News flash: Exercise is good for you. But is there such a thing as too much exercise?

That’s a question raised by the exponential growth in ultra-endurance athletics, such as ultramarathon running and extra-long triathlons. According to one report, worldwide participation in ultrarunning increased almost 1700% from the late 1990s to 2020.

Needless to say, you have to be super fit to run or bike 100 miles. But there are some health risks associated with this kind of extreme exercise.

Read: Could Running Be Good for Your Knees?

When a body is made to endure hours of high-intensity exercise, there are many ways the musculoskeletal system can start to break down.

Orthopedic surgeon Robert Klapper, MD, says ultrarunners face many types of injuries: shin splints, stress fractures, sprains, tendonitis and tendinosis, muscle and tendon tears, ligament tears, plantar fasciitis and, over time, osteoarthritis (OA), particularly in the knees and hips.

With the exception of OA, these injuries are usually short-term and will heal, given time and some physical therapy. 

OA occurs when cartilage, which acts as a shock absorber between bones, gets damaged or worn away, allowing bone-on-bone contact. There is some evidence recreational runners are less likely to develop OA than the general population. But for ultrarunners, the incidence of OA is higher. 

“Doing the same thing over and over, you’re exposing the tendons and the muscles and ligaments to just the same angles and the same stress,” said Klapper. “It’s OK to run, but to stay healthy and strong, you need to mix it up with some cross training.”

For swimmers, Klapper says the weak spot is the shoulder. Depending on the study, anywhere from 27% to 91% of competitive swimmers report having had shoulder injuries. Overuse—mile after mile of the same freestyle windmill motion—can lead to so-called “swimmer’s shoulder”: anything from mild, short-term inflammation to tears in the rotator cuff that can lead to surgery and severe damage.

For cyclists, sitting in the saddle for hours at a time can be a pain in the butt (literally) and lead to stiffness in the neck, back and hips. A little stretching and maybe a better-fitting seat can usually take care of those issues. 

For some riders, however, knee issues can develop from prolonged stress on the tendons connecting to the quadriceps, the big thigh muscle that does so much of the work for a cyclist. 

In rare instances, Klapper says ultra cyclists can damage something called a plica: a malformation in the membrane that protects the knee joint. According to autopsy results, as much as half the population has some form of plica. When it’s injured, it can cause significant pain, but because it’s rare, it’s often misdiagnosed. It can usually be treated with physical therapy, but sometimes requires surgery.

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Cardiovascular exercise—exercise that makes your heart and lungs work harder than usual—is, in almost all cases, good for heart health. The idea that endurance athletics leads to heart attacks or cardiac arrest is not born out by the facts, said cardiologist Sumeet Chugh, MD, associate director of the Smidt Heart Institute, director of the Division of Clinical Electrophysiology and director of the Center for Cardiac Arrest Prevention in the Department of Cardiology at Cedars-Sinai.

In the very rare case that an athlete suffers cardiac arrest during a race, explained Chugh, “The culprit is usually a preexisting heart condition and not the endurance exercise itself.

“The heart adapts to the extra work required of it by these ultra-distance athletes,” he said. “It’s called ‘athlete’s heart’: the walls of the left ventricle might thicken, the right ventricle might get a little bigger.” 

These adaptations are mostly positive, according to Chugh.

“But exercise at the very upper limits of endurance,” he said, “does increase the likelihood of getting atrial fibrillation (AFib), which results in symptoms like irregular heartbeat, palpitations, chest pain and shortness of breath. More importantly, it can result in a higher risk of stroke.”

Read: Why Do Women Gain More Benefit From Exercise Than Men?

Pulmonologist Zab Mosenifar, MD, senior consultant in the Cedars-Sinai Department of Medicine, has run more than 100 marathons. At age 75 he continues to run at least 5 miles every day in the hills above his home. 

Mosenifar points out all of us inhale particles in the air. When we exercise, we inhale a lot more air, and therefore, a lot more particles, particularly in a place like smoggy Los Angeles.

“At rest, I’m inhaling maybe 7 or 8 liters of air every minute,” he explained. “But with extreme exercise, that can go up to 140 to 150 liters. Then you’re sucking all sorts of things from the air.”

Some of those particles can become irritants, which may cause inflammation in the airway and can lead to exercise-induced asthma or bronchospasms. According to Mosenifar, most of these side effects are thought to be temporary and not to cause permanent lung damage.

However, research shows chronic, long-term exposure to fine particulates is associated with lung and heart disease, including cancer, and is especially problematic for people with asthma and chronic obstructive pulmonary disease (COPD). So, it may be best not to exercise outdoors when the air pollution index is high.

Mosenifar scoffed at the notion that exercise can strengthen lungs. 

“Lungs don’t get any stronger,” he said. 

He also pointed out that, for an endurance athlete, the extra blood needed by the legs, respiratory muscles and heart means there is less available for the gastrointestinal (GI) system. As a result, many ultra athletes experience forms of GI distress, including GI bleed.

“I’m obviously an incredible advocate of running,” said Mosenifar, “but I’m also comfortable saying that running at high intensity for a long time probably is not a good idea.”

Read: A Daily Dose of Exercise as You Age

Iron deficiency is the most common mineral deficiency in the world, but it’s more prevalent in endurance athletes, particularly women. Studies have found that 15% to 35% of female endurance athletes are iron-deficient, versus 3% to 11% of male athletes.

Endurance athletes lose iron in a number of ways, including sweat (the average ultra athlete sweats 0.5-2.0 liters every hour for 6-24+ hours of racing.) Runners can also lose iron through something called foot-strike hemolysis: the destruction of red blood cells in their feet as they repeatedly hit the ground. 

At the same time, endurance athletes need more iron than sedentary folks for their bone marrow to create the red blood cells that carry oxygen to their working muscles.

Iron deficiency causes athletes to feel fatigued. They lose energy, strength, concentration, and the ability to recover quickly from workouts.

“Blood work is the easiest way to test for iron levels,” said clinical dietitian Saori Yen. “But athletes’ iron levels can be falsely elevated in tests due to inflammation caused by hard workouts. So, the recommendation for athletes is to have blood drawn in the morning in a rested and hydrated state after an overnight fast.”

An iron-rich diet can replenish iron stores, she said. Red meat, liver, shellfish, lentils and green leafy vegetables are all high in iron. Iron supplements are available, but they can have some unpleasant GI side effects. 

For all current and prospective endurance athletes out there: Know the risks and listen to your bodies.

“Don’t mask what your body is telling you with analgesics,” said Klapper. “If it hurts, you need to stop and do something different.”