Osteoporosis happens when your body loses bone faster than it can rebuild it. This imbalance between bone breakdown and bone formation is the core mechanism behind the disease, but the reasons it develops in the first place range from hormonal shifts and genetics to medications, nutrient deficiencies, and everyday habits like smoking and heavy drinking.
How Bone Loss Actually Works
Your skeleton is not a static structure. It constantly tears itself down and rebuilds through a process called bone remodeling. Specialized cells called osteoclasts dissolve small sections of old or damaged bone, and then a second group of cells called osteoblasts fill those gaps with fresh bone tissue. In a healthy adult, these two processes stay tightly coupled so that each remodeling cycle ends with no net change in bone mass or strength.
Osteoporosis develops when that balance tips toward destruction. Osteoclasts remove bone faster than osteoblasts can replace it, and over time each remodeling cycle leaves behind a small deficit. Those deficits accumulate, and eventually bones become porous, lighter, and fragile enough to fracture from minor stress. Nearly every cause of osteoporosis traces back to something that disrupts this balance, either by speeding up breakdown or slowing down formation.
Hormonal Changes: Estrogen and Testosterone
The most common trigger for osteoporosis is the drop in estrogen that comes with menopause. Estrogen acts as a brake on osteoclast activity, so when levels fall, bone resorption accelerates. Both breakdown and formation speed up during this period, but breakdown outpaces formation by a wide margin. Up to 20% of a woman’s bone mass can be lost during and shortly after menopause, and roughly 1 in 10 women over 60 worldwide are affected by osteoporosis.
Men are not immune. Testosterone supports bone in two ways: it directly stimulates the cells responsible for building new bone tissue, and it gets converted into estrogen inside bone cells, which protects both the dense outer layer and the spongy interior. When testosterone drops, whether from aging, medical treatment, or other conditions, men lose bone too. Men who are deficient in the enzyme that converts testosterone to estrogen almost universally develop weakened bones, highlighting how important that conversion pathway is.
Genetics and Family History
Your genes set the ceiling for how much bone you build during your teens and twenties, a peak that determines how much you have in reserve as you age. Women with a family history of hip fracture are twice as likely to experience one themselves. While no single gene causes osteoporosis, inherited factors influence bone size, density, the rate of bone turnover, and how efficiently your body absorbs calcium. If a parent or sibling had osteoporosis or a fragility fracture, your own baseline risk is meaningfully higher.
Not Enough Calcium or Vitamin D
Calcium is the primary mineral in bone, and vitamin D is essential for absorbing it from food. When either one falls short, your body pulls calcium from the skeleton to maintain blood calcium levels, weakening bones over time. Close to 30% of men and 60% of women over age 19 in the United States don’t consume enough calcium, and more than 90% fall short on vitamin D.
Current dietary guidelines recommend that women over 50 get 1,200 mg of calcium per day, while men over 50 need 1,000 mg (rising to 1,200 mg after age 71). For vitamin D, the recommendation is 600 IU daily for adults through age 70 and 800 IU after that. These amounts are achievable through a combination of diet and supplements, but the gap between what people actually consume and what they need is striking.
Medications That Weaken Bone
Long-term use of glucocorticoids (steroids like prednisone, commonly prescribed for asthma, rheumatoid arthritis, and inflammatory bowel disease) is the most frequent drug-related cause of osteoporosis. These medications suppress osteoblast activity while boosting osteoclast-driven breakdown, and the damage starts early. Fracture risk rises even at low doses and can increase within the first month of treatment. Daily doses as low as 2.5 mg of prednisone, taken for three months or more, are enough to significantly raise fracture risk.
Other medications linked to bone loss include certain breast cancer treatments that block estrogen, prostate cancer drugs that suppress testosterone, some anti-seizure medications, and proton pump inhibitors used for acid reflux when taken for years at a time. If you take any of these long-term, bone density monitoring becomes important.
Other Medical Conditions
Several diseases accelerate bone loss through different pathways. An overactive parathyroid gland (hyperparathyroidism) pumps out excess parathyroid hormone, which signals osteoclasts to ramp up bone resorption. The damage concentrates in the dense outer layer of bone, particularly at sites like the wrist and hip. An overactive thyroid gland has a similar effect, speeding up overall bone turnover to the point where formation can’t keep pace.
Digestive disorders are another underrecognized cause. Celiac disease, Crohn’s disease, and other conditions that damage the intestinal lining impair absorption of calcium and vitamin D. In celiac disease specifically, the combination of nutrient malabsorption and chronic inflammation activates osteoclasts and can trigger secondary hyperparathyroidism, where the body overproduces parathyroid hormone in response to chronically low calcium. Osteoporosis is sometimes the first and only visible sign of undiagnosed celiac disease.
Type 1 diabetes, rheumatoid arthritis, and chronic kidney disease also contribute to bone loss through a mix of inflammation, hormonal disruption, and impaired mineral handling.
Smoking and Alcohol
Smoking weakens bones through several routes at once. Nicotine directly interferes with osteoblasts, the cells that build new bone. At the concentrations typical in regular smokers, nicotine inhibits osteoblast production and suppresses the growth of new blood vessels that bone tissue depends on. Smoking also lowers estrogen levels by reducing the conversion of other hormones into estrogen in fat tissue, compounding the effect.
Alcohol’s impact depends on quantity. A meta-analysis of dose-response data found that three or more standard drinks per day significantly increases hip fracture risk: 33% higher at three drinks daily and 59% higher at four, compared to non-drinkers. Moderate drinking (one to two drinks per day) did not show the same clear increase, so the threshold for harm appears to sit around three drinks daily.
Age, Body Weight, and Physical Inactivity
Aging itself is a cause. After peak bone mass is reached around age 30, everyone loses bone gradually. By the time you reach your 70s and 80s, decades of small remodeling deficits add up regardless of other risk factors. Being underweight (a BMI below about 20) compounds this because less body weight means less mechanical stress on the skeleton and less fat tissue producing estrogen.
Physical inactivity matters because bone responds to load. Weight-bearing activity, anything from walking to strength training, signals osteoblasts to reinforce bone at the sites under stress. A sedentary lifestyle removes that stimulus. People who are bedridden or immobilized lose bone rapidly, sometimes several percent per month in the affected limbs, illustrating how powerfully mechanical loading shapes bone density over time.