Bone density is a measure of how solid your bones are. I'm sure we all know of elderly people who shattered a hip just by falling over. It's important to keep your bones strong!
The act of bones becoming thinner is called osteopenia. Bones are made of important minerals such as calcium, so for example if you never ate anything at all with calcium in it your bones would slowly deplete and get weaker over time. Eventually you get the point of osteoporosis where bones can break very easily.
Many scales on the market can now tell you your current bone mineral mass. This can help you keep track of your bone density. This is sometimes called bone mineral density, bone mass or BMD.
The average number for a male is that the bone is 15% of his weight. For a woman, bone should be about 12% of her weight.
So for example I am 130 pounds so 12% of my weight would be 15.6 pounds.
The scale isn't telling you your actual bone weight, though. It's telling you your bone mineral mass - how much mineral is currently in your bone. In my case my value is 6.3 pounds.
The important thing here is not to be "better" or "worse" than someone else - but to watch your OWN numbers for any decrease. You want to keep your value at least even by the following steps.
To build up bone density, it is important that you take in calcium with Vitamin D in proportion. The two chemicals have to work together to build up your bone mass.
Next, make sure you go for regular walks and do weight training. Both of these build up your bone density. Find a friend to go for walks with, or get an iPod and listen to your favorite music or even a good story. Whatever it takes to get you moving, take those steps. A set of small weights by the couch is great for commercial-time arm curls.
By keeping an eye on your bone mineral density and knowing immediately if it begins to change, you can proactively care for your bones and keep them strong. Make sure you go for those walks and do even basic strength exercises!