Bone Abstracts

In adults, majority of bone marrow (BM) space of long bones is filled with fat tissue. Adipocytes are also present within trabecular bone areas such as vertebral bodies. Despite its prevalence the roles of BM fat in energy and bone metabolism have been largely overlooked. To characterize bone marrow metabolic activity we measured regional glucose uptake in femoral and vertebral bone marrow during fasting and insulin stimulation in normal weight healthy subjects.

Nine healthy adults (age 47±6 years, BMI 23.7±1.9 kg/m²) volunteered for the study. The subjects were imaged with positron emission tomography (PET) using ¹⁸F- fluorodeoxyglucose (¹⁸F-FDG) tracer to measure glucose uptake (GU) in skeletal muscle, abdominal subcutaneous fat, abdominal visceral fat and vertebral and femoral bone marrow. PET imaging was performed at fasting state and during hyperinsulinemic euglycemic clamp to measure basal and insulin-stimulated GU.

Fasting GU in femoral BM was significantly higher than in subcutaneous fat (4.93±1.58 vs 2.82±0.38 μmol/l per min, respectively, P<0.05) but did not significantly differ from visceral fat. Skeletal muscle GU was 56% higher than that of femoral BM (P<0.01). Interestingly, glucose uptake in vertebral BM that contains bone and hematopoietic cells and adipocytes, was five-fold higher than in femur (P<0.001). Insulin stimulation during clamp induced a four-fold increase in femoral BM GU (20.43±6.00 μmol/l per min, P<0.001 vs fasting state), which remained higher than that of sc and visceral fat. Surprisingly, insulin did not stimulate glucose uptake in vertebral bone marrow (25.98±3.46 clamp vs 24.78±4.59 μmol/l per min at fasting).

This study shows that glucose metabolism differs significantly between vertebral and femoral BM. GU in vertebral BM cells appears to be insulin independent. Conversely, insulin stimulates GU in the mainly fatty femoral BM to similar extent as in brown fat. Moreover, the overall GU in femoral BM both in fasting state and during hyperinsulinemic euglycemic clamp is higher than in other fat depots. Thus, our data supports the hypothesis of bone marrow fat as functionally distinct ‘yellow fat’.