An estimated ten million Americans have osteoporosis, an age-related disease in which the bones gradually become brittle and weak. Now, scientists are looking to animals for clues on how to combat this condition.
Getting down to bear bones. I'm Bob Hirshon and this is Science Update.
For most animals, inactivity is a main cause of osteoporosis—a disease that makes bones porous and brittle. But according to Seth Donahue, a biomedical engineer at Michigan Technological University, bears are the exception.
He says that during hibernation, bears lie immobile for five to seven months each year. Yet they don't suffer the same kind of bone loss that humans would if they did the same.
They don't lose bone strength, they don't lose the mineral content, and their porosity doesn't increase—it actually stays the same.
That's why Donahue and his colleagues are studying bears to see how they're able to do it. They analyzed blood and bone samples from bears, looking for chemical markers involved with bone loss and growth.
We found that as in humans and other animals during disuse, the bone resorption does increase in the bears; however, the unique thing about the bears is that the bone formation does not appear to be impaired by disuse. In other words, they can maintain normal levels of bone formation during hibernation.
Donahue says they're figuring out the role that hormones play in preserving the bears' bones, and whether the same mechanisms are at work in people. I'm Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
For many different tissues, our bodies follow a "use it or lose it" policy. Muscles are the most obvious example. If you exercise a lot, your body builds up muscle to support the work it's doing. But if you just sit around the couch all day, you'll lose muscle mass. Your body's point of view is simple: It's not going to waste energy building and maintaining huge biceps just to lift the remote control.
The same thing happens with bone. Normally, our bones are constantly cycling through a process of rebuilding and renewal. Old bone tissue is broken down and absorbed back into the body, while new bone material is constructed from protein and minerals like calcium.
Unless, of course, you don't use your bones enough. This often happens as we get older, and under other conditions (for example, astronauts on long space missions can suffer from bone problems caused by disuse). In addition, other biological factors can contribute to bone weakening as we age, including hormonal changes and a natural shift in the balance between absorbing old bone material and building up new material.
Studies show that after being inactive for a while, humans (and most other animals) need to be active for two or three times as long to make up for the bone loss. In other words, if you're laid up in bed for a week, you need to be active for two or three weeks before your bones bounce back. But bears, which sleep for half the year, don't have that kind of time. So they've developed strategies for keeping their bones in shape even during hibernation.
What strategies? It's too early to say for sure, but Dr. Donahue thinks that hormones that regulate the body's levels of the mineral calcium may be the key. These hormones may behave differently in bears than in humans, and seem also to have slightly different structures. Hormone therapy is already a promising treatment for human osteoporosis, and it's possible that studying bear hormones could make it work a lot better.
Now try and answer these questions:
- What is osteoporosis? Why does inactivity contribute to it?
- Why do bears need to maintain their bone structure differently than humans or other animals?
- How might understanding how bears keep their bone structure help human patients? Be specific.
- What are some obstacles that might stand in the way of applying this knowledge to humans?
The National Osteoporosis Foundation has more information about osteoporosis and those affected by it.
Nature's Best: The Human Body, an anatomy site for high-school students, includes a section on the skeletal system, which covers specific bones as well as bone structure and composition.