Protecting our bones is important, but figuring out how to do that through diet can be confusing. With that in mind, I contacted a few experts to answer some frequently asked questions. This is the first of two columns on the impact of nutrition and diet on bone health. The first part is about general bone health and the role calcium plays. The second part focuses on other foods and nutrients that help or hurt bone health.

Q: What is osteoporosis, and what does it do to our bones?

According to Tejaswini Rao, a professor of nutrition at State University of New York-Buffalo, "Osteoporosis, or porous bones, is a disease characterized by low bone mass, leading to fragile bones and resulting in an increased susceptibility to fractures of the hip, spine and wrist."

Q: At what age do our bones start to deteriorate?

Our bones reach their peak density in the mid-20s (on average), says Dorothy Teegarden, a professor of nutrition at Purdue University. "Roughly after age 35, both men and women start to lose bone at a slow rate of 1 to 2 percent per year. This bone loss is accelerated in women around and after menopause to about 5 percent per year for five to seven years, and then the bone loss in women returns to a baseline of 1 to 2 percent per year. Men don't go through the accelerated menopausal bone loss but continue to lose bone at the rate of 1 to 2 percent per year," says Dr. Primal Kaur, director of the Osteoporosis Clinic at Temple University School of Medicine in Philadelphia.

Q: Is osteoporosis mostly a female problem?

It's true that osteoporosis is less common in men because they start out with higher bone density than women and do not go through menopause, which accelerates bone loss, says Rao. But after age 65, men and women lose bone at the same rate. By 75, one-third of men have osteoporosis as do a third of women.

Q: What is the bone-calcium connection?

"Calcium is the material that gives bone strength, much as bricks do for a building, but the bones need an underlying structure of proteins that are special for bone to hold on to the calcium," according to Dr. Ruth Freeman, a professor of medicine at Albert Einstein College of Medicine Montefiore Medical Center. "Basically there are two types of cells active in bone: one, the osteoclast, makes pockets in the bone, chewing it up so that good bone can replace any damaged bone. The second type, the osteoblast, comes in and fills in the bone pockets that the osteoclast created. Over about 10 years' time, all bone is replaced to remove little damaged areas. But after age 40 the osteoblasts don't adequately refill the pockets created by the osteoclasts, so there is a net loss of bone. Anything, therefore, that speeds up turnover of bone, like loss of estrogen, will increase the loss of bone in people who are over age 40."

Estrogen is important in maintaining bone mass in adult women; in part it slows bone remodeling (replacing old bone with new bone). And while bone remodeling is normally a good thing, it takes place over weeks and months, which leaves a woman more susceptible to bone fractures. Estrogen helps to slow this process down. So basically, the estrogen maintains the proper balance between the activity of bone-forming cells (the osteoblasts) and bone-resorbing cells (the osteoclasts).

Q: Are low-fat dairy products higher in calcium than whole-milk products?

Not significantly so. There is slightly more calcium in skim milk because the fat has been removed, leaving more room for other components of the milk, like calcium.

Q: How much calcium do I need?