Can Exercise Make You Sick?

Exercise is at the top of the list of healthy lifestyle habits, but sometimes a workout leaves you feeling worse than when you started. It's been known to cause headaches or an upset stomach. It can send you sprinting to the bathroom with vomiting or diarrhea and even leave you vulnerable to infection.

But none of this means you should skip your usual workout.

"We all know the benefits of exercise — a longer lifespan, more energy, reduced stress. Exercise has so many benefits when we do it right," says Sherri Sandel, DO, director of hospital medicine and medical education at Northern Westchester Hospital in Mount Kisco, New York, and an assistant professor at the Donald and Barbara Zucker School of Medicine at Hofstra/Northwell.

What you can do is take steps to exercise healthfully.

Endurance athletes — those who train and work out for long periods of time — commonly experience digestive woes like nausea, vomiting and diarrhea. While uncomfortable and potentially embarrassing, these issues don't often indicate a serious problem, according to research published in May 2014 in the journal Sports Medicine.

More likely, it means someone has had too much to eat or drink before working out, Dr. Sandel says. "When you do rigorous exercise, blood flow moves away from the stomach to the muscles that are being worked," she explains. And, without adequate blood flow, digestion slows down.

Sometimes, the reverse occurs. If your exercise session involves a lot of running or jumping, you may speed up your digestive system. And if you've had coffee or a sugary sports drink before working out, you may speed up your intestines even more, leading to diarrhea.

Dr. Sandel recommends not eating in the two hours leading up to a workout. And while you need to be hydrated, be careful not to drink too much water before or during your session. If you're working out for an hour or less, small sips of water when you're thirsty are OK. If you're exercising for a longer time, she says you need to drink something with electrolytes and suggests coconut water.

If you're an endurance athlete competing in a marathon or an Ironman event, for instance, don't eat high-fiber foods the day you're competing, avoid high-fructose foods and drinks and guard against dehydration, which can also cause digestive distress, the Sports Medicine study suggests.

Read more: 5 Tips to Avoid Exercise-Induced Nausea and Vomiting

Your digestive system isn't your only concern. Some people experience headaches during or after exercising. These are often called exertion headaches. Though they're usually not dangerous, it's important to call your doctor the first time you have any headache that feels severe or unusual.

A headache caused by exercise usually occurs on both sides of the head. It may feel as though your head is throbbing. Taking extra time to warm up and cool down may help prevent it. Fortunately this type of headache doesn't usually stick around long. Three to six months after they first start happening, exercise headaches usually disappear, according to the American Migraine Foundation.

If you have a very sudden, very intense headache, call 911 and get emergency help right away. This kind of headache may signal a serious health condition, according to the American Migraine Foundation.

Read more: Headaches the Day After Physical Activity

Most of the time, exercise is thought to help boost the immune system, Dr. Sandel says. But if you're getting sick or you've been injured, exercise may slow healing.

The science isn't conclusive yet, but it seems that if you participate in high-intensity activity on multiple days, you may dampen your immune response. This can leave you more vulnerable to infection, according to research published in Frontiers in Physiology in June 2016 and in May 2017 in the Journal of Applied Physiology.

The studies suggest that resting between exercise bouts may give the immune system a chance to recover. A massage might also help boost immune system recovery, according to the Journal of Applied Physiology study.

"Anything you do in the extreme, especially if you're not used to it, will take a toll on your body," Dr. Sandel says. "If you don't feel good doing something, stop."