Bone fat, also known as bone marrow adipose tissue (BMAT), is the fat stored inside the marrow cavity of bones. Once considered an inactive filler that replaced blood-forming red marrow with age, BMAT is now understood to be an active tissue. It participates in numerous bodily processes and can make up 50-70% of the marrow cavity in adults, making it a significant component of bone health.
The Dual Nature of Bone Fat
Bone marrow adipose tissue has both beneficial and detrimental qualities. It serves as a localized energy reserve, as the breakdown of its lipids fuels bone-building cells and supports blood cell production. BMAT also functions as an endocrine organ by secreting hormones like adiponectin and leptin that influence metabolism.
Conversely, an excessive accumulation of BMAT is linked to negative outcomes. When bone marrow adipocytes become too numerous, they can physically crowd the marrow space and hinder the function of blood-forming stem cells. High BMAT levels are also linked to reduced bone mass, suggesting a shift in cellular activity that favors fat storage over bone formation.
Two main subtypes of BMAT have been identified. Regulated BMAT (rBMAT) is found with active, blood-producing marrow in areas like the femur and vertebrae. Constitutive BMAT (cBMAT) appears earlier in life and is more densely packed in the distal parts of bones, such as the tibia. These distinct populations respond differently to bodily signals and contribute uniquely to the bone marrow environment.
How Bone Fat Differs from Other Body Fats
Bone fat is distinct from subcutaneous fat (under the skin) and visceral fat (around internal organs). BMAT is uniquely located within the mineralized environment of bone, where it shares space with blood-forming and bone-remodeling cells.
The metabolic behavior of BMAT also sets it apart. While excess visceral fat is strongly linked to metabolic diseases, the role of BMAT is more complex. Its levels are regulated by a different set of rules than other fat stores in the body.
Subcutaneous fat provides insulation and padding, while excess visceral fat releases inflammatory substances. BMAT arises from the same mesenchymal stem cells in the bone marrow that can also become bone-forming osteoblasts. This shared origin creates a competitive relationship where the development of one cell type can occur at the expense of the other, linking fat accumulation to skeletal health.
Factors That Influence Bone Fat Levels
Several factors influence the amount of bone fat. The most significant is aging, as active red marrow decreases over time and is replaced by yellow marrow, which is mostly BMAT. This age-related increase is observed in both men and women.
Hormonal changes are another driver of BMAT accumulation. The decline in estrogen after menopause is associated with a significant increase in BMAT in women. In men, lower testosterone levels have also been linked to higher amounts of bone fat.
Lifestyle and medical factors also play a part. Severe calorie restriction, as seen in anorexia nervosa, paradoxically leads to an expansion of BMAT. Certain medications, such as thiazolidinediones used to treat diabetes, can also increase BMAT levels. Conversely, physical exercise may reduce BMAT, as mechanical loading on bones can push stem cells toward bone formation instead of fat storage.
Bone Fat’s Connection to Overall Health
The accumulation of bone fat has direct implications for skeletal integrity. A high volume of BMAT is consistently associated with lower bone mineral density and an increased risk of fractures. This occurs because marrow adipocytes can secrete substances that promote osteoclasts, the cells that break down bone tissue, creating an environment that favors bone resorption over formation.
This connection is particularly evident with osteoporosis. In many cases of age-related and postmenopausal osteoporosis, an increase in BMAT is a prominent feature. The expansion of fat within the bone marrow is thought to contribute to the disease by displacing bone-forming cells and creating a microenvironment that undermines skeletal strength.
Beyond the skeleton, BMAT is linked to broader metabolic health issues. Increased bone marrow fat is observed in individuals with type 2 diabetes and cardiovascular disease. The adipocytes release signaling molecules that affect systemic glucose and lipid metabolism, showing BMAT is an active participant in the body’s overall metabolic state.
Understanding these connections is opening new avenues for research. Scientists are exploring therapies that could target BMAT to improve both bone and metabolic health.