How Exercise Affects Your Immunity

So you're feeling a bit run-down. Do you power through your WOD in the name of strength, or sit it out so your body can rest? Good question.

Quick biology refresher: Your immune system is a complex network of cells and proteins that serves as your first and best line of defence against harmful viruses and bacteria. To strengthen it, you want to focus on wellness practices—in this case, exercise—that directly affect the network.

"Exercise, be it cardio or strength training, improves the circulation of important immune cells, which reduces your risk of infection", says David C. Nieman, DrPH, an American College of Sports Medicine fellow and biology professor at Appalachian State University's North Carolina Research Campus. He recently published a paper on the topic in "Exercise Immunology Review".

Movement, particularly 30 minutes of moderate to vigorous exercise, calls important immune cells (neutrophils, monocytes, natural killer cells, killer T cells) into action by pulling them off peripheral "bases" (the spleen, lymph nodes, bone marrow) and into the blood and lymph vessels where they circulate throughout the body at a higher rate than normal. This allows the cells to better monitor viruses and bacteria. "Activity also stimulates the function of macrophages [another important immune cell], improving viral defence", says Neiman.

You want all of those soldiers out and about, because they need to bump into a virus before they can respond to it and prepare a plan of attack, says Neiman. When you skip your daily walk or workout to stay in bed, or sit at a desk day after day, you're stuck with a skeleton crew. The big guns are never called to the scene, which can leave your immune system under-protected.

It's not just whether you move but also how you move that matters. Remember when we said exercise puts stress on the body? There's a sweet spot for activity: just enough to mobilise your army but not enough to stop it in its tracks (due to too much inflammation). Here's how to find it.

Anything that gets your circulation going will help call fighter cells to action, and "it really doesn't take much", says Nieman, who recommends taking a near-daily walk faster than window-shopping pace. How long should you go? Nieman says 30 to 75 minutes of consistent cardio is ideal. Frequent cardio not only delivers the instant circulation benefit, it can also strengthen your heart to keep that blood flowing well on the regular.

Running offers similar perks, so if you prefer to pick up the pace, go for it. Just know that going much harder or longer than you're used to could put your immune system at risk (there's that stress thing again). Case in point: A landmark 1987 study, still the largest of its kind, found that marathon runners were almost six times more likely to get a cold, flu or sore throat the week after the race than non-runners were.

If you're training for a distance event, make sure you stick to a training plan that gradually builds up your mileage and speed over time (that might be 12 weeks for a half-marathon and 20 for a full, depending on your fitness level). Virtual races might be safer than live ones because there's less exposure to germs, adds Nieman. And how you eat for your training runs and race can make a big difference too. Because glucose (a sugar in carbohydrates) is a major nutrient for immune cells, staying fuelled with the proper amount of carbs before, during and after endurance activity can help keep you healthy, he says. Even if you're going for less than 75 minutes, natural sugar sources, like fruit and/or whole grains, pre- and post-activity, can't hurt.

Don't think you need to get your heart rate up, up, up to get your immunity there too. After a strength session, your body releases white blood cells and other ruthless warriors to repair muscle tissue that stick around to bolster your defence, according to research also published in "Exercise Immunology Review".

"Aerobic and resistance exercise both recruit similar types of immune cells", says Nieman, though moderate cardio might be slightly more effective because more muscle mass is involved. But the nice thing about strength training compared to cardio is that it's difficult to go too hard in terms of immunity. "We studied the immune response after two hours of intense weightlifting and didn't see anything to suggest negative effects", says Nieman. That's probably because you aren't draining glycogen stores, thus avoiding a high-stress situation.

Of course, mixing up your workouts will ensure you don't overwork any one muscle group, reducing stress hormones and inflammation that can build up to work against your immune system. So your best bet is to do your heavy lifting on alternating days just three times a week and recover with a light hike, easy jog or yoga session in between.

If you're all about that WOD, there's good news. "Short bursts of high-intensity work don't have the same downside as continuous intense effort over a long period of time does", says Nieman. The built-in rest intervals, he says, seem to blunt the negative effects that long endurance activity might invite.

Exhibit A: Nieman conducted a study on tennis players—who sprinted, rested, sprinted, then rested again—and saw the same immunity-boosting benefits as those associated with brisk walking. Other classic HIIT exercises, such as squat jumps, jumping lunges and burpees, are fair game.

To make sure you're really getting those come-back-down intervals, try a 30-minute HIIT workout that alternates between 60 seconds of all-out effort and 75 seconds of active rest, suggests Nieman. Just avoid doing HIIT every day so your muscles, and any stress markers, have time to recover.

Nieman believes that a well-rounded fitness routine—complete with cardio, strength training and HIIT—is your best attack for, well, your best attack. That way you get the benefits from all three modes of training, which also support your overall health and wellness.

The one thing you don't want to do? Push yourself to train when you're feeling off or straight-up sick. "There's no data to support that exercise is therapeutic during an infection", says Nieman. So do the right thing for yourself and everyone else and take a rest day … or two.