Bone density is a measure of the amount of mineral content, primarily calcium phosphate, packed within a volume of bone tissue. This mineral content gives bones their hardness and rigidity, making them capable of withstanding the stresses of daily life. Low bone density, often referred to as bone mass, is a condition that weakens the skeletal structure and increases the risk of fractures. The health of your bones relies on a continuous process of renewal where old bone is broken down and new bone tissue is created.
The Structure and Measurement of Bone Density
The human skeleton is composed of two distinct types of bone tissue: cortical and trabecular. Cortical, or compact bone, forms the dense, hard outer layer of most bones, accounting for about 80% of the total bone mass in the adult skeleton. Trabecular, or cancellous bone, is the spongy, porous tissue found primarily at the ends of long bones and inside the vertebrae.
Trabecular bone is metabolically more active than cortical bone, making it more sensitive to changes in diet, hormones, and lifestyle. The micro-architecture of both tissues, especially the connectivity of the trabeculae, is a determinant of overall bone strength.
To quantify the amount of mineral content in these structures, clinicians use Dual-Energy X-ray Absorptiometry, commonly known as a DEXA or DXA scan. The DEXA scan is a non-invasive procedure that uses low-dose X-rays to measure bone mineral density, typically at the hip and spine. The results are presented as a T-score, which compares a patient’s bone density to the average peak bone density of a healthy young adult of the same sex. A T-score of 0 represents this young adult reference value, and the more negative the score, the lower the bone density.
The Lifespan Trajectory of Bone Mass
Bone mass acquisition is a dynamic process that starts from birth and continues through early adulthood, with most individuals achieving their Peak Bone Mass (PBM) between the late 20s and early 30s. After PBM is reached, the body begins a slow, natural decline in bone density where bone breakdown starts to slightly outpace bone formation.
Bone loss accelerates significantly in women following menopause due to hormonal changes. Estrogen helps regulate the balance between bone formation and resorption, and the sharp decline in estrogen levels causes bone resorption to drastically increase. Postmenopausal women can experience a rapid annual bone loss rate, especially at the hip and spine, making them highly susceptible to conditions like osteoporosis. While men also experience age-related bone loss, the decline is typically more gradual and begins later in life.
Lifestyle and Nutritional Factors
Controllable factors like nutrition and physical activity play a large role in building and maintaining bone density throughout life. Calcium is a mineral that forms the structural component of the bone matrix, and adequate intake is necessary for bone strength. The recommended daily intake for adults between 19 and 50 years old is 1,000 milligrams, increasing to 1,200 milligrams per day for women over 50 and men over 70.
Vitamin D is equally important because its primary function is to optimize the intestinal absorption of dietary calcium. Most adults under 70 years of age should aim for 600 International Units (IU) of Vitamin D daily, though some research suggests a higher intake of 800 to 1,000 IU may be beneficial for older adults.
Physical activity, particularly weight-bearing and resistance exercise, provides the mechanical loading necessary to stimulate bone growth. When muscles pull on the bone during activities like walking, jogging, or lifting weights, it creates strain that signals bone cells to produce more tissue. This mechanical stress is a potent stimulus for strengthening the skeleton. Engaging in 30 minutes of weight-bearing activity, four or more days a week, is recommended for maintaining bone health.
Certain lifestyle choices can negatively affect bone density by disrupting the balance of bone remodeling. Tobacco smoking is associated with decreased bone health, possibly by inhibiting the activity of bone-forming cells and contributing to earlier menopause in women. Excessive alcohol consumption can also interfere with calcium absorption and Vitamin D metabolism, leading to a decrease in bone mass.
Screening and Medical Management
The DEXA T-score is used to clinically classify bone mineral density according to World Health Organization criteria. A T-score of -1.0 or higher is considered normal bone density for a young adult reference population. A T-score between -1.0 and -2.5 indicates low bone mass, a condition known as osteopenia.
A T-score of -2.5 or lower at the hip or spine is the diagnostic threshold for osteoporosis, signifying a severely increased risk of fracture. However, the diagnosis may be made even with a higher T-score if a patient has already experienced a low-trauma fracture. Screening with a DEXA scan is generally recommended for all women aged 65 and older, men aged 70 and older, and younger individuals with specific risk factors.
Medical management for diagnosed osteoporosis typically begins with antiresorptive medications like bisphosphonates, such as alendronate, which work by inhibiting the bone-resorbing cells called osteoclasts. Other treatment options include denosumab, an injectable medication that targets a protein involved in bone breakdown, and anabolic agents like teriparatide, which actively stimulate new bone formation.
Hormone therapy, specifically estrogen replacement, can also be used to treat osteoporosis in postmenopausal women, particularly those who are also seeking relief from menopausal symptoms. However, it is typically not considered a first-line treatment for bone density alone due to associated risks. For patients with osteopenia, treatment decisions are often guided by a comprehensive fracture risk assessment tool, such as FRAX, which considers the T-score alongside other risk factors like age, medical history, and lifestyle.