What Is Osteoporosis? Bone Density, Risk Factors, and Treatment

Understanding Bone Structure and Remodeling

Bone is a dynamic living tissue, constantly being broken down and rebuilt through a process called bone remodeling. Specialized cells called osteoclasts resorb (dissolve) old bone, while osteoblasts build new bone to replace it. In healthy adults, this process maintains bone mass and repairs micro-damage. Peak bone mass is achieved in the late 20s to early 30s, after which a gradual net loss begins as osteoclast activity starts to outpace osteoblast activity.

Osteoporosis occurs when this balance tips dramatically in favor of bone resorption, leading to progressive loss of bone mineral density (BMD) and deterioration of the microarchitectural structure of bone—the intricate lattice of trabeculae (spongy bone) and compact cortical bone. As bone becomes less dense and more porous, it loses its strength and resilience, making fractures more likely to occur from minor mechanical stress—a fall from standing height, a cough, or even bending forward.

Measuring Bone Density: DEXA Scans and T-Scores

Dual-energy X-ray absorptiometry (DEXA or DXA) scanning is the gold-standard method for measuring bone mineral density. It uses two different X-ray energy levels to distinguish bone from soft tissue and measures BMD at key sites—typically the lumbar spine (L1–L4 vertebrae) and the proximal femur (hip). The scan is brief, non-invasive, and delivers very low radiation exposure (less than a transcontinental flight).

The results are reported as a T-score: the number of standard deviations above or below the mean BMD of a healthy young adult of the same sex. A T-score between -1.0 and +1.0 indicates normal BMD. A T-score between -1.0 and -2.5 indicates osteopenia (below-normal bone density but not yet osteoporosis). A T-score of -2.5 or lower indicates osteoporosis. The Z-score—comparing BMD to age-matched peers—is used in premenopausal women and men under 50 to identify secondary causes of bone loss. The FRAX tool (Fracture Risk Assessment Tool) integrates BMD with clinical risk factors to estimate the 10-year probability of a hip or major osteoporotic fracture, which guides treatment decisions.

Risk Factors and Epidemiology

Osteoporosis affects an estimated 200 million people worldwide and is responsible for approximately 8.9 million fractures annually. Women are disproportionately affected—particularly postmenopausal women—because estrogen plays a critical role in suppressing osteoclast activity. The sharp decline in estrogen at menopause accelerates bone loss dramatically, with women losing up to 20% of their bone density in the 5–7 years following menopause. However, osteoporosis is not exclusively a women's disease: approximately 20% of patients with osteoporotic fractures are men, and men often have worse outcomes following hip fracture.

Additional risk factors include: advanced age; family history of osteoporosis or fragility fractures; low body weight (BMI below 19 kg/m2); cigarette smoking; excessive alcohol consumption (more than two drinks/day); prolonged use of glucocorticoids (a major secondary cause); other secondary causes including hyperthyroidism, hyperparathyroidism, malabsorption syndromes (celiac disease, inflammatory bowel disease), and hypogonadism; low dietary calcium and vitamin D intake; and a sedentary lifestyle. Certain ethnicities (Caucasian and Asian populations) have higher osteoporosis risk than Black and Hispanic populations, though all groups are affected.

Prevention: Calcium, Vitamin D, and Exercise

Building maximum peak bone mass in youth and young adulthood is the most powerful long-term strategy for preventing osteoporosis. Adequate calcium intake throughout life is essential—calcium is the primary mineral component of hydroxyapatite, the crystal that gives bone its strength. The recommended daily calcium intake is 1,000 mg for adults under 50 and 1,200 mg for women over 50 and all adults over 70. Dairy products (milk, yogurt, cheese), fortified plant milks, leafy greens (kale, bok choy), and tinned fish with bones are excellent food sources.

Vitamin D is essential for calcium absorption in the gut and for bone mineralization. Adults generally require 600–800 IU daily, with many experts recommending higher intakes (1,000–2,000 IU) for individuals with limited sun exposure or malabsorption. Weight-bearing and resistance exercise stimulates osteoblast activity and increases bone density—activities include walking, running, dancing, tennis, and weight training. Exercise also reduces fall risk by improving balance, coordination, muscle strength, and reaction time—all important because most osteoporotic fractures result from falls rather than from bone fragility alone.

Pharmacological Treatment

Bisphosphonates—including alendronate (Fosamax), risedronate, and zoledronic acid—are the most commonly prescribed osteoporosis medications. They bind to hydroxyapatite on bone surfaces and are taken up by osteoclasts, inhibiting their bone-resorbing activity. Oral bisphosphonates (alendronate, risedronate) must be taken on an empty stomach with a full glass of water, remaining upright for 30 minutes to minimize gastrointestinal side effects. Intravenous zoledronic acid is given once yearly. Long-term use (beyond 3–5 years) carries a small risk of atypical femoral fractures and osteonecrosis of the jaw.

Denosumab (Prolia) is a monoclonal antibody that inhibits RANKL—a key signaling molecule for osteoclast development. Given as a subcutaneous injection every six months, it is highly effective but must be continued without gaps because bone density loss accelerates rapidly upon discontinuation. For patients with very low bone density or who have already sustained vertebral fractures, anabolic agents that build new bone are preferred: teriparatide (a fragment of parathyroid hormone) and abaloparatide are daily self-injections used for up to 24 months; romosozumab (Evenity) is a monthly injection that both stimulates bone formation and inhibits resorption. These powerful anabolic treatments are typically followed by antiresorptive therapy to preserve gains.