Bone marrow fat (BMF) is the adipose tissue located within the bone cavity, making up about 70% of the bone marrow volume in adults and approximately 10% of total body fat mass in lean individuals. For a long time, BMF was mistakenly considered an inert filler or passive space-occupier within the bone. Current scientific understanding, however, recognizes BMF as a highly dynamic and metabolically active endocrine organ. Its function is far more complex than simple storage, leading researchers to investigate its specific roles in overall health and disease, distinguishing it from peripheral fat depots.
Unique Characteristics of Bone Marrow Fat
Bone marrow fat is structurally distinct from white adipose tissue (WAT). Unlike WAT, BMF is broadly categorized into two major subtypes: constitutive and regulated marrow adipose tissue.
Constitutive BMF (cBMF) is found primarily in the distal bones, such as the lower leg, and develops early in life, remaining relatively stable throughout adulthood. Regulated BMF (rBMF), by contrast, is located in the central skeleton, interspersed among the blood-forming cells, and is highly responsive to metabolic and hormonal changes.
The adipocytes within BMF also have unique cellular properties, including a different fatty acid composition compared to peripheral fat. This dynamic nature and unique location within the rigid, highly vascularized bone environment underscore why BMF is considered a specialized fat depot with distinct functions.
BMF’s Role in Systemic Energy Regulation
Bone marrow fat functions as an energy reservoir, actively participating in the body’s overall energy balance, especially during periods of high demand. BMF adipocytes are capable of storing and releasing fatty acids, which can serve as an energy source for the cells within the bone marrow itself and potentially for distant organs. This metabolic activity is influenced by systemic cues.
The adipocytes in bone marrow also act as an endocrine gland, secreting signaling molecules known as adipokines, such as leptin and adiponectin. These molecules travel through the bloodstream and influence energy metabolism, insulin sensitivity, and appetite regulation in other tissues. The presence of insulin receptors on marrow adipocytes highlights their responsiveness to circulating hormones.
During times of caloric restriction or fasting, there is often a paradoxical increase in BMF volume, even as peripheral fat stores shrink. This expansion in BMF may represent a strategic shift in energy storage, perhaps acting as a local fuel source for the bone microenvironment or as a last-resort reserve. BMF’s metabolic response to stress suggests it plays a specialized, protective role distinct from the primary energy-storage function of white adipose tissue.
Influence on Blood Cell Production
Bone marrow fat is an integral part of the bone marrow niche, the specialized microenvironment that supports the production of all blood cells. The adipocytes actively communicate with the neighboring hematopoietic stem cells (HSCs) and progenitor cells. This local interaction is mediated through direct cell-to-cell contact and the secretion of various factors.
BMF adipocytes release factors like stem cell factor, which is crucial for maintaining and promoting the recovery of HSCs, particularly after cellular stress. They also secrete other molecules, including adiponectin, which can influence the proliferation and differentiation of blood cell lineages. However, an excessive accumulation of BMF can sometimes suppress blood cell production, illustrating a fine balance where an increase in fat volume physically crowds out the hematopoietic tissue. This complex interplay means BMF directly modulates the bone marrow’s capacity to sustain the body’s blood and immune systems.
How Disease and Nutritional Stress Affect Bone Marrow Fat
The volume and metabolic profile of bone marrow fat change significantly in response to various diseases and nutritional stresses, reflecting its role as a sensor of systemic health. BMF volume generally increases with age, which correlates with reduced bone mineral density and conditions like osteoporosis. This age-related expansion suggests a shift in the bone marrow’s composition that can compromise skeletal strength.
In conditions of metabolic disease, such as obesity and type 2 diabetes, BMF accumulation is also observed. This increase in fat is often associated with a less healthy marrow environment and may contribute to the progression of these diseases, potentially driving osteoporosis and fractures. Conversely, in states of severe caloric deprivation, such as anorexia nervosa, BMF volume expands dramatically, indicating its role as a survival mechanism in extreme energy deficits. The contrasting responses in obesity (energy surplus) and anorexia (energy deficit) underscore the unique regulatory function of BMF.