like a filter that can prevent the skin from making vitamin D. As a result, people with darker
skin need more time in the sun than people with lighter skin to make the same amount of
vitamin D.
Fish is naturally rich in vitamin D. Vitamin D is added to other foods including milk, soy
milk and breakfast cereals. Teenagers are recommended to get 5 micrograms (200 IU/day)
of vitamin D every day. A 3 ½ ounce portion of cooked salmon provides 360 IU of vitamin
D.
Lack of vitamin D, ordeficiency, can be caused by two different things: not enough sunlight
exposure, and lack of vitamin D in the diet. Vitamin D deficiency results in problems with
bone growth and hardening. This leads to bone softening diseases such as rickets in children
and osteomalacia in adults. Osteomalacia is a bone disease in which the bones do not harden
properlyandtheycanbreakeasily. Ricketsisatypeofosteomalacia. AnX-rayofachildthat
has rickets is shown inFigure16.20. Lack of vitamin D may also be related to osteoporosis.
Bone Fractures
Even though they are very strong, bones can fracture, or break. Fractures can happen at
different places on a bone. They are usually due to excess bending stress on the bone.
Bending stress is what causes a pencil to break if you were to bend it too far. Soon after a
fracture, the body begins to repair the break. The area becomes swollen and sore. Within
a few days bone cells travel to the break site and begin to rebuild the bone. It takes about
2 to 3 months before compact and spongy bone form at the break site.
Sometimes the body needs extra help in repairing a broken bone. In such a case a surgeon
will piece a broken bone together with metal pins. Moving the broken pieces together will
help keep the bone from moving, and give the body a chance to repair the break. A broken
ulna has been repaired with pins inFigure16.21.
Cartilage Injuries
Osteoarthritisis a condition in which the cartilage at the ends of the bones breaks down.
The break down of the cartilage leads to pain and stiffness in the joint. Decreased movement
of the joint because of the pain may lead to muscles that are attached to the joint to become
weaker, and ligaments may become looser. Osteoarthritis is the most common form of
arthritis. It has many causes, some of the more common causes include old age, sport
injuries to the joint, bone fractures, and overweight and obesity. Total hip replacement is a
common treatment for osteoarthritis.