Fat is stored not only beneath the skin and around organs, but also deep within the skeletal structure in a distinct tissue known as Bone Marrow Adipose Tissue (BMAT). This specialized fat depot exists within the central cavity of bones, making it anatomically unique from subcutaneous or visceral fat. BMAT is recognized as a metabolically active organ that plays a complex role in the body. Understanding BMAT’s location and function is transforming the view of bone, revealing its dynamic involvement in local skeletal health and whole-body metabolism.
Bone Marrow Adipose Tissue Location
Fat storage occurs within the marrow cavity, the soft tissue found inside bones. Bone marrow is divided into two main types: red marrow, which generates blood cells (hematopoiesis), and yellow marrow, which is predominantly composed of adipocytes. BMAT is the specific name for the fat tissue that makes up the yellow marrow.
In infants, most bones contain red marrow, but maturation leads to the progressive replacement of red marrow by yellow marrow, starting in the extremities. In healthy adults, BMAT is concentrated in the yellow marrow of long bones, such as the femur and tibia. Red marrow, where blood production continues, is primarily found in the spine, pelvis, ribs, and sternum. BMAT is substantial, accounting for approximately 70% of the total bone marrow volume and up to 10% of the body’s total fat mass.
Scientists identify two BMAT subtypes based on location and responsiveness: constitutive BMAT (cBMAT) and regulated BMAT (rBMAT). Constitutive BMAT is stable, found primarily in distal bones like the lower leg, and develops rapidly after birth. Regulated BMAT is more dynamic, found in sites retaining hematopoietic activity like the vertebrae, and its volume changes significantly in response to age, diet, and systemic hormones.
Local Metabolic Support and Structure
BMAT plays a direct role in maintaining the health and function of the bone microenvironment. The adipocytes within the bone marrow are situated in close proximity to skeletal cells, including osteoblasts (which build bone) and osteoclasts (which break it down), as well as hematopoietic stem cells (HSCs). This anatomical arrangement allows BMAT to function as a localized reservoir of energy for these surrounding cells.
The stored lipids within BMAT can be broken down to release free fatty acids, providing a readily available fuel source for the highly active bone and blood-forming cells. This energy supply is particularly important during periods of high metabolic demand or nutritional stress, such as fasting or caloric restriction. BMAT can be mobilized to support the energy needs of HSCs when they are rapidly proliferating.
BMAT also contributes to the physical structure of the bone cavity by occupying space within the medullary canal. Its expansion can occur at the expense of red marrow or bone mass. The relationship between BMAT and bone strength is complex, as the adipocytes and their precursors share a common origin with osteoblasts from mesenchymal stem cells. Increased differentiation toward BMAT often correlates with decreased differentiation toward bone-forming cells, a phenomenon observed in conditions like osteoporosis.
BMAT as a Systemic Regulator
BMAT is recognized as a dynamic endocrine organ because it actively secretes signaling molecules that influence whole-body health, distinguishing it from simple energy storage. The adipocytes in the bone marrow release various hormones and signaling proteins, known as adipokines and cytokines, which travel through the bloodstream to affect distant tissues. This secretory function links the skeletal system directly to systemic metabolic processes.
One key molecule secreted by BMAT is adiponectin, an adipokine generally associated with improved insulin sensitivity and metabolic health. Unlike other fat depots, BMAT appears to be a significant source of circulating adiponectin, especially during states of caloric restriction or fasting, when other fat depots shrink. The increase in adiponectin from BMAT suggests a mechanism by which the bone marrow communicates the body’s energy status to other organs like muscle and liver.
The systemic influence of BMAT is evident in its connection to bone health and metabolic diseases. An increase in BMAT volume, which commonly occurs with aging, is often inversely correlated with bone mineral density, a hallmark of osteoporosis. The molecules secreted by BMAT, such as certain cytokines and factors like RANK ligand, can locally affect the balance between bone formation and resorption, potentially contributing to bone loss.
Furthermore, BMAT volume and function are altered in conditions like obesity, type 2 diabetes, and anorexia nervosa, suggesting its direct involvement in whole-body metabolic regulation. While BMAT expansion is seen in both high-calorie and calorie-restricted states, its specific response to diet and disease is unique and distinct from peripheral fat depots. This highlights BMAT’s role as a unique metabolic sensor, dynamically adjusting its volume and secretory profile to maintain internal balance.