Bone loss happens when your body breaks down old bone faster than it can build new bone. This imbalance accelerates with age, but hormonal changes, nutritional gaps, certain medications, and lifestyle habits all play significant roles. Understanding these causes helps explain why some people lose bone density decades before others.
How Your Body Builds and Breaks Down Bone
Your skeleton isn’t static. Throughout your life, specialized cells are constantly tearing down old bone and replacing it with new tissue in a process called remodeling. Two cell types drive this cycle. Bone-breaking cells attach to the bone surface and use enzymes and acids to dissolve the existing matrix, leaving small scooped-out pits behind. Bone-building cells then move in and fill those pits with fresh collagen and minerals.
In healthy adults, this demolition and reconstruction stays roughly in balance. Bone loss begins when the breaking-down side outpaces the building side. That can happen because the bone-breaking cells become overactive, because the bone-building cells slow down or die off, or both. As you age, the number of bone-building cells declines relative to demand. The result is a slow, steady net loss of bone tissue that, over years, can weaken your skeleton enough to raise your fracture risk significantly.
Estrogen and Menopause
Estrogen is one of the most powerful regulators of bone turnover. It works on both sides of the equation: it triggers the death of bone-breaking cells (shortening their lifespan) while protecting bone-building cells from dying prematurely. Estrogen also suppresses inflammatory signals that would otherwise ramp up the production of new bone-breaking cells. The dominant effect of estrogen is blocking the formation of these destructive cells in the first place.
At menopause, estrogen levels drop sharply. This removes the brakes on bone breakdown. The body activates far more remodeling sites across the skeleton, and at each site, the bone-breaking cells live longer and dig deeper. The result is an accelerated phase of bone loss that begins at menopause and continues for several years before settling into a slower, sustained decline. Men experience a more gradual drop in bioavailable estrogen with aging, which contributes to bone loss as well, though typically later in life and at a slower pace.
Not Enough Calcium or Vitamin D
Your body needs calcium to keep bones dense and strong, but it also needs calcium for nerve signaling, muscle contraction, and other essential functions. When your diet falls short, your body pulls calcium directly from your bones to maintain normal blood levels. Over time, this borrowing weakens the skeleton.
Adults aged 19 to 50 need about 1,000 mg of calcium per day. Women over 50 and men over 70 need 1,200 mg. Teenagers and young adults, who are still building peak bone mass, need 1,300 mg.
Vitamin D is equally important because it controls how much calcium your gut can absorb from food. Without adequate vitamin D, even a calcium-rich diet won’t fully protect your bones. Adults up to age 70 need 600 IU of vitamin D daily, and those over 70 need 800 IU. Many people fall short of both nutrients, especially those who avoid dairy, get limited sun exposure, or have digestive conditions that impair absorption.
Medications That Weaken Bones
Long-term use of corticosteroids (commonly prescribed for asthma, autoimmune diseases, and inflammatory conditions) is one of the best-known drug-related causes of bone loss. These medications attack bone density from multiple angles. They increase the death rate of bone-building cells and mature bone cells embedded in the skeleton. They reduce the production and development of new bone-building cells. And they impair the gut’s ability to absorb calcium by depleting the energy and transport proteins intestinal cells need to move calcium into the bloodstream.
The combined effect is rapid bone loss that can begin within the first few months of treatment. Other medications linked to bone loss include certain seizure drugs, some cancer treatments (particularly those that lower sex hormones), and proton pump inhibitors used for acid reflux when taken for years.
Overactive Parathyroid and Other Medical Conditions
The parathyroid glands produce a hormone that regulates calcium levels in the blood. When these glands become overactive, they release too much of this hormone, which stimulates bone-breaking cells to pull calcium and phosphorus out of the skeleton and dump them into the bloodstream. Over time, this leads to high bone turnover, weakened bones, bone pain, and in severe cases, bone deformities and fractures. In extreme situations, the constant bone destruction creates visible holes in the skeleton known as brown tumors.
Other conditions that accelerate bone loss include hyperthyroidism (an overactive thyroid gland), celiac disease and other malabsorption disorders that prevent nutrient uptake, rheumatoid arthritis, chronic kidney disease, and type 1 diabetes. Each condition disrupts bone metabolism through different pathways, but the end result is the same: more bone removed than replaced.
Smoking and Alcohol
Smoking impairs bone health enough that it’s considered a standalone risk factor for osteoporosis. Nicotine and other compounds in tobacco interfere with the blood supply to bone tissue and disrupt the hormonal environment that supports bone building.
Heavy alcohol use directly suppresses bone-building cells. Research on bone tissue from men with osteoporosis confirmed that alcohol leads to delayed and impaired bone-building cell activity while the bone-breaking cells continue functioning normally. In animal studies, the amount of bone surface covered by active bone-building cells dropped significantly with chronic alcohol exposure, and the thickness of new bone walls was reduced by 52 percent compared to controls. Alcohol also decreases activated vitamin D, which reduces calcium absorption from food (though this effect reverses quickly once drinking stops).
How Bone Loss Is Measured
Bone density is measured with a DXA scan, a low-radiation X-ray that typically focuses on the hip and spine. The result is reported as a T-score, which compares your bone density to that of a healthy 30-year-old at peak bone mass.
- T-score of -1 or higher: healthy bone density
- T-score between -1 and -2.5: osteopenia, a less severe form of low bone density
- T-score of -2.5 or lower: osteoporosis
Each single-point drop in the T-score increases your fracture risk by 1.5 to 2 times. Someone with a T-score of -3 faces roughly three to four times the fracture risk of someone with a score of -1.
Who Should Get Screened
The U.S. Preventive Services Task Force recommends routine bone density screening for all women aged 65 and older. Postmenopausal women younger than 65 should also be screened if they have risk factors such as low body weight, a parent who fractured a hip, cigarette smoking, or heavy alcohol use. For men, there isn’t yet enough evidence to issue a blanket screening recommendation, though men with known risk factors (long-term corticosteroid use, low testosterone, heavy drinking) are often screened on an individual basis.
These screening guidelines apply to adults 40 and older who haven’t already been diagnosed with osteoporosis or had a fragility fracture. People with underlying conditions known to cause bone loss, such as cancer, metabolic bone diseases, or hyperthyroidism, typically follow a separate screening timeline set by their specialist.