Every so often, after a marathon, headlines will pop up about a marathon runner collapsing and dying during or shortly after the race. It’s rare, but it happens: A 2025 study found records of 176 cardiac arrests during long-distance running events in the U.S. out of about 29 million people who finished the races. 

Deaths like these raise the question of whether such an intense physical feat as a marathon can possibly be dangerous, or at least unhealthy, for the heart. We asked scientists who study marathoners about long-distance races and heart health, plus what runners can do to keep themselves safe.

Is all that running actually good for you?

Experts are pretty unequivocal: Yes. “Exercise is good for the heart,” says David Oxborough, professor of echocardiography and cardiovascular physiology at Liverpool John Moores University in the U.K. “We know that regular moderate-intensity exercise improves your cardiovascular risk, it reduces mortality, and it has real positive benefits.”

Of course, training for a marathon—which can take months or years of steady increases in endurance—is different from the act of running the race itself, says Daniel Muniz, senior lecturer in exercise physiology at the University of Hertfordshire in the U.K. “Running a marathon is a big physiological stress. You're going to hurt after…and if you look at markers in your physiology, everything is going to be all over the place.” Markers of muscle damage and heart damage are elevated, at least for a little while. (Oxborough, who has studied some of these elevated markers, says that they come back down swiftly and don’t seem to be linked to long-term damage.)

But the consistent training that running a marathon requires carries positive effects for physical and mental health, Muniz continues. “The benefits far outweigh the negatives. I think that's very fair to say.” 

Why do people die running marathons, then?

Dying during a marathon is extremely uncommon, says Oxborough. What’s more, it doesn’t seem random. People who die during endurance running events tend to fall into two categories, often identified after the fact: either younger people who have undiagnosed genetic heart problems, or middle-aged men who have heart disease without any symptoms.  

“If you've got underlying disease, then there is a risk of sudden cardiac death. If you're under the age of 35, that is usually caused by an inherited cardiac disease, such as cardiomyopathy. As you go over 35, the main cause is more acquired disease, like coronary artery disease,” he says.

How can you find out if your heart is healthy enough for a marathon? And should you?

“You should, absolutely,” says Oxborough. “We do pre-participation cardiac screening for young people in competitive sport. We have charities in the U.K. It's now becoming much more accepted in the U.S., and it wasn't before. This is a simple 12-lead ECG and a questionnaire to young people.” 

That’s enough to start the process of figuring out whether people might have an inherited disease, the experts say. If they do, it doesn’t mean they have to stop training. It just might shape how they decide to train and manage their risk.

What about older people?

The problem with coronary artery disease is that many people have no symptoms in the early stages. 

If you’re thinking of running a marathon and you’re over 35 or 40, here’s what Oxborough recommends: “Go speak to your GP. Have a 12-lead ECG done. Have your blood done. Make sure your cholesterol levels are normal. Have your blood pressure taken, and if any of them show any sign that they may be a higher risk, then we recommend an exercise stress test. And if you're very lucky, if you're able to access it, a CT coronary angiogram,” he says. These are tests that will help establish your baseline and what is likely to be happening in your arteries before you really ramp up training.  

Not all of these are required, but “a baseline test of risk is really important, and that you can do at your GP,” he says.

Can anyone run a marathon?

In principle, yes. “I am of the opinion that most people with training should be able to run a marathon. The important bit there is: with appropriate training for a number of months or or perhaps a year,” says Muniz. 

This is not a physical challenge where you can go out and achieve it tomorrow without practice. “But if you start training and you start running for 20 minutes, or run and walk, and then transition to run for 20 minutes, 30 minutes, 40 minutes, and then you do that for a number of months, I think most people should be able to eventually run a marathon,” he says.

Hepatitis B can be a silent killer. It can pass from mother to child during birth and can spread through contact with infected blood or semen. Among babies infected before they turn 1, up to 90% develop a chronic, potentially lifelong infection, for which there are treatments but no cure. People may not even know they have the virus until their liver fails or they develop cancer, years later. Early-in-life hepatitis B infections, left untreated, lead to a 1 in 4 lifetime chance of developing liver cancer. 

In the U.S., hepatitis B vaccination became routinely recommended for all infants in 1991; later guidance specified that the first dose should happen within 24 hours of birth. However, in Dec. 2025, the U.S. Centers for Disease Control and Prevention (CDC) under the Trump Administration moved to cease recommending this practice for babies born to mothers who test negative, instead advising a shared decision-making model where parents and clinicians decide together what is medically appropriate: a dose shortly after birth, or a first dose "no earlier than 2 months of age." (For babies born to mothers who test positive or whose status is unknown, the CDC continues to recommend a dose shortly after birth.)

A new study, published today in Annals of Internal Medicine, uses computer modeling to simulate the hepatitis B infection status of an entire year’s worth of children born in the U.S. under the new recommendations over their first 20 years of life. The researchers find that if only about 26% of children are vaccinated—an estimate of what would occur under shared decision-making—chronic hepatitis B cases would rise by 225%.

When should kids get the first dose of hepatitis B vaccine? 

The American Association of Pediatrics continues to recommend vaccination at birth against hepatitis B for all infants, and 15 states have sued the Trump Administration over vaccination schedule changes. 

“It's been challenged in many states and by professional societies, but we wanted to add some evidence to the conversation about what the potential impact of those changes could be,” says Dr. Amir Mohareb, an infectious-disease researcher at Harvard Medical School and Massachusetts General Hospital who is an author of the new study. 

The simulation modeled how many of 3.6 million children, born the same year in the U.S., would develop hepatitis B under a number of scenarios, including if the former CDC recommendations continued to be followed, if the initial dose (also called a “birth dose”) was delayed, and if a shared decision-making scenario was employed. 

The model suggests that if the first dose is delayed until 2 months old, and only 75% of infants get it, then there would be a 19% increase in chronic cases compared to if every child received their first dose at birth. With a 26% vaccination rate at 2 months, the number climbed to 225%. 

This study modeled only at a single year of births, notes Mahoreb. “Imagine that happening year after year after year after year. That could quickly become thousands and thousands of new infections, and even a small number of new infections could result in lifelong chronic infection that could be detrimental to families, lead to more cases of cancer, and [become] costly to the health care system,” Mohareb says.

Two other recent modeling studies have looked at the impacts of changing hepatitis B vaccine recommendations. One found that if vaccination no longer occurred at birth, a substantial increase in screening pregnant women for the virus would be necessary to keep infection numbers in infants down. Another, looking at the economic ramifications, found that even if every child eventually received the vaccine, delaying the first dose by 2 months yielded an additional $16.4 million in health care spending on new infections.

“All three of these studies complement each other, in a good way, and provide some modicum of evidence so that we don't need to risk doing a clinical trial or testing this on people,” Mahoreb says. “We have done those experiments.” Decades ago, he continues, clinical trials already proved the efficacy of the existing approach, “so that we would have the benefit now of advancing medical progress and not needing to reopen that question.”