You may not realize it, but your bones are living tissue. And that tissue is always breaking down and re-growing.

When the growth of new bone tissue does not keep up with the loss of old bone tissue you may develop a medical condition called osteoporosis.

Osteoporosis literally means "porous bones." Normal bones have many tiny holes. With osteoporosis, the bones become so thin in places that even a simple stretch or a cough can result in a bone fracture.

Some fractures, like hip fractures, can lead to death. The National Center for Biotechnology Information in the United States says “some reports show that up to 50 percent of patients with hip fracture die within six months.”

About 200 million people worldwide have osteoporosis. Health experts say that number will increase greatly as the world’s population gets older. After all, everyone's bones weaken with age.

The International Osteoporosis Foundation says that one in three women over the age of 50 will experience an osteoporotic fracture. The organization says for men over 50, the probability is one in five.

Usually people get osteoporosis as a result of hormonal changes, or from a lack of calcium or vitamin D. White and Asian people are at higher risk for the disease. Women are more likely to get it than men. Old people, small people and those with a family history of osteoporosis are also at higher risk for the disease.

There are behaviors that can help prevent osteoporosis. Eating healthful foods, being physically active and avoiding tobacco and alcohol use are important.

However, one of the most important things that will protect you against osteoporosis when you are older is how much bone mass you get when you are young.

The better your bone mass, the less likely you are to develop osteoporosis when you get older. According to several health websites, our bones are at their strongest between the ages of 20 to 30.

Bones, like muscles, react to pressure. When bones experience increased impact or force, they grow new cells.

So, it is important that children do high-impact activities like running, playing sports like basketball and volleyball and simple jumping. Jumping jacks and jumping rope are great exercises for a child’s bone growth. These impact exercises will help to build up bone mass needed later in life.

As we said earlier, inactivity is bad for your bones. However, even people who exercise often can have low bone density. This can lead to osteoporosis.

For example, meet bicyclist Dean Hargett. Every week, Hargett bikes more than 160 kilometers. This exercise is great for his heart, but it doesn't do much for his bones.

Hargett was surprised when the doctors told him that he had low bone density.

"It alarmed me. I don't want to have fragile bones. I want to be able to be strong and healthy throughout the rest of my life."

So, swimming and cycling might be great for your heart, but they do not do a lot for re-growing your bones. And even if you are very active and exercise often, that does not necessarily mean you have strong bones.

Pam Hinton is a researcher in nutrition, exercise and bone health at the University of Missouri in the United States.

Hinton studied how two kinds of exercises affected men’s bone health: resistance and jump-training. Resistance training is a form of exercise in which you work against force. The study took place over a 12-month period. The results showed that resistance exercises did more than just slow the rate of bone loss.

"We actually saw an increase in bone mass with either type of exercise, so that was a very encouraging and exciting result."

Bicyclist Dean Hargett is encouraged by the news. He now knows he has to do more than cycle to stay healthy.

He also has a warning -- do not take bone strength for granted. In other words, do not make the mistake of thinking your bones are always going to be strong and healthy.

"Don't take your bone strength for granted. They can wither just like any other part of your body."

So, to help your bones stay healthy, the National Osteoporosis Foundation in the U.S. suggests doing weight-bearing and muscle-strengthening exercises.

Examples of high-impact weight-bearing exercises are: dancing, hiking, jogging or running, jumping rope, stair climbing and tennis.

For muscle-strengthening exercises, they suggest: lifting weights, using weight machines and lifting your own body weight.

And remember, what you put into your body matters too.

Get enough calcium and vitamin D. New research shows that vitamin K may also be good for your bones. Good sources of vitamin K include dark leafy greens such as kale and spinach.

Lower your use of salt. Avoid soda and too much alcohol. Both can affect how your body uses calcium. And, if you smoke, quit.