This is due to the angle of the sun, which is too oblique to provide enough sunlight for the body to manufacture the vitamin. In circumstances such as these, supplements are essential. Moderate weight bearing exercise is also important for avoiding osteoporosis. Say at age 30 you become sedentary, there's not a lot of incentive for your bones to stay pumped up, because there's no work for them to do. Good sources of such exercise include walking, running or playing tennis.
Ignorance is osteoporosis' biggest ally.
The more we learn about this disease, the less we have to fear its unexpected arrival.