Kidney stones, those hard deposits of minerals and acid salts, can cause excruciating pain as they pass through the urinary tract. These stones are no rare affliction—about one in 11 people will have a kidney stone at least once by age 70, and recurrences are common. Your risk of developing a kidney stone is increased if you have diabetes, gout or Crohn’s disease. But even if you are perfectly healthy, normal activities like eating meat, salt or sugar…not drinking enough fluids…or taking a calcium supplement can lead to a kidney stone.

You can lower that risk, though, by taking a very simple step—just get a particular amount of exercise. It doesn’t have to be very much! In fact, according to a new study, a regular evening stroll or some light gardening may suffice.

GET MET

For the study, researchers looked at an average of eight years’ worth of data on more than 84,000 women enrolled in the Women’s Health Initiative. (Attention, men—there’s no reason to think this study won’t be valuable for you, too.) None of the participants had a history of kidney stones when the study began. Periodically, participants completed questionnaires about their diets, physical activity and health—including whether they developed kidney stones, as 3% did.

In analyzing participants’ exercise habits, the researchers used a measurement called a metabolic equivalent for task (MET), which estimates the amount of energy expended in various activities. The formula on which MET is based has to do with the amount of oxygen consumed per kilogram of body weight per minute, which sounds complicated—but the basic idea is that it allows you to express energy expenditure in a way that is comparable among people of differing body weights and fitness levels.

One MET hour is the energy expended in an hour when you are at rest. An hour of reading, writing or using a computer is worth 1.5 MET hours…slow-paced walking and light chores like kitchen cleanup count as 2.5…bowling, yoga and light gardening net about a 3.0…golfing, moderately brisk walking and doing household repairs all merit a 3.8…baseball, volleyball and horseback riding are all about 5.0…slow jogging gets a 6.0…tennis and basketball earn an 8.0…and an hour of really intense sports, such as rugby or martial arts, is equal to 10 MET hours.

Encouraging findings: It turned out that you don’t need to be a hard-core athlete to lower your risk for kidney stones. After adjusting for other risk factors (age, diet, body mass index, etc.), the researchers discovered that, compared with sedentary participants, those who regularly engaged in a…

  • Small amount of physical activity, from 0.1 to 4.9 MET hours per week, were 14% less likely to develop kidney stones.
  • Moderately small amount of activity, from 5.0 to 9.9 MET hours per week, had a 22% reduction in risk.
  • Moderate amount of activity, from 10.0 to 19.9 MET hours per week, had a 31% reduction in risk. This does not require a big effort! It’s equal to about four hours of light gardening, three hours of walking at an average pace or two hours of slow jogging.

Interestingly, getting even more weekly exercise—20, 30 or more MET hours—did not provide any extra protection against kidney stones beyond that afforded by the moderate 10-to-19.9 MET level.

WHY A WORKOUT HELPS

There are several possible explanations for why physical activity might reduce kidney stone risk. People who exercise may drink more fluids, which dilutes urine and helps flush away tiny crystals in the kidneys before they can turn into stones. Working out also improves bone density, keeping more calcium in the bones instead of letting it lodge in the kidneys. Also, people who exercise generally sweat more, so salt (a component of kidney stones) leaves the body via the sweat instead of becoming concentrated in the urine. Exercising reduces cardiovascular risk factors (such as high blood pressure and high cholesterol), which also seem to have an impact on stone formation. And working out combats obesity, which itself is a significant risk factor for kidney stones, perhaps because it alters urinary pH and electrolytes.

This matters for men, too: Because this study focused only on women, more research is needed to determine how much exercise men need to do to lower their kidney stone risk. But exercise might actually benefit men even more, given that men are more than twice as likely as women to get kidney stones.

Let MET get you motivated to move: Click here to see how many METs your favorite physical activities account for and to get inspired to boost your weekly MET numbers. Your kidneys will thank you!