Bone marrow (BM) is the porous tissue found inside bones that consists of adipose tissue, hematopoietic cells, and other stem cells. BM adipose tissue has been shown to have high basal glucose uptake in humans, exceeding that of white adipose tissue. Obesity has been shown to suppress bone metabolism and increase the risk of insulin resistance and type 2 diabetes. Exercise training is known to improve bone mineral density and whole-body insulin sensitivity reducing the risk of type 2 diabetes. However, the effects of exercise training on BM metabolism remain unclear.

The aim of this thesis was to study the effects of short-term and long-term exercise training on BM metabolism using positron emission tomography (PET). We studied the effects of two weeks of exercise training on BM glucose and free fatty acid metabolism in healthy and insulin resistant participants. We also studied the effects of six months of regular exercise training on BM glucose metabolism in monozygotic twin pairs that were discordant for body weight. Lastly, to gain insight into the mechanisms in more detail, we studied the effects of high-fat diet and exercise training on BM glucose metabolism in rats.

We found that BM metabolism differs regarding anatomical location in both humans and rats, being higher in vertebrae and sternum and lower in long bones of the extremities. Already two weeks of exercise training improved BM glucose and free fatty acid metabolism regardless of glycemic status. Regular exercise training also improved femoral BM glucose metabolism. Interestingly lumbar vertebral BM glucose uptake was higher in participants with higher body weight and exercise training decreased lumbar vertebral BM metabolism even without reduction in weight. In rats, BM glucose metabolism was decreased by age but not significantly affected by the interventions.

In conclusion, this thesis suggests that BM is metabolically active and responds to exercise training. The response depends on its biological function and anatomical location.