There are also some important lifestyle issues which have an effect on the normal formation of bone. Physical activity and good nutrition appear to be the most important of these "environmental" factors. It appears that poor activity levels and nutrition during the years of bone formation may prevent the normal growth of bones which may cause them to be less dense. Smoking during these years may also decrease the amount of bone which is formed. A significant illness during the teenage years which causes prolonged bed rest and lack of exercise will also prevent the complete acquisition of bone density. Persons who are affected by any of these factor are likely to enter adult life with a bone mineral density (BMD) which is less than their healthier peers.
Remember, it is the difference between how much healthy bone is formed during the first 28 or so years of life and the rate at which it is remodeled and removed later in life which determines how much osteoporosis or osteopenia a person has.