Bone marrow fat (BMF), also known as marrow adipose tissue (MAT), is a distinct type of fat found within bone cavities. Unlike subcutaneous or visceral fat, BMF is an integral component of the bone marrow environment. It comprises a significant portion of adult bone marrow volume, up to 70% in adults, and approximately 10% of total body fat in healthy individuals over 25. This fat depot is distributed throughout the skeleton, with higher concentrations in distal sites like the arms and legs compared to central areas such as the spine, pelvis, and sternum.
The Dual Role of Bone Marrow Fat
Bone marrow fat is an active and multifaceted component. One primary function of BMF is its role as a local energy reserve. The adipocytes store lipids, which can be mobilized and utilized by nearby bone cells and hematopoietic cells, the precursors to blood cells, when energy demands arise. This local energy supply supports the high metabolic activity of the bone marrow.
BMF also functions as an endocrine organ, secreting various hormones and signaling molecules, collectively known as adipokines. These secreted factors, such as adiponectin and leptin, communicate with other tissues throughout the body, influencing systemic processes. For instance, adiponectin is associated with improved metabolism and insulin sensitivity, while leptin influences energy homeostasis and inflammation. This endocrine activity highlights BMF’s role in broader physiological regulation.
Connection to Bone Health
A relationship exists between bone marrow fat and skeletal integrity. An inverse correlation is observed: as the amount of fat within the bone marrow increases, bone mass tends to decrease. This shift can elevate the risk of conditions like osteoporosis, characterized by weakened bones, and an increased susceptibility to fractures.
The underlying mechanism involves the shared lineage of bone and fat cells. Both bone-forming cells, called osteoblasts, and fat-storing cells, known as adipocytes, originate from a common type of precursor cell called mesenchymal stem cells (MSCs). An imbalance in the differentiation of these MSCs can lead to a preferential formation of fat cells over bone cells. For example, increased expression of peroxisome proliferator-activated receptor gamma (PPARĪ³) in the bone marrow with aging can favor adipogenesis while suppressing osteogenesis, contributing to age-related bone loss.
Influence on Metabolism and Blood
Beyond its local effects on bone, bone marrow fat exerts systemic influences on the body’s metabolism and blood cell production. The hormones and signaling molecules released by BMF can impact insulin sensitivity, a process where the body’s cells respond effectively to insulin. High levels of certain BMF-derived factors may contribute to systemic inflammation and reduced insulin sensitivity, which are associated with conditions such as type 2 diabetes and obesity. For example, patients with type 2 diabetes exhibit higher levels of saturated fat in their bone marrow cavities.
Bone marrow fat also shares the marrow space with hematopoietic stem cells, which are responsible for producing all types of blood cells, including red blood cells, white blood cells, and platelets. While BMF can provide energy to these cells, excessive accumulation of marrow fat can negatively impact hematopoiesis, the process of blood cell formation. This can disrupt the delicate balance required for healthy blood cell production, potentially affecting the immune system and oxygen transport.
Factors That Alter Bone Marrow Fat
Several factors contribute to changes in bone marrow fat levels throughout life. A primary driver is aging, as BMF increases as individuals grow older. This age-related accumulation is a consistent observation in both humans and mice.
Diet and exercise also play roles in modulating BMF. High-fat diets can lead to an increase in marrow adipose tissue. Conversely, weight-bearing exercise, such as running, has been shown to suppress the accumulation of BMF while simultaneously promoting bone formation.
Certain medical conditions and treatments can also alter BMF. For instance, conditions like anorexia nervosa, obesity, and treatments such as chemotherapy can influence BMF levels. The responses of BMF to these factors underscore its relevance to overall health.