Low bone density in your teens, twenties, or thirties typically results from one of three things: your body never built enough bone during the critical growth window, a medical condition is quietly breaking bone down faster than it’s being rebuilt, or a medication is interfering with the process. A Z-score of -2.0 or lower on a bone density scan is the threshold that defines “low bone density for age” in anyone under 50, and it’s more common in younger adults than most people realize.
Understanding why this happened is the first step toward protecting your skeleton before the natural bone loss of aging even begins.
Peak Bone Mass and Why the Timing Matters
Your bones don’t just maintain a fixed strength throughout life. They actively build density from childhood through your mid-twenties, reaching what’s called peak bone mass. For women, that peak arrives around age 22. For men, it comes later, closer to age 25 or 27. After that window closes, you’re essentially living off the bone “savings account” you built during those years.
Anything that disrupts this building phase has outsized consequences. A teenager with an eating disorder, a young adult with undiagnosed celiac disease, or someone who spent years on certain medications may arrive at 30 with a bone bank that was never fully funded. Body weight and BMI play a significant role in how much bone you ultimately accumulate. Interestingly, research tracking young adults found that self-reported physical activity levels during this window did not significantly influence total bone mineral density, though exercise still matters for bone health through other pathways like muscle strength and balance.
Medical Conditions That Steal Bone Early
Dozens of conditions can cause what’s called secondary osteoporosis, meaning the bone loss is a downstream effect of another problem. In younger adults, the most common culprits fall into a few categories.
Hormonal and Endocrine Disorders
Your bones depend on a steady supply of hormones to maintain their rebuilding cycle. An overactive thyroid gland speeds up bone breakdown. Overactive parathyroid glands pull calcium directly out of bone to raise blood levels. Cushing syndrome, where the body produces too much cortisol, suppresses the cells responsible for building new bone. Conditions that lower sex hormones, like premature ovarian insufficiency in women or low testosterone in men, remove one of the strongest signals bones rely on to stay dense. Even type 1 and type 2 diabetes alter bone quality, though through different mechanisms.
Gut and Absorption Problems
Your bones can only use nutrients that actually make it into your bloodstream. Celiac disease, Crohn’s disease, and ulcerative colitis all damage the intestinal lining in ways that reduce absorption of calcium and vitamin D. Chronic liver disease has a similar effect. This is why someone can eat a perfectly adequate diet and still develop low bone density: the nutrients pass through without being absorbed.
Chronic Inflammatory Conditions
Rheumatoid arthritis, cystic fibrosis, and other conditions that keep the immune system chronically activated create a chemical environment in the body that favors bone breakdown. The inflammation itself directly interferes with the cells that build bone, and the medications used to treat these conditions (particularly steroids) can compound the problem.
Energy Deficiency and Bone Loss in Athletes
One of the most overlooked causes of low bone density in young people is chronically not eating enough to match energy expenditure. This pattern, now called Relative Energy Deficiency in Sport (REDs), was originally recognized only in female athletes but affects men too. When the body doesn’t get enough fuel, it dials down functions it considers non-essential, and reproductive hormones are among the first to drop. In women, this shows up as missed periods. In men, it manifests as low testosterone.
The bone consequences go beyond just hormone suppression. Athletes with REDs show reduced activity of the cells that build new bone and increased activity of the cells that break it down. The result is lower bone density and weakened bone structure compared to athletes eating enough or non-athletes of the same age. This isn’t limited to elite competitors. Recreational runners, dancers, gymnasts, and anyone restricting calories while training heavily can develop the same pattern.
Medications That Weaken Bone
Glucocorticoids (steroids like prednisone) are the most well-known bone-damaging medications, and young adults with asthma, autoimmune diseases, or inflammatory bowel disease may take them for months or years. These drugs attack bone from multiple angles. They suppress the cells that build new bone, promote the death of existing bone cells, and even redirect the stem cells that would normally become bone-building cells to instead become fat cells. The effect is a net loss of bone that begins within the first few months of use.
Other medications linked to bone loss include certain treatments for epilepsy, some antidepressants, hormone-blocking therapies used in cancer treatment, and proton pump inhibitors used for acid reflux when taken long term.
Genetic Causes
Some people are dealt a difficult hand genetically. Osteogenesis imperfecta, often called “brittle bone disease,” is the most recognized genetic cause, but mild forms can go undiagnosed until adulthood. Research using advanced genetic sequencing on young adults with unexplained osteoporosis found mutations in the genes responsible for building collagen, the protein that gives bone its flexibility and framework. In some patients, the bone fragility looked identical to mild osteogenesis imperfecta, with the only distinguishing feature being that classic osteogenesis imperfecta also affects the eyes and ears.
Other genetic mutations affect signaling pathways that regulate how quickly bone is built and broken down. These aren’t common, but they’re worth investigating when a young person has fractures or very low bone density with no obvious medical explanation. Ehlers-Danlos syndrome and thalassemia are two other inherited conditions associated with reduced bone density.
Nutritional Gaps That Add Up
Calcium and vitamin D are the raw materials your bones need, and falling short on either one over time will show up on a bone density scan. The recommended daily intake of vitamin D for adults aged 19 to 50 is 600 IU (15 micrograms), and the calcium target is around 1,000 milligrams per day. Many young adults miss both, particularly those who avoid dairy, get limited sun exposure, or follow restrictive diets.
Eating disorders deserve special mention here. Anorexia nervosa is one of the most potent risk factors for low bone density at a young age because it combines caloric restriction, low body weight, hormonal disruption, and nutritional deficiency all at once. The bone loss can be severe and, in some cases, only partially reversible even after recovery. Obesity, somewhat counterintuitively, is also listed among nutritional conditions associated with secondary osteoporosis, partly because excess body fat produces inflammatory signals that interfere with bone metabolism, and partly because people with obesity sometimes have poorer diet quality despite higher calorie intake.
How It’s Detected and Measured
Bone density is measured with a DXA scan, a low-radiation X-ray that takes about 10 to 15 minutes. If you’re under 50, your result is reported as a Z-score rather than the T-score used for older adults. The Z-score compares your bone density to the average for healthy people of your same age, sex, and ethnicity. A Z-score of -2.0 or lower means your bone density is significantly below what’s expected for someone like you, and it should prompt a search for an underlying cause.
A Z-score between -1.0 and -2.0 still warrants attention, especially if you have risk factors like a family history of fractures, a chronic medical condition, or long-term medication use.
Building and Protecting Bone When You’re Young
The single most important thing you can do is identify and treat any underlying cause. Low bone density from untreated celiac disease, for example, often improves significantly once you go gluten-free and your gut starts absorbing nutrients properly again. Stopping or reducing a bone-damaging medication, correcting a hormone imbalance, or recovering from an eating disorder can all shift the trajectory.
Weight-bearing and high-impact exercise stimulates bone to remodel and strengthen. Research on osteogenic loading, which involves brief sessions of exercises that place high force on the skeleton, has shown bone density increases of 3 to 11 percent at various sites over a 24-week period. Practical examples include jumping exercises, running, stair climbing, and resistance training with progressively heavier weights. Activities like swimming and cycling, while excellent for cardiovascular health, don’t load the skeleton enough to drive significant bone adaptation.
Meeting your daily calcium and vitamin D targets through food or supplements provides the building blocks. Avoiding smoking and limiting alcohol both matter, since smoking directly impairs bone-building cells and heavy drinking interferes with calcium absorption and hormone levels. For young adults whose peak bone mass window hasn’t fully closed, these interventions aren’t just protective. They’re an opportunity to add density that will serve you for decades.