When is exercise bad for your health?

Physical activity is an essential component to good health. Indeed, government guidelines state that to obtain the most health benefits, adults need at least 150 minutes of moderate-intensity aerobic movement, such as brisk walking, or at least 75 minutes of vigorous-intensity exercise, such as running, each week. Adults also need to perform muscle strengthening moves, for example, by lifting weights or performing body-weight exercises, at least 2 days a week.

Unfortunately, only about one-quarter of US adults (24%) are reaping the benefits of meeting these minimum aerobic and muscle-strengthening standards, the CDC says. (For the latest on How exercise can make you a better doctor, read MDLinx.)

However, a growing research base suggests that more is not necessarily better for health, and that there is an upper limit beyond which exercising becomes detrimental.

Exercise-related glucose intolerance

You might want to rethink a plan of getting healthy by going gonzo at the gym: It turns out that there is an upper limit to the amount of intensive exercise that can be performed without disrupting metabolic homeostasis, according to a study published in Cell Metabolism. For this study, healthy participants were subjected to 4 weeks of progressively increasing training loads.

Following the week with the most extreme exercise load, researchers found a striking reduction in participants’ mitochondrial function, decreased glucose tolerance, and increased insulin secretion. This same type of mitochondrial dysfunction may play a role in the development of type 2 diabetes, researchers noted. The same researchers also assessed blood glucose profiles in elite endurance athletes who were otherwise healthy and found that they, too, had impaired glucose control when compared with a matched control group.

“From a health perspective, we do not advise against intensive exercise training, as former elite athletes have lower mortality rates and seem to live longer compared with the general population,” the authors concluded. “Nevertheless, both athletes and those looking to improve their health through exercise should carefully monitor the response to training, as too much exercise might have negative effects.”

Extreme exercise and heart disease

Over-exercising is harmful to your cardiovascular system, too, recent research suggests. For one study published in Mayo Clinical Proceedings, researchers followed nearly 3,200 young adults, all of whom were exercisers, for 25 years. Participants were sorted into three groups: those who exercised less than recommended physical activity guidelines (57%), those who met the guidelines (34%), and those who exceeded the guidelines by at least three times over (8%).

Researchers found those who exercised the highest amount were actually 27% more likely to develop coronary subclinical atherosclerosis by middle age than those who exercised less than the guidelines suggested. Further analysis by race showed that White over-exercisers were 80% more likely to develop this type of atherosclerosis when compared with similar Black participants.

Researchers examining the frequency of physical activity in 1.1 million healthy women came to similar conclusions about the risks of extreme exercising, according to a study published in Circulation. Researchers tracked the women’s hospital admissions and death records for an average of 9 years and found that those who described their activity as “moderate” had significantly fewer first coronary heart disease (CHD), cerebrovascular, or venous thromboembolic events (VTE) than women who were inactive did. But as for the women who reported “strenuous physical activity daily,” they had higher risk of all three coronary conditions, researchers noted.

Moderation may be best

It’s becoming more clear that long-term extreme daily exercise poses health risks. But what about the super trendy sprint interval training workout and its cousin, high-intensity interval training (HIIT), both of which involve intense short bursts of exertion? These are touted as a timesaving way to boost fitness, but evidence suggests more moderate, endurance-targeted workouts might be a better goal.

In one provocative new study, researchers compared men who did short, intense, interval training 3 days a week vs men performing longer, gentler workouts 5 days a week. They found that both groups improved their aerobic capacity and glucose tolerance, according to the report in Medicine & Science in Sports & Exercise. However, only the moderate exercisers lowered their blood pressure, improved their postprandial lipid tolerance, and reduced abdominal fat. These improvements in cardiovascular risk factors did not occur among the sprint-interval exercisers.

Using heart rate zones

Aiming for your target heart rate zone can help take the guesswork out of your fitness regimen and prevent overexertion. According to the American Heart Association, this number varies by age. But for the average 40-year-old person, the target zone ranges from 90 to 153 bpm, with an average maximum of 180 bpm.

Moderate activity involves maintaining your heart rate at about 50% to 70% of its maximum. You can monitor your pulse, use a fitness tracker, or use this low-tech gauge: If your activity is moderate, you'll be able to talk, but not sing. If you’re just starting back to an exercise routine, you’ll want to aim for the lower range of your target zone (50% of maximum heart rate) and gradually build up.

For high-intensity exercise, 70% to 85% of your maximum heart rate is the ideal zone. You will only be able to speak a few words. For more tips on fine-tuning your routine, see 7 healthy tips to reboot your exercise program on MDLinx.