Osteoporosis happens when your body breaks down bone faster than it can rebuild it. This imbalance develops over years or decades, driven by a combination of hormonal changes, genetics, nutritional gaps, and lifestyle factors. One in three women and one in five men over 50 are affected worldwide, making it the most common bone disease. Understanding what causes it starts with how your bones constantly rebuild themselves, and what goes wrong when that process tips out of balance.
How Bone Constantly Rebuilds Itself
Your skeleton isn’t a fixed structure. It’s a living tissue that tears itself down and rebuilds in a continuous cycle called remodeling. Two cell types run this process. Bone-dissolving cells go first, creating small pits by releasing acid that dissolves bone mineral and enzymes that break down the protein framework. After a brief transition phase, bone-building cells move in and fill those pits with fresh bone material that gradually hardens with calcium and other minerals.
In healthy adults, this cycle stays roughly in balance. You lose a little bone and gain it back. But when the dissolving cells become overactive, or the building cells can’t keep up, each remodeling cycle removes slightly more bone than it replaces. Over thousands of these cycles across years, bones become progressively thinner, more porous, and more fragile. That net loss is osteoporosis.
The Role of Estrogen and Hormones
Estrogen is the single biggest hormonal protector of bone. It works by suppressing a signaling molecule that activates bone-dissolving cells while also promoting the release of a protein that blocks those cells from forming in the first place. When estrogen levels drop, particularly after menopause, this braking system weakens. The body ramps up production of inflammatory signals that accelerate bone breakdown. The result is both faster bone turnover and a shift toward net loss with every cycle.
This is why postmenopausal women face the steepest decline in bone density. Roughly one-tenth of women are affected by age 60, rising to two-fifths by age 80 and two-thirds by age 90. Men experience a more gradual decline in testosterone, which also supports bone health, so their bone loss tends to be slower and later. Other hormonal conditions can trigger the same imbalance at any age: an overactive thyroid gland, an overactive parathyroid gland (which pulls calcium from bone), and Cushing disease (which floods the body with stress hormones) all accelerate bone loss.
Peak Bone Mass Sets the Baseline
Not everyone enters middle age with the same amount of bone to lose. Your skeleton reaches its maximum density, called peak bone mass, in your late teens to mid-twenties. Research using national health survey data found that women typically reach peak density at the hip around age 19 and at the spine around age 20, while men peak slightly later, around 21 at the hip and 24 at the spine.
The higher your peak bone mass, the more you can afford to lose before crossing into dangerous territory. A lower peak in youth may be the single most important factor leading to osteoporosis later in life. This is why childhood and adolescent nutrition, physical activity, and overall health have lifelong consequences for bone strength, even though symptoms won’t appear for decades.
Genetics Control More Than You’d Expect
Twin and family studies estimate that 50 to 85 percent of the variation in bone mineral density is determined by genetics. That’s a remarkably large genetic contribution. No single gene is responsible. Instead, hundreds of small-effect gene variants collectively influence how dense your bones become and how quickly you lose that density. Many of these variants affect the same signaling pathways that control how bone-dissolving and bone-building cells communicate with each other.
If a parent or sibling has had osteoporosis or a fragility fracture, your own risk is meaningfully higher. You can’t change your genes, but knowing your family history helps you and your doctor decide when screening makes sense.
Calcium and Vitamin D Deficiency
Bone is largely made of calcium phosphate, the mineral compound that gives it hardness and rigidity. When your diet doesn’t supply enough calcium, your body pulls it from your skeleton to maintain the blood calcium levels your heart, muscles, and nerves depend on. Over time, this borrowing weakens bones.
Vitamin D is essential because it controls how well your gut absorbs calcium from food. Without enough vitamin D, even a calcium-rich diet can fall short. Chronic deficiency of either nutrient forces the body into a state of ongoing mineral withdrawal from bone, compounding whatever other risk factors are already at play.
How Physical Activity Protects Bone
Bones adapt to the forces placed on them. When you walk, run, jump, or lift something heavy, the bending and twisting loads create tiny strains throughout the bone. Specialized sensor cells embedded deep in bone tissue detect these strains and release chemical signals that stimulate bone-building cells to add new material, particularly along the outer surface where stress is greatest.
A sedentary lifestyle removes this stimulus. Without regular mechanical loading, the remodeling cycle tilts toward breakdown. This is why prolonged bed rest, spinal cord injuries, and even the weightlessness of space travel cause rapid bone loss. Aging compounds the problem: the sensor cells in bone become less responsive to mechanical signals over time, so the same amount of activity produces a weaker bone-building response in older adults than in younger ones. That doesn’t make exercise less important with age. It makes it more important.
Medications That Weaken Bone
Glucocorticoids (steroids like prednisone, often prescribed for asthma, autoimmune diseases, and inflammatory conditions) are the most common medication-related cause of osteoporosis. They attack bone on multiple fronts: suppressing the signals that help bone-building cells mature, increasing the death rate of both bone-building cells and the sensor cells that coordinate remodeling, and disrupting the communication network within bone that detects mechanical stress. Bone strength can drop rapidly, with increased fracture risk appearing at doses as low as 2.5 to 7.5 mg of prednisone daily. Long-term use beyond six months carries significant risk.
Other medications linked to bone loss include blood thinners like heparin, anti-seizure drugs, certain chemotherapy agents, and hormone-suppressing drugs used in prostate and breast cancer treatment. If you’re on any of these long-term, bone density monitoring is worth discussing.
Diseases That Cause Secondary Osteoporosis
When bone loss results from an underlying medical condition rather than aging alone, it’s called secondary osteoporosis. The list of contributing diseases is long. Rheumatoid arthritis drives bone loss through chronic inflammation. Celiac disease and other malabsorption conditions prevent the gut from taking in enough calcium and vitamin D. Chronic kidney disease disrupts the body’s ability to activate vitamin D. Chronic liver disease interferes with the same pathway. Diabetes, both type 1 and type 2, is associated with altered bone quality through mechanisms that are still being studied.
Blood disorders, including certain cancers that affect the bone marrow, can also displace normal bone tissue. In many of these conditions, the disease itself and the medications used to treat it (often glucocorticoids) work together to accelerate bone loss.
How Osteoporosis Is Detected
Osteoporosis is silent until a fracture happens. There’s no pain, no obvious symptom. It’s diagnosed with a bone density scan, a painless imaging test that measures how much mineral is packed into a section of bone, usually at the hip and spine. Your result is reported as a T-score, which compares your bone density to that of a healthy young adult. A T-score of negative 1 or higher is normal. Between negative 1 and negative 2.5 indicates reduced bone density (sometimes called osteopenia). A T-score of negative 2.5 or lower means osteoporosis.
One detail that often catches people off guard: an initial fracture dramatically increases the risk of another. A wrist fracture at 55 or a compression fracture in the spine isn’t just a one-time event. It’s a signal that bone has already weakened enough to break under relatively minor stress, and the risk of a second fracture climbs from that point forward.
Why Women Are Affected More Than Men
Three factors converge. Women generally build less bone to begin with, reaching a lower peak bone mass than men. They lose bone more rapidly after menopause due to the sharp drop in estrogen. And they tend to live longer, giving osteoporosis more years to progress. In the European Union alone, an estimated 22 million women aged 50 to 84 have osteoporosis compared to 5.5 million men. In China, about half of women over 50 are affected versus roughly one-quarter of men in the same age group.
Men are not immune, though. By their 70s and 80s, the gap narrows. Male osteoporosis is underdiagnosed partly because it’s still widely perceived as a women’s disease, which means men with fragility fractures often don’t get screened.