evidence: Bones are composed of protein, collagen and calcium. In healthy bones, these elements combine to ensure that the bones have a strong structure. In people with osteoporosis, however, the density of the bones decreases. This causes the structure of the bones to weaken in a process which can worsen over time. [...] Lacking sufficient levels of calcium and vitamin D is a primary risk factor for developing osteoporosis. The body cannot make calcium on its own, and the production of sufficient levels of vitamin D (which the body needs in order to absorb calcium from food) requires exposure to plenty of sun. It is vital that one’s diet contains sufficient sources of both nutrients. People with osteoporosis are often advised to take supplements of both vitamin D and calcium in order to replenish the body’s levels of these substances and prevent further bone damage.