Osteoporosis happens when your body breaks down bone faster than it can rebuild it. The result is bones that become porous, fragile, and prone to fractures. It affects one in three women and one in five men over 50 worldwide, making it the most common bone disease. The causes range from hormonal shifts and nutritional gaps to medications, lifestyle habits, and underlying medical conditions, often working in combination.
How Bone Normally Rebuilds Itself
Your skeleton is not a fixed structure. It constantly tears itself down and rebuilds through a process called remodeling. Two types of cells drive this cycle. Osteoclasts act as the demolition crew: they release enzymes that dissolve old or damaged bone, leaving microscopic pits on the bone surface. Osteoblasts then move in and deposit new, stronger tissue in those same spots. In a healthy adult, this cycle stays balanced. You lose old bone and gain new bone at roughly the same rate.
Osteoporosis develops when this balance tips toward destruction. The osteoclasts keep working at full speed (or faster), while the osteoblasts can’t keep up. Over months and years, the internal scaffolding of bone thins out, and the overall structure weakens. Nearly every cause of osteoporosis traces back to this imbalance in one way or another.
Estrogen Loss at Menopause
The single biggest driver of osteoporosis in women is the drop in estrogen that comes with menopause. Estrogen directly regulates how long osteoclasts survive. It triggers a natural self-destruct signal in these bone-dissolving cells, keeping their numbers in check. When estrogen levels fall, osteoclasts live longer and break down more bone than osteoblasts can replace.
This is why bone loss accelerates sharply in the first five to seven years after menopause. Women can lose up to 20% of their bone density during that window. Men experience a more gradual version of this process as testosterone (which partly converts to estrogen in the body) declines with age, but the effect is slower and less dramatic.
Aging Beyond Hormones
Even apart from hormonal changes, getting older shifts the internal environment of your skeleton. The bone marrow, which houses the stem cells that become osteoblasts, gradually fills with fat cells instead. This change in the marrow’s composition means fewer bone-building cells are produced and the ones that remain work less efficiently. At the same time, the signaling environment inside aging bone favors resorption over formation. The net effect is a slow, steady loss of bone density that begins in your 30s and continues for the rest of your life.
Not Enough Calcium or Vitamin D
Calcium is the mineral that gives bone its hardness, and your body needs a steady supply of it for functions far beyond your skeleton, including muscle contraction and nerve signaling. When your diet falls short, your body pulls calcium directly from your bones to keep blood levels stable. Over time, this quiet borrowing weakens the bones from within.
How much you need depends on your age. Adults 19 to 50 need about 1,000 mg per day. Women over 50 and men over 70 need 1,200 mg. Teenagers and young adults need even more, around 1,300 mg, because that’s when bones are still building peak density.
Vitamin D makes this whole system work because it controls how well your gut absorbs calcium from food. Without adequate vitamin D, you could be eating plenty of calcium-rich foods and still not getting enough into your bloodstream. People who live in northern latitudes, spend little time outdoors, or have darker skin are especially prone to vitamin D shortfalls.
Medications That Weaken Bone
Long-term use of glucocorticoids (steroids like prednisone, often prescribed for asthma, autoimmune diseases, and inflammatory conditions) is one of the most well-documented drug-related causes of bone loss. These medications attack the problem from both sides: they directly suppress the function of osteoblasts and osteocytes (the cells that maintain and build bone), while simultaneously prolonging the lifespan of osteoclasts. The result is a transient spike in bone breakdown paired with a sustained drop in bone formation.
Bone loss from steroids can begin within the first few months of use and tends to be most severe in the spine. Other medications linked to bone loss include certain seizure drugs, some cancer treatments, and long-term use of proton pump inhibitors for acid reflux.
Underlying Medical Conditions
When osteoporosis is triggered by another disease, it’s called secondary osteoporosis. Several conditions interfere with bone health through different mechanisms. Celiac disease and inflammatory bowel disease damage the gut lining, reducing absorption of calcium and other nutrients essential for bone. Hyperthyroidism speeds up metabolism across the body, including bone turnover, tipping the balance toward excess resorption. Kidney disease disrupts the activation of vitamin D, which the kidneys are responsible for converting to its usable form. Rheumatoid arthritis drives chronic inflammation that directly stimulates osteoclast activity. Liver disease, cancer, and multiple myeloma also raise osteoporosis risk through various pathways.
These conditions matter because they can cause significant bone loss in people who wouldn’t otherwise be considered at risk, including younger adults and men.
Smoking and Alcohol
Nicotine directly inhibits the formation of new bone and impairs the growth of new blood vessels that supply bone tissue with nutrients. Smoking also has indirect effects: it suppresses appetite, alters body weight, disrupts parathyroid hormone levels (which regulate calcium), and increases oxidative stress that damages bone cells. Smokers consistently show lower bone density than nonsmokers of the same age.
Alcohol’s relationship with bone is more nuanced. Heavy drinking clearly raises fracture risk, but moderate consumption (two drinks per day or fewer) does not appear to increase the risk of osteoporotic fractures. The damage from heavy alcohol use comes from its toxic effects on osteoblasts and its tendency to impair balance, increasing fall risk.
Physical Inactivity and Low Body Weight
Bone responds to the mechanical stress you place on it. Weight-bearing exercise, anything where your muscles and gravity pull on your skeleton, stimulates osteoblasts to build more tissue. A sedentary lifestyle removes that stimulus, and bones weaken in response. This is why prolonged bed rest or immobilization after injury can cause rapid bone loss in just weeks.
Being significantly underweight is also a risk factor, partly because low body weight means less mechanical load on the skeleton and partly because it often coincides with poor nutrition. People with eating disorders are at particularly high risk of developing osteoporosis at a young age.
Peak Bone Mass and Genetics
Your bone density peaks somewhere around age 30. After that, you gradually lose more bone than you build. How high that peak reaches depends heavily on genetics. Family history of osteoporosis or fractures is one of the strongest predictors of your own risk. If your parents or siblings had fragility fractures (breaks from minor falls or everyday movements), your baseline bone density is likely lower.
Ethnicity plays a role as well. White and Asian women face the highest rates of osteoporosis, though the disease affects all racial and ethnic groups. Frame size matters too: people with smaller, thinner frames have less bone mass to draw from as they age, so they reach the danger zone sooner.
How Bone Density Is Measured
A bone density scan (DEXA scan) compares your bone mineral density to that of a healthy 30-year-old. The result is expressed as a T-score. A score of negative 1 or higher is considered healthy. Between negative 1 and negative 2.5 indicates osteopenia, a milder form of bone thinning that often precedes osteoporosis. A T-score of negative 2.5 or lower means osteoporosis. These thresholds help identify bone loss before a fracture happens, which is critical because osteoporosis itself causes no pain or symptoms until a bone actually breaks.