Does exercise really help bones?
It has long been common wisdom that weight-bearing exercise helps improve bone health. That’s why virtually the entire medical establishment recommends such exercise for people of all ages, from youth to old age. So it’s no surprise that an article in the New York Times entitled “Exercise Is Not the Path to Strong Bones” generated lots of buzz, lots of traffic to the Times site, and lots of dismay among people who doggedly exercise at least in part to help keep their bones strong. It was written by Gina Kolata, an ace science reporter and “self-proclaimed exercise addict.”
The gist of her article was that exercise’s effect on bone is minuscule and “too small to be clinically significant.” Kolata tried to clarify some of her points in a follow-up article, “A Second Look at a ‘Misconception’ on Exercise and Bones,” which undoubtedly left many readers still bewildered about what to do. In particular, should middle-aged and older people who are worried about osteoporosis and bone fractures skip the weights and hang up their exercise shoes?
Here’s what Kolata got right and what she got wrong. We include some comments by noted bone researcher Dr. Bess Dawson-Hughes, director of the Bone Metabolism Laboratory at Tufts University, whom we asked about Kolata’s articles.
What Kolata got right
• Changes in bone mineral density (BMD) resulting from weight-bearing exercise are small in middle-aged and older adults. The most common way to assess bone is via BMD, measured by a special X-ray (DEXA). As seen in a study in the Journal of Bone and Mineral Metabolism in 2004, postmenopausal women with osteopenia or osteoporosis who undertook a walking program four hours a week for a year had just a 2 percent increase in BMD in the spine. That’s typical of what such studies have found.
Keep in mind that most gains in bone strength from exercise and diet occur during childhood and adolescence, when bones are still growing and developing. Bone density peaks in young adulthood and then starts to decline in middle age, usually earlier in women than in men. The trick is to slow this loss, or possibly even halt or reverse it somewhat.
• The decrease in fracture risk resulting from exercise may largely be the result of stronger muscles, rather than actually affecting the bones themselves. Exercise not only strengthens muscles but also improves balance and the ability to recover from a stumble, which can all reduce the risk of falls and resulting fractures. This is important, since with age comes a decline in both muscle strength and balance. A study in the European Review of Aging and Physical Activity in 2007 found that regular exercise can preserve proprioception (the sense of your body’s position in space) and reduce the risk of falls. According to Dr. Dawson-Hughes, “I don’t like the dismissive tone she [Kolata] has, so I would indicate that, in addition to the fall risk reduction, the BMD and bone quality improvements seen with exercise are expected to contribute to the skeletal benefits of exercise.”
• Not all weight-bearing exercise is equally good for bone density—casual walking, for instance, won’t do much. It generally takes high-impact weight-bearing as well as strength training for both the upper and lower body to gain measurable bone strength. (The benefit is site-specific, so that when you run, the bone growth occurs primarily in your legs; when you play tennis, more occurs in your dominant arm.) Brisk walking is good for aerobic fitness as well as muscle endurance and balance, but bones respond better to heavy loads, as when jumping or lifting weights.
What Kolata got wrong
• The small changes in BMD resulting from exercise are clinically significant. “Kolata assumed that a 1 to 1.5 percent difference in BMD between exercise and placebo groups over a year is all you get if you continue exercising,” says Dr. Dawson-Hughes. If people continue to exercise, however, gains are likely to accrue. And even small changes in BMD can mean a lot. For instance, a 2 percent increase in BMD would lead to an estimated 10 percent lower fracture risk. “For a lifestyle change, 10 percent lower risk is not nothing.” If the gains continue, you may end up with a 10 percent increase in BMD (more than what most osteoporosis drugs accomplish), which would result in an estimated 50 percent reduction in fracture risk, according to Dr. Dawson-Hughes.
The authors of a 2007 study in the journal Bone noted that small improvements in bone from exercise can lead to big gains in bone strength because it largely occurs in parts of the bone where the strain is greatest. And this is the case even when there’s no increase in BMD. It’s also important to note that while exercisers may boost their BMD only slightly, at least they are not losing more, which would otherwise be expected.
• Exercise improves bone quality, which Kolata just glossed over, calling this a “mysterious property.” Bone strength depends not just on bone quantity, as measured by BMD, but on bone quality and structure, including the microarchitecture of bone, size of the mineral crystals, and quality of collagen (connective tissue), none of which are seen by DEXA. There hasn’t been a lot of research on exercise and bone quality, but some insight is provided by the above-mentioned study in Bone, which looked at women (ages 35 to 40) who did high-impact exercise for a year. They had a significant increase in the circumference of the femur (thigh bone), with the most active women experiencing a small but significant improvement in measures of bone quality that can affect bone strength.
More recently, an interesting study in the Journal of Bone and Mineral Research in 2015 had men, ages 65 and older, hop on the same leg for two minutes every day for a year; the other leg served as the control. The result was a significant increase in bone mineral content in parts of the hip bone, including the thinnest area most likely to fracture in a fall—but only on the side of the hopping leg. According to the researchers, such localized bone changes could increase bone strength more than increases in BMD and may explain why people who exercise have a lower risk of fractures. Using BMD to assess bone health underestimates other changes in bone strength that occur with exercise.
In other words, you don’t have to build a large quantity of bone to improve the quality of bone and make it more resistant to fracture. “The arrangement of minerals in bone is hugely important,” says Dr. Dawson-Hughes. “Most of the measures of bone quality are fairly new. We don’t yet have much data from randomized trials in humans. But to claim no effect of exercise on bone quality or that we don’t know about bone quality is to be overly pessimistic about an area of research that’s active and exciting.”
Bottom line: Don’t be disillusioned about exercise as a way to help keep bones healthy. Just remember that it’s important to include some moderate-to-intense strength training and at least short bursts of high-impact activities like hopping or jumping jacks (for advice, see New Tricks for Old Bones). It’s vital to keep exercising, since bone loss will continue when you stop. And don’t forget other bone-health steps, such as consuming adequate calcium and vitamin D (the American Geriatrics Society recommends at least 800 IU of D a day for people at increased risk for falls) as well as lots of plant-based foods, which contain other bone-building nutrients. Don’t smoke and don’t drink more than a moderate amount of alcohol. Even if you follow every possible bone-friendly step, at age 70 your bones won’t be as strong as at age 20. But every little bit helps when it comes to bone health.