Why Weight-Training Is a Must for Your Bones
There are a number of vitamins and minerals that are essential for strong and healthy bones. One of these, of course, is vitamin D, produced by your skin when it's exposed to sunlight. However none of these co-factors will result in optimal bone density unless you also engage in a regimen of weight lifting exercise.

You might have heard that when astronauts return to earth, they often experience bone problems. Problem is, bones are living tissues, and the materials that make them up are constantly being broken down and then replaced. Without weight or stress on the astronauts' bones, there is no stimulation to replace the material being broken down.

This is why weight-training exercise is so important. Applying force along the axis of a bone stimulates its growth.

from Anti-Aging Fitness Program