Bones become osteoporotic when the body breaks down old bone faster than it builds new bone, and this imbalance tips far enough that bones become porous and fragile. More than 200 million people worldwide have osteoporosis, but it doesn’t happen to everyone equally. The reasons some people develop it come down to a mix of genetics, hormones, nutrition, lifestyle, aging, and underlying medical conditions, often working together over decades.
How Healthy Bone Maintains Itself
Your skeleton is not a static structure. It constantly tears itself down and rebuilds through a process called remodeling. Three cell types run this operation: cells that dissolve old bone (osteoclasts), cells that lay down new bone (osteoblasts), and cells embedded in the bone matrix (osteocytes) that act as sensors, detecting mechanical stress and coordinating the other two. In a healthy skeleton, the amount of bone removed roughly equals the amount replaced. Osteoporosis develops when that balance breaks and resorption consistently outpaces formation.
A key signaling molecule controls how many bone-dissolving cells your body produces. This molecule, called RANKL, acts as a master switch for osteoclast survival, activation, and growth. Its counterpart, a decoy receptor, binds RANKL and neutralizes it. When the system is balanced, bone mass stays stable. When something tips RANKL signaling upward, whether hormones, inflammation, or aging, your body produces more bone-dissolving cells and bone density drops.
Genetics Set the Starting Line
Genetic factors account for roughly 60 to 85 percent of the variation in bone mineral density between individuals. That’s an enormous influence. Dozens of genes play a role, including those that code for vitamin D receptors, estrogen receptors, collagen proteins that form the structural scaffold of bone, and the RANKL signaling pathway itself. Some people inherit a skeletal architecture and cellular machinery that maintain density efficiently into old age. Others inherit variants that make their bones more vulnerable to the hormonal shifts, nutritional gaps, or lifestyle factors described below.
Certain genetic conditions carry especially high risk. Osteogenesis imperfecta, Turner syndrome, Ehlers-Danlos syndrome, and thalassemia all predispose people to reduced bone mass, sometimes beginning in childhood.
Peak Bone Mass: What You Build by Age 20 Matters
By the end of your second decade of life, you’ve reached what’s called peak bone mass, the maximum amount of bone your skeleton will ever contain. Think of it as a bone bank account. The more you deposit during childhood and adolescence, the more you can afford to lose later without crossing into osteoporosis territory. People who reach a lower peak due to poor nutrition, inactivity, chronic illness, or hormonal problems during growth years start adulthood at a disadvantage.
Several conditions common in young people can suppress peak bone mass: eating disorders like anorexia nervosa, celiac disease that impairs nutrient absorption, juvenile arthritis, cystic fibrosis, and chronic kidney disease. Medications matter too. Long-term use of corticosteroids, certain anti-seizure drugs, proton pump inhibitors, and some antidepressants during the growing years can reduce the bone mass a young person ultimately achieves.
Estrogen Loss Is the Biggest Single Trigger
The sharp rise in osteoporosis among women after menopause is not coincidental. Estrogen directly suppresses RANKL production in bone lining cells, keeping bone-dissolving activity in check. When estrogen drops after menopause, RANKL expression jumps roughly threefold. Animal studies show that replacing estrogen brings RANKL back down to normal levels, confirming this isn’t just a correlation.
The result of estrogen loss is a simultaneous increase in osteoclast numbers and a decrease in osteoblast numbers. Bone resorption accelerates while bone formation slows. This is why the first five to ten years after menopause represent the period of fastest bone loss for most women. Men experience a more gradual decline in sex hormones, which is one reason osteoporosis tends to appear later and less frequently in men, though they are far from immune.
What Changes Inside Aging Bone
Even beyond hormonal shifts, aging itself changes how bone cells behave. The stem cells in bone marrow that can become either bone-building cells or fat cells increasingly choose the fat pathway as people age. Studies in older mice and humans show that osteoblast numbers decline while fat cells (adipocytes) in the marrow increase, directly reducing the body’s capacity to form new bone.
On top of this, aging stem cells accumulate damage and enter a state of senescence, where they stop dividing and begin secreting inflammatory molecules that create a toxic local environment. This inflammatory signaling further suppresses bone formation and tips the remodeling balance toward loss. It’s a self-reinforcing cycle: aging bone marrow produces fewer bone builders, more inflammation, and more fat, all of which accelerate the decline.
