Bone mineral density (BMD) is a measure of how much calcium and other minerals are packed into a segment of bone. It’s reported in grams per square centimeter and serves as the primary indicator doctors use to assess bone strength, diagnose osteoporosis, and estimate fracture risk. A higher number means denser, stronger bone; a lower number signals that bones have become more porous and fragile.
How Bone Density Is Measured
The standard test for bone mineral density is a DXA scan (sometimes written DEXA), which stands for dual-energy X-ray absorptiometry. The machine sends two low-power X-ray beams through your body at slightly different energy levels. Because bone, fat, and lean tissue each absorb these beams differently, the scanner can isolate exactly how dense your bones are at specific sites, typically the hip and lower spine. The scan is painless, takes about 10 to 15 minutes, and exposes you to far less radiation than a standard chest X-ray.
What T-Scores and Z-Scores Mean
Your DXA results come back as two numbers: a T-score and a Z-score. The T-score compares your bone density to that of a healthy 25- to 35-year-old adult of the same sex and ethnicity. That age range represents when bones are at their strongest, so it provides a fixed benchmark. The World Health Organization uses these T-score thresholds to classify bone health:
- Above -1.0: Normal bone density
- Between -1.0 and -2.5: Osteopenia (lower than normal, but not yet osteoporosis)
- -2.5 or below: Osteoporosis
The Z-score compares your density to people your own age, sex, race, height, and weight. It’s especially useful for younger adults, premenopausal women, and children, where a T-score comparison against peak bone mass wouldn’t be appropriate. A Z-score significantly below zero suggests something beyond normal aging may be driving bone loss, prompting doctors to look for underlying causes.
How Bone Density Changes Over a Lifetime
Your skeleton isn’t static. Bone is constantly being broken down and rebuilt in a process called remodeling. During childhood and adolescence, new bone forms faster than old bone is removed, so density climbs steadily. Most of the bone mass at the hip and spine accumulates by late adolescence, with some sites continuing to add small amounts of mineral into a person’s 30s. After that, the balance tips: breakdown gradually outpaces rebuilding, and density begins a slow decline.
For women, this decline accelerates sharply after menopause. Estrogen plays a key protective role by suppressing the cells that break down bone (osteoclasts), keeping remodeling in check. When estrogen levels drop during menopause, those bone-removing cells multiply and become more active, and remodeling speeds up in a way that favors loss over rebuilding. This is why women lose bone density more rapidly than men in midlife and why osteoporosis is more common in women.
Men experience a more gradual decline, partly because they start with greater peak bone mass and don’t undergo the same hormonal shift. That said, men over 70 can still develop osteoporosis, particularly if other risk factors are present.
Who Should Get Screened
The U.S. Preventive Services Task Force recommends routine bone density screening for all women 65 and older. Postmenopausal women younger than 65 should also be screened if they have one or more risk factors, including low body weight, a parent who fractured a hip, smoking, or heavy alcohol use. For men, there isn’t yet enough evidence to support a blanket screening recommendation, though doctors often order a DXA scan for men with clear risk factors like long-term steroid use or a history of fragility fractures.
What Causes Bone Density to Drop
Age and menopause are the most common drivers, but several other factors accelerate bone loss. Low calcium and vitamin D intake, physical inactivity, smoking, and excessive alcohol all weaken bones over time. Certain chronic conditions, including thyroid disorders, celiac disease, and rheumatoid arthritis, also contribute.
A number of medications can cause what’s known as secondary bone loss. Long-term corticosteroids (often prescribed for asthma, autoimmune conditions, or inflammatory diseases) are the most well-known culprits. But the list extends to acid-suppressing drugs used for reflux, some antidepressants, certain diabetes medications, hormone-blocking cancer therapies, anti-seizure drugs, and some blood thinners. If you take any of these long term, your doctor may monitor your bone density more closely.
Exercise That Strengthens Bone
Bone responds to mechanical stress by becoming denser, a principle sometimes called osteogenic loading. Not all exercise is equal here. Two categories have the strongest evidence: weight-bearing aerobic activities like jogging, stair climbing, and brisk walking, and progressive resistance training, particularly for the lower body. Resistance training targeting the legs and hips appears to be the single most effective exercise type for maintaining density at the femoral neck, one of the most common fracture sites.
Combining several forms of exercise, including aerobics, resistance work, balance training, and even dancing, tends to produce better results than any single type alone. Tai chi has also shown a positive effect on slowing bone loss at the spine and hip. The key is that the activity needs to reach sufficient intensity. A leisurely stroll provides cardiovascular benefits but may not generate enough force to stimulate bone adaptation.
Calcium and Vitamin D Needs by Age
Calcium is the primary building block of bone mineral, and vitamin D is essential for absorbing it from food. Current daily recommendations are:
- Adults 19 to 50: 1,000 mg calcium, 600 IU vitamin D
- Women 51 to 70: 1,200 mg calcium, 600 IU vitamin D
- Men 51 to 70: 1,000 mg calcium, 600 IU vitamin D
- All adults 71 and older: 1,200 mg calcium, 600 IU vitamin D
Dairy products, fortified foods, leafy greens, and canned fish with bones are good dietary sources of calcium. Vitamin D comes from sun exposure, fatty fish, egg yolks, and fortified milk or cereal. Many people, especially those living in northern climates or spending most of their time indoors, fall short on vitamin D and may benefit from supplementation. Getting these nutrients from food first is generally preferred, with supplements filling the gap rather than replacing a balanced diet.
What Osteopenia and Osteoporosis Feel Like
Low bone density itself doesn’t cause symptoms. You can’t feel your bones becoming less dense, which is why screening matters. Most people discover they have osteopenia or osteoporosis only after a DXA scan or, in more advanced cases, after a fracture from a minor fall or even a sneeze. The most common fracture sites are the hip, spine, and wrist. Spinal compression fractures can cause gradual height loss and a hunched posture over time, sometimes with chronic back pain, but many occur without any noticeable event.
Because bone loss is silent, the window for prevention and early intervention depends entirely on knowing your numbers. A DXA scan gives you a concrete starting point, and repeat scans every one to two years can track whether your density is stable, improving, or declining.