What is bone mineral density (BMD) and how is it measured?
Bone mineral content (BMC) and bone mineral density (BMD) describe the mineral concentration in your bones, which is key to assessing bone strength. A standard medical measurement called a DEXA (DXA) scan is the primary method for evaluating BMD.
A DEXA scan is a quick, non-invasive procedure using low-dose X-rays, typically on the hip and spine, to measure mineral content. Results are given as T-scores and Z-scores.
Understanding your T-score and Z-score
Your DEXA scan report uses T-scores and Z-scores to interpret your bone density.
T-Score: Comparison to a healthy young adult
For postmenopausal women and men 50 and older, the T-score is the main diagnostic tool, comparing your BMD to a healthy young adult at peak bone mass. Scores are measured in standard deviations (SD).
- Normal Bone Density: T-score of -1.0 or higher.
- Osteopenia (Low Bone Mass): T-score between -1.0 and -2.5, a sign of bone loss and increased fracture risk.
- Osteoporosis: T-score of -2.5 or lower, indicating significant bone density loss and high fracture risk.
Z-Score: Comparison to an age-matched group
The Z-score compares your bone density to others your age, sex, and ethnicity. This is more relevant for children, premenopausal women, and men under 50. A Z-score of -2.0 or lower may suggest an underlying cause of bone loss.
Factors that influence bone mineral density
While aging naturally causes some bone loss, various factors can accelerate it. These include:
- Age: Bone density peaks around 30 and declines, particularly for women after menopause.
- Hormonal Changes: Reduced estrogen or testosterone can lead to bone loss.
- Genetics: A family history of osteoporosis increases risk.
- Lifestyle: Smoking, excessive alcohol, and inactivity negatively impact bone health.
- Diet: Insufficient calcium and vitamin D can weaken bones.
- Medications and Medical Conditions: Certain drugs (like steroids) and conditions (like thyroid disorders) can affect BMD.
Comparison of DEXA results
This table compares T-score classifications:
| DEXA T-Score Range | Classification | What It Means |
|---|---|---|
| -1.0 and above | Normal | Healthy bone density comparable to a young adult's. |
| -1.0 to -2.5 | Osteopenia | Low bone mass, increased fracture risk. |
| -2.5 and below | Osteoporosis | Significantly low bone density, high fracture risk. |
Strategies to improve bone health and prevent fractures
If you have low bone density, you can take steps to strengthen bones and reduce fracture risk, which also benefits overall bone health.
- Calcium-rich diet: Aim for 1,000–1,200 mg daily from sources like dairy, leafy greens, and fortified foods.
- Adequate Vitamin D: Essential for calcium absorption. Get 600–800 IU daily from sunlight, fatty fish, or fortified foods.
- Weight-bearing exercise: Activities like walking, jogging, and weightlifting build bone density.
- Avoid smoking and limit alcohol: These habits decrease bone mass.
- Prevent falls: Use measures like grab bars and good lighting to reduce fall risk.
Conclusion
Bone health is assessed using DEXA scan T-scores and Z-scores, not a simple "bone mineral percentage". A T-score of -1.0 or higher is normal. Low scores indicate osteopenia or osteoporosis. A balanced diet with calcium and vitamin D, regular weight-bearing exercise, and lifestyle changes can improve bone health and lower fracture risk. Discuss concerns with a healthcare provider for personalized advice.
