Low bone density in a young person is not normal, but it’s more common than most people realize. Among 18-year-olds, roughly one in five already shows signs of lower-than-expected bone mineral density. The causes range from nutritional gaps and hormonal disruptions to genetic conditions and medication side effects, and identifying the right one matters because bones are still actively building through your mid-30s.
How Young Bones Are Supposed to Build
Your skeleton isn’t a finished structure. Bones continuously add mineral density from childhood through early adulthood, reaching what’s called peak bone mass between ages 25 and 35, depending on the specific bone. Most of that accumulation happens during adolescence, but the process keeps going well into your twenties. Think of it as a savings account: the more bone you deposit during these years, the more you have to draw on later in life.
When something interferes with this building phase, whether it’s poor nutrition, a hormonal problem, or a chronic illness, you don’t just stall. You can end up with a bone density that’s significantly below what’s expected for your age. Doctors measure this using a Z-score, which compares your bones to other people of the same age, sex, and ethnicity. A Z-score at or below negative 2.0 signals bone density below the expected range and usually warrants further investigation.
Not Eating Enough Calcium or Vitamin D
This is the most straightforward and most fixable cause. Calcium is the primary mineral that gives bones their hardness, and vitamin D is what allows your gut to absorb it. Adults up to age 50 need about 1,000 milligrams of calcium daily, and most people need 600 international units of vitamin D. Adolescents actually need more calcium than adults, around 1,300 milligrams per day, because their skeletons are growing so rapidly.
Many young people fall well short of these targets. Diets low in dairy, leafy greens, or fortified foods leave bones without raw materials. Vitamin D deficiency is especially common in people who spend most of their time indoors, live in northern latitudes, or have darker skin. Without enough vitamin D, it doesn’t matter how much calcium you consume because your body can’t use it efficiently. Years of running this deficit during the critical bone-building window can leave you with measurably thin bones by your twenties.
Hormonal Disruptions
Estrogen and testosterone are not just reproductive hormones. They directly regulate bone remodeling, the process where old bone is broken down and new bone is laid in its place. When levels of either hormone drop too low, the breakdown side of the equation outpaces the building side, and bone density falls.
In young women, the most common hormonal cause of bone loss is losing their menstrual period. This can happen from extreme dieting, excessive exercise, polycystic ovary syndrome, or conditions affecting the pituitary gland. When periods stop, estrogen plummets, and the skeleton’s bone-building processes are disrupted. In young men, a condition called hypogonadism (where the testes produce too little testosterone) leads to the same outcome. Over time, men with hypogonadism can develop significant bone loss.
Overactive parathyroid glands or adrenal glands can also throw off bone metabolism. The parathyroid controls calcium levels in the blood, and when it’s overactive, it pulls calcium out of bones to raise blood levels. Excess cortisol from the adrenal glands (whether from a medical condition or from taking steroid medications) suppresses new bone formation.
Underfueling in Athletes
Young athletes, both male and female, face a specific risk called Relative Energy Deficiency in Sport (RED-S). This happens when calorie intake consistently falls short of what training demands. The body responds by dialing down functions it considers nonessential, including reproductive hormones and bone maintenance.
In female athletes, the classic warning sign is losing periods. That hormonal drop weakens the skeleton and makes stress fractures far more likely. In the most severe cases, young women can develop full osteoporosis while still in their teens or twenties. Male athletes aren’t immune. RED-S in men leads to poor bone health and stress fractures through similar energy-deficit mechanisms, though it often goes unrecognized because there’s no obvious signal like a missed period.
Anyone with a history of repeated stress fractures, disordered eating, or menstrual irregularity should have their bone density evaluated. These fractures aren’t just bad luck from overtraining. They’re often the skeleton’s way of signaling that something deeper is wrong.
Chronic Diseases That Steal Bone
Several medical conditions can quietly erode bone density for years before anyone notices. The mechanism varies, but the result is the same: bones that are thinner and more fragile than they should be for your age.
- Celiac disease damages the lining of the small intestine, reducing absorption of calcium and other minerals even if your diet is adequate. Many people with celiac are diagnosed only after a bone density scan reveals unexplained thinning.
- Inflammatory bowel disease (Crohn’s and ulcerative colitis) causes chronic inflammation that interferes with bone metabolism, compounded by poor nutrient absorption.
- Kidney or liver disease disrupts the body’s ability to activate vitamin D and regulate calcium, both of which are essential for maintaining bone.
- Rheumatoid arthritis drives bone loss through persistent inflammation, and the steroid medications used to control it can accelerate the problem.
Gastrointestinal surgery is another underappreciated cause. Any procedure that reduces the size of the stomach or removes part of the intestine, including weight-loss surgeries, limits the surface area available to absorb nutrients. Calcium absorption takes a permanent hit, and without careful supplementation, bone density declines.
Medications That Weaken Bones
A number of commonly prescribed drugs can cause bone loss, and young people on long-term treatment are particularly vulnerable because they’re still trying to build peak bone mass.
Glucocorticoids (steroid medications like prednisone) are the most well-known culprits. They suppress new bone formation and are prescribed for everything from asthma to autoimmune conditions. Even a few months of use can measurably reduce bone density. Antiseizure medications are another significant category. They interfere with vitamin D metabolism, indirectly starving bones of calcium.
Other medications with documented effects on bone include certain injectable contraceptives (specifically the progestin-only shot), blood thinners like heparin with long-term use, opioids, some antidepressants, acid-blocking medications used for reflux, and cancer chemotherapy drugs. If you’re taking any of these medications long term, bone density monitoring may be appropriate.
Genetic and Developmental Causes
Some young people have low bone density because of how their skeleton was built from the start. Juvenile primary osteoporosis is a rare condition that causes significant bone thinning in childhood. Children with this condition often experience multiple fractures in the long bones of the arms and legs, particularly near the growth plates, along with fractures in the spine that can cause vertebrae to collapse. The result is bone pain and movement problems that can be severe.
In some cases, the condition is linked to a specific gene mutation, but many children with juvenile osteoporosis have no identifiable genetic cause. When no mutation or underlying condition explains the bone loss, doctors call it idiopathic juvenile osteoporosis. Research suggests that additional genes involved in the condition simply haven’t been identified yet.
Osteogenesis imperfecta, sometimes called brittle bone disease, is another genetic condition that produces extremely low bone density from birth. It varies widely in severity, from mild cases that cause occasional fractures to severe forms that are apparent at birth.
Lifestyle Factors That Add Up
Beyond nutrition, several everyday habits influence how much bone your body builds during the critical years. Physical inactivity is a major one. Bone responds to mechanical loading, meaning it gets stronger when you put weight on it. Activities like running, jumping, and resistance training stimulate bone formation. A sedentary lifestyle during adolescence and early adulthood means missing one of the strongest natural signals for bone growth.
Smoking and heavy alcohol use both suppress bone formation. Smoking interferes with the cells responsible for building new bone, while alcohol in excess disrupts calcium balance and hormone levels. Being significantly underweight, regardless of the reason, is another independent risk factor. Lower body weight means less mechanical stress on the skeleton, and it often coincides with inadequate calorie and nutrient intake.
Because bone density peaks in your late twenties to mid-thirties, every year of these habits during adolescence and early adulthood costs you part of your lifetime bone reserve. The good news is that bones in young people are still responsive. Correcting the underlying cause, whether it’s a nutritional gap, a medication, or an undiagnosed condition, can often improve bone density in ways that aren’t possible later in life.