Why are more people fracturing more bones more often? An article in Nutrition Reviews, volume 49 number 1 (January 1991) which addresses this question.

There is a definite increase in fractures in the population: both in absolute number and in percentages of hospital admissions. And it's not only in the elderly, a rapidly growing segment of the population, but in all age groups. Projections predict a 20% increase in fracture rates over the next two decades.

The answers to the first question-why so many fractures?-is threefold. Inactivity heads the list, with decreased nutrient intake and increased use of drugs close behind.

Healthy gravity stress, the result of activity and exercise, is most important for bone strength. People with sedentary jobs and/or who don't exercise run a higher risk of fracture than those who are active. The physical stress on the bones maintain and even build their strength. The best example of this problems is an astronaut. While in the weightless (gravity-free) environment of outer space, the astronaut loses bone mass very quickly. Measurements show extremely large amounts of bone loss in astronauts in space for relatively short times. One reason it is presently impossible to send humans to the planet Mars is due to the extreme loss of bone (and muscle) which would result in death before they could return.