk or easy jog, instead of a more intense workout?

Dr. Peake: Epidemiological evidence suggests that regular moderate exercise protects against upper respiratory illnesses, whereas regular intense exercise increases the risk of upper respiratory illnesses.

Q. Because moderate exercise likely results in less physiological stress throughout the body than more vigorous exercise?

Dr. Peake: That’s fair to say. The stress hormones that regulate activity of the immune system respond to intensity and duration. Generally speaking, the more strenuous the exercise, the longer it takes for the immune system to return to normal afterward.

Q. For people who train hard and would prefer not to repeatedly catch colds, is there any way to maintain a healthy immune response?

Oliver Neubauer: Ingesting carbohydrates during vigorous exercise may help, because carbohydrates maintain blood sugar levels. Having stable blood sugar levels reduces the body’s stress response, which in turn, moderates any undesirable mobilization of immune cells.

Q. How much carbohydrate? And when?

Dr. Neubauer: Most people only need carbohydrates during high-intensity or prolonged exercise that lasts for 90 minutes or more. For them, between 30 and 60 grams — which is 1 or 2 ounces — of carbohydrates per hour during exercise could minimize immune disturbances related to exercise. Consuming carbohydrates in the first few hours immediately after strenuous exercise also helps to restore immune function.

Q. Any additional advice for those of us who work out and wish to stay well?

Dr. Peake: Washing your hands often and avoiding contact with sick people will also help.