教えるのは、あなたが望む専門分野の現役ビジネスマン達。

When we think about the body's skeleton, we usually picture the bones, one of the human body's hardest, most durable substances. When we think about the bones'function in the human body, we usually assume that they are responsible for giving the body its shape, and protecting and supporting organs, tissues and other body parts.

However, the skeleton is much more dynamic than that. In addition to the functions listed above, bones repair and renew themselves constantly, and they are involved in the body's mineral balance. Bone is mostly (65%) made up of hard minerals such as calcium, phosphate, magnesium and other minerals; the other 35% is made of a soft protein called collagen¹. Bones consist of two distinct structures: trabecularbone (which appears spongy-looking) is found on the inside, and is surrounded by cortical bone (a tough, dense outer layer) on the outside. The combination of hard and soft materials, plus the combination of structures, makes bones strong and flexible².

In addition to consisting of two types of materials and two types of structures, bones rely on two types of cells, osteoclastsand osteoblasts, both of which are scattered throughout the bones, for their constant renewal process. In this process, old or damagedbone is taken away and resorbedby the body and new, healthy bone takes its place. The osteoclastsbreak down the old bone, and then the osteoblastsput down the new bone³. Throughout childhood and into young adulthood (twenties), new bone formation surpasses bone resorption. However, after the age of thirty this process begins to reverse and bone resorptionexceeds new bone formation. This means that people reach their peak bone mass by age thirty, after which bone density slowly begins to decrease⁴. For women, the rate of decrease in bone density accelerates after menopause due to the lack of estrogen. Men's bone density also diminishes, although at a slower rate, as testosterone levels decrease with age⁵.

The skeleton also contributes to the body's mineral balance. The human body contains approximately 900 to 1800 grams of calcium, nearly 99 percent ofwhich can be found in the teeth and skeleton⁶. Although the remaining one percent is a very small amount, it plays a critical role in functions such as blood clotting, nerve transmission and muscle contraction (including the heartbeat)⁷. The blood level of calcium is kept in a very narrow range, andwhen needed, bones release calcium.

Since people reach their peak bone mass by age thirty, it is important to build up as much bone as possible during childhood and the early adult years in order to prevent diseases such as osteoporosis (a disease in which the bones become thin and brittle, and as a consequence break easily). Peak bone mass depends about 80% on genetics and 20% on lifestyle factors (diet, exercise, habits); however, as mentioned above sexual hormones (estrogen and testosterone) are also a critical factor⁸. It is important that both children and adults receive adequateamounts of calcium and vitamin D. In addition, regular weigh-bearing exercise (such as walking, hiking, jogging, and dancing) is important for building and maintaining a strong skeleton. Finally, it is important to avoid tobacco use and limit alcohol intake⁹. Hopefully understanding the skeleton's important role in maintaining our health will motivate us to lead a healthy lifestyle!

Discussion questions

1. Please summarize the article. What is the main point of the article?
2. What is the role of the skeleton in the human body?
3. What does bone consist of and what is its structure?
4. What are osteoclastsand osteoblasts? What are their functions?
5. How do bones contribute to the body's mineral balance?
6. What factors determine peak bone mass?
7. What can be done to help ensure that bones develop properly and remain healthy?

References:

1. Becker, Carolyn. Prevention of osteoporosis: Maximizing peak bone mass. Accessed 25 July 2006.
2. Strange, Carolyn. Boning up on osteoporosis. WebMD Public Information from the FDA. Accessed 25 July 2006.
3. Becker, Carolyn. Prevention of osteoporosis: Maximizing peak bone mass. Accessed 25 July 2006.
4. Becker, Carolyn. Prevention of osteoporosis: Maximizing peak bone mass. Accessed 25 July 2006.
5. Becker, Carolyn. Prevention of osteoporosis: Maximizing peak bone mass. Accessed 25 July 2006.
6. Strange, Carolyn. Boning up on osteoporosis. WebMD Public Information from the FDA. Accessed 25 July 2006.
7. Strange, Carolyn. Boning up on osteoporosis. WebMD Public Information from the FDA. Accessed 25 July 2006.
8. Becker, Carolyn. Prevention of osteoporosis: Maximizing peak bone mass. Accessed 25 July 2006.
9. Becker, Carolyn. Prevention of osteoporosis: Maximizing peak bone mass. Accessed 25 July 2006.

---