Bone is a living and growing tissue that is constantly being built and broken down.

Bones make up the skeleton, which support the structure of the body, protect organs and stores the minerals, calcium and phosphorous. Calcium and phosphorous are released from bone and involved in essential functions, including blood clotting, nerve transmission and the movement of fluid in and out of cells. The body must have enough calcium and phosphorous from the diet to perform these functions, and if it doesn't, bone will be broken down to get it!

Bone grows in length and density during childhood and reaches maximum length during adolescence, 16-20 years. Bone reaches its peak density between the ages 20 to 30. After that age, more bone is broken down than is being built and bone becomes lighter and more brittle.

Building strong bones before the age of 30 is important. After age 30, minimizing bone loss is important.

Two bone disorders have become a new problem for people living with HIV. Osteopenia occurs when there is a loss of bone mass and bone density and osteoporosis is the diagnosed condition when there is a more severe loss of bone mass and bone density, resulting in weak and brittle bones that are more likely to break. It is unclear if this increase in osteoporosis for people living with HIV is caused by HIV or by the medications used to treat HIV.

Prevent your bones from becoming weak. Be sure to get enough calcium and vitamin D rich foods in your diet every day. Good sources of calcium include dairy products and green, leafy vegetables such as broccoli and kale. Orange juice, soymilk, cereals, breakfast bars and other calcium-fortified products are also good sources of calcium.

Vitamin D allows the calcium you eat from foods and supplements to be absorbed into your blood-stream. It can be found in dairy products, egg yolks, fatty fish, fish liver oil, and breads and cereals fortified with vitamin D.

Vitamin D is also made in the skin from sunlight exposure. Depending upon the time of the year and where you live, about 10 to 15 minutes in the sun two to three times a week with your hands, arms and face exposed helps meet your body's requirements for vitamin D.

Physical activity is another necessary factor in bone health. Just like muscles, bones become stronger the more you use them. Weight-bearing and resistance exercise can help to:

• Achieve peak bone mass in children and adolescents

• Help to maintain or slightly increase bone density in adulthood

• Help to minimize age-related bone loss in older adults

It is important to keep an active lifestyle with a variety of activities to have healthy bones. Weight-bearing activities are those in which bones and muscle work against gravity. Weight-bearing activities include running, dancing, soccer and volleyball. Swimming and bike riding are not weight-bearing activities. For older adults, activities such as intense walking and low-impact aerobics may be just fine. Resistance exercise, such as weightlifting, is another type of activity that strengthens both muscles and bones.

Alcohol and smoking negatively affect bone health. In men and pre-menopausal women, alcohol affects certain hormones that cause calcium to be withdrawn from the bone. Over time, this can lead to a large loss in calcium from the bone.

Alcoholics often have weak bones, tend to lose their balance, have trouble walking and therefore are more likely to fall and break a hip. Smokers, more than non-smokers, have lower bone density and need to be concerned about their bone health. In post-menopausal women, moderate consumption of alcohol may have a positive affect of increasing bone density by increasing the conversion of testosterone into estradiol, a hormone that prevents bone loss.

Marcy Fenton, M.S., R.D., is a nutritionist at AIDS Project Los Angeles who can be reached by calling (213) 201-1611 or by e-mail at mfenton@apla.org. Courtney Sanders, a dietetic intern from West Los Angeles Veterans Administration Medical Center, developed this article during her rotation with AIDS Project Los Angeles' HIV Nutrition Education Program.