Calcium, Vitamin D, and the Absorption Problem
Bones are mineralized with calcium and phosphorus crystals, so a steady supply of both is essential. But calcium absorption depends heavily on vitamin D. Without adequate vitamin D, your intestines absorb only 10 to 15 percent of the calcium you eat. With sufficient vitamin D, that absorption rate climbs to 30 to 40 percent. That’s a two-to-fourfold difference from the same dietary calcium intake.
Vitamin D deficiency is remarkably common. One estimate found it affects 70 percent of children in the United States. Adults fare little better, particularly those with limited sun exposure, darker skin, obesity, or digestive conditions that impair fat absorption (vitamin D is fat-soluble). A person can eat plenty of calcium-rich foods and still develop weakened bones if their vitamin D status prevents that calcium from reaching the skeleton. This is why calcium and vitamin D deficiencies so often appear together as drivers of osteoporosis.
Exercise Tells Bones to Stay Strong
Bones respond to mechanical loading. When you walk, run, jump, or lift weight, the osteocytes embedded throughout your skeleton detect the strain and send chemical signals to surrounding cells that ramp up bone formation. This is why weight-bearing and resistance exercise consistently protect against bone loss, and why prolonged bed rest, paralysis, or a sedentary lifestyle accelerates it.
The sensing mechanism is sophisticated. Osteocytes sit within a network of tiny channels in the bone and use surface receptors to detect pressure, fluid flow, and deformation. They then relay those mechanical signals to osteoblasts and osteoclasts, essentially telling the skeleton how much structural support the body actually needs. A skeleton that isn’t regularly loaded gets the signal that less bone is needed, and remodeling shifts accordingly.
Diseases and Medications That Steal Bone
A long list of medical conditions can trigger what’s called secondary osteoporosis, bone loss driven by an identifiable disease or drug rather than aging and hormones alone. The major categories include:
- Endocrine disorders: overactive parathyroid glands, Cushing syndrome, hyperthyroidism, and diabetes all disrupt the hormonal signals that regulate bone turnover.
- Digestive diseases: inflammatory bowel disease, celiac disease, and chronic liver disease interfere with nutrient absorption, starving bones of calcium and vitamin D.
- Chronic inflammatory conditions: rheumatoid arthritis, COPD, and cystic fibrosis produce inflammatory molecules that directly stimulate bone resorption. Inflammatory cytokines like interleukin-1 and tumor necrosis factor are known to amplify RANKL signaling.
- Kidney disease: the kidneys activate vitamin D and regulate calcium and phosphorus balance, so chronic kidney disease disrupts bone mineralization at multiple levels.
- Neuromuscular conditions: cerebral palsy, multiple sclerosis, Parkinson’s disease, and post-stroke disability reduce weight-bearing activity, removing the mechanical stimulus bones need.
Medications are a major and often overlooked contributor. Glucocorticoids can cause bone loss at doses as low as 2.5 mg of prednisone per day, and even inhaled corticosteroids for asthma carry some risk. Anti-seizure drugs, certain antidepressants, proton pump inhibitors used for acid reflux, hormone-blocking cancer therapies, and antiretroviral drugs for HIV all increase bone loss through various mechanisms. Smoking and heavy alcohol use compound the problem further.
How Osteoporosis Is Measured
Bone density is assessed with a specialized scan that compares your results to those of a healthy 30-year-old of the same sex. The result is a T-score. A score of negative 1.0 or higher is normal. Between negative 1.0 and negative 2.5 indicates low bone mass, sometimes called osteopenia. A T-score of negative 2.5 or below is osteoporosis. If that low score comes with a fracture from a minor fall or impact, it’s classified as severe osteoporosis.
These thresholds help explain why some people fracture a hip from a standing-height fall while others walk away unharmed. The difference in bone density between the two groups can be decades in the making, shaped by every factor described above: the genes they inherited, the bone mass they built as teenagers, the hormonal environment of midlife, the nutrients they absorbed, how much they moved, and whether any diseases or medications quietly chipped away at their skeleton along the way.