To build bone, you have to beat it up.
Your bones are nothing like the frame of a building. If a skyscraper's steel skeleton is shaken by an earthquake, it weakens. But shocks to bone only make it stronger. Bone is living tissue, and it responds to your activities. Mechanical stress -- the impact of your feet pounding pavement, the weight of a barbell, or the shock that travels up your arm when you whack a baseball -- creates microscopic fractures. Your bone not only repairs the tiny fractures, but it also responds by building more bone on top of them.
Doctors have known this for over a century, ever since a German surgeon named Julius Wolff proposed the idea, which became known as Wolff's Law.
Most of us, however, are ignorant of how our bones behave. "I don't think this information has really hit people," says Nicholas DiNubile, an orthopedic surgeon in suburban Philadelphia. He consults for the Pennsylvania Ballet and the 76ers basketball team about their bones.
If you hope to live to a ripe old age and be active during your golden years, you must pay heed to Wolff's Law.
This is especially true for women. After menopause, women start to lose a lot of bone. The end result can be osteoporosis: bones so brittle that even the stress of ordinary activities can snap them. Men aren't immune, either. The rapid bone loss that leads to osteoporosis starts about 10 years later in men -- around age 60.
DiNubile says it is crucial to build up as much bone as you can during the first 20-30 years of your life, so that when you reach the age when bone loss accelerates, the effect won't be devastating. He compares it to investing in the stock market. If you have $1 million and you lose 90% of it in a market crash, you're still left with more than most people earn in a year. If you've only invested $10,000, what's left probably couldn't cover a month's rent.
To build healthy bones, you need the raw material calcium. Eating foods rich in calcium is a good start, but you also have to make sure your bones can use it. You may get enough calcium in your diet, but you rob your bones of it if you drink lots of soda. The phosphorus in soft drinks inhibits calcium absorption.
Then you have to punish your skeleton a bit. Low-impact activities such as walking, swimming, and cycling are good for your heart and muscle tone, but they don't do much for your bones. High-impact activities such as running and weightlifting build bone. "I think strength training is the key here," DiNubile says.
Moderation is important, too. "It's exercise as medicine," DiNubile says. All medicines must be taken at the right dose. Too little has no effect; too much causes awful side effects.
Women who overdo it may develop a condition called amenorrhea, which is triggered when overall body fat drops below 15%-17%. This is basically exercise-induced menopause. An amenorrheic woman stops menstruating and her estrogen levels take a dive. With low estrogen levels, bone loss speeds up, just like it does in post-menopausal women. Many female athletes, DiNubile says, are pleased with their lean, cover-girl bodies, "Yet they're not healthy. They've got disease.
"You need to train smart," he says. You should maintain a healthy weight -- not too plump, but not too hard -- and allow your bone enough time between workouts to heal and grow.
Clinton Rubin, PhD, a scientist at the State University of New York in Stony Brook, thinks he may have found another way to build bone: vibration. His experiments have shown that gentle, high-frequency vibrations greatly increase bone growth. In a recent study, he had sheep stand on a vibrating platform for 20 minutes, five days a week, over the course of one year. The density and volume of their leg bones increased by more than 30%.
Rubin's research is funded by NASA's National Space Biomedical Research Institute. NASA is interested because bone loss is a bane to astronauts. According to Rubin, we lose about 2% of our bone per decade here on Earth. Astronauts lose about 2% of their bone per month in space. Under no stress from gravity, bone dissolves. "It's doing exactly what it thinks it should do," Rubin says. That is, a weightless body doesn't really need bones. It's responding to the environment.
At present, both American astronauts and Russian cosmonauts try to fight bone loss with intense exercise programs. They spend four hours a day doing high-impact, bone-building workouts, Rubin says. Still, the bone loss doesn't stop.
Rubin thinks a healthy skeleton depends on the body's natural vibrations, in addition to the strain placed on bone by exercise. Even when sitting upright or standing still, your muscles are working to maintain your posture. The muscles vibrate at 10-50 hertz, and Rubin thinks these subtle vibrations stimulate bone growth. In space, a human body doesn't strain against gravity to hold its posture. The muscles are relaxed, and bone is not constantly subjected to muscular vibrations.
If Rubin is right, a vibrating platform could remove one more obstacle to sending astronauts on a mission to Mars. What's more, vibration might become a first-line treatment for osteoporosis, as well as a way to prevent it. Better treatments for osteoporosis are sorely needed. DiNubile says he worries about what will happen when today's young women go through "the change" some 40 years from now. "They're not even close on calcium intake," he says. What's more, teens tend be sedentary and guzzle massive quantities of soft drinks. All things considered, it looks like an epidemic of shattered hips may plague us mid-century.
A lot of hope was pinned on estrogen replacement therapy for postmenopausal women, but now doctors are finding out that it's unsafe. "They will not have that protector of bones," DiNubile says.
Rubin says he thinks vibration therapy would be the perfect answer -- no drugs, no side effects. "It basically relies on the normal physiology of the skeleton," he says. "What could be better?"