Collagen, a protein, gives bones their elasticity while calcium salts make them hard. Bone marrow, located in the center of our long bones, makes blood cells. To develop properly and grow strong, our bones need calcium , vitamin D , and regular exercise. Sources of calcium such as milk, yogurt, ice cream, and broccoli provide the nutrients for continuous bone building and remodeling. Vitamin D helps our bodies absorb this calcium through the gut.
Vitamin D comes either from a chemical action of sunlight on our skin or from fortified milk.
Finally, regular exercise makes bones stronger by stressing them. When bones are stressed, they respond by fortifying themselves. This cycle of stress and response to stress promotes strengthening. When we don't get enough calcium, vitamin D, and exercise, our bones deteriorate. For example, young children who are deprived of calcium and vitamin D may develop a type of bone weakness called rickets .