Like skin, bone renews itself constantly and old bone is replaced by new. This remodelling and renewing is triggered by factors such as exercise, inactivity, diet, hormones and even the Earth's gravity.
Within limits, the strength and shape of our bones is determined by the stresses they undergo. For example someone who carries heavy loads or exercises regularly will have thicker and stronger bones.
Bones are living tissues and are continuously growing and being replaced with new bone. This new bone being laid down more in places where the bones are under the most stress such as the legs and spine of a runner.
At any given time 5% of your total bone mass in the body is being remodelled. By the age of 20, the average young adult will acquire roughly 98% of his or her total bone mass. This percentage is crucial as at this age it will be the highest it will be in their life time.
With this in mind children and young adults should engage in regular weight bearing exercise up until they stop growing to ensure maximum bone mass prior to the inevitable reduction which occurs as they age. If they have low bone mass at a young age they far more likely to suffer from osteoporosis in their senior years.
Bone loss for females usually begins at 30 and accelerates after the age of 45 due to the decrease in oestrogen's and accounts for as much as 8% of bone loss every 10 years.
For Males, bone loss begins after 60 with a 3% loss every 10 years.
Astronauts in space loose 1% of bone mass a week due to the lack of gravity.