“Exercise Is Not the Path to Strong Bones,” according to the headline of a recent article in the New York Times, by Gina Kolata. In fact, the article continues, exercise “has little or no effect on bone strength.”
Allow me to disagree.
The problem, in my view, isn’t so much that the article provides the wrong answers; it’s that it asks the wrong question. In discussing the studies that have (indeed) shown increased bone density in response to training, Kolata writes: “Those studies failed to find anything more than a minuscule exercise effect—on the order of 1 per cent or less, which is too small to be clinically significant.”
It’s true: The study she refers to in this case, by a group at the University of Missouri, US, led by Pamela Hinton, found increases on the order of 1 per cent after 12 months of moderate resistance training or jump training two or three times a week in men.
Is 1 per cent significant? To understand the context, remember that once you finish puberty, the normal course of aging involves a steady and inexorable loss of bone. Studies have found that even young and middle-aged men are typically losing between 0.4 and 1.5 per cent every year in their bone density.
The stakes are even more dramatic when you consider a decade or a lifetime rather than a single year. That’s why, when treating and monitoring patients at risk of osteoporosis, simply maintaining (or even slowing the decline of) bone density from year to year can be considered a successful outcome, and is associated with lower fracture risk.
(The Times article mentions that studies of exercisers have found lower fracture risk in older adults, but attributes this to stronger muscles reducing the likelihood of falls. While there’s undoubtedly some truth to this, there are also studies of non-exercise interventions—drug therapies—where bone density stays stable or declines but fracture risk also declines.)
How do we know that the subtle effects of exercise add up over time? One way is to compare cohorts who do different types of exercise, as Hinton did in a 2009 study. When she compared habitual runners, cyclists, and resistance trainers, the cyclists had significantly lower bone density—as expected, because cycling doesn’t provide jarring impacts or build muscles that keep bones under tension, which are thought to be the two key elements of bone-stimulating exercise.
Another interesting point from this study is that total lean body mass (i.e., muscle) was a good predictor of bone density in the cyclists and resistance trainers, but not in the runners. So in general, having more muscle is good for your bones, because strong muscle applies tension. But that effect is swamped by the jarring impacts of activities like running: it seems to be more important to get jarring impacts than to have lots of muscle.
Getting back to the original point: Yes, it’s true that doing a few months of exercise isn’t going to produce a dramatic increase in bone strength. It’s also true that simply doing “weight-bearing” activity, like standing or walking, is unlikely to have any bone benefits for most people. Bone doesn’t respond to exercise with dramatic increases like we imagine muscle does.
Then again, for most people, neither does muscle. The benefits of exercise may be near-magical, but they’re often very subtle and accrue over long periods of time. If you’re at risk of osteoporosis, starting an exercise program won’t produce a rapid turnaround. But not exercising will pretty much guarantee that your bones get steadily weaker with each passing year.
