High Fat Diet-Induced Obesity Negatively Affects Whole Bone Bending Strength but not Cortical Structure in the Femur

Although body mass index is positively associated with bone mineral density, suggesting obesity is protective against fracture, elderly obese individuals experience greater fracture risk at certain sites than non-obese peers, suggesting bone structural or material changes contribute to fragility. Diet-induced obesity rodent studies have reported detrimental changes to bone microstructure and some apparent-level material properties, but tissue-level material changes are not well understood. Because adipose tissue is highly vascularized, and bone remodeling depends critically on functional vascular supply, concurrent effects on osteovascular perfusion and structure may provide insight about obesity-related bone fragility. This study aimed to determine the effects of obesity on both tissue-level bone properties and osteovascular properties that could negatively impact bone strength. Five-week-old male C57Bl/6J mice were fed either high fat diet (HFD) or control fat diet (CFD) for 17 weeks and received daily treadmill exercise or remained sedentary for eight weeks at ages 14-22 weeks. HFD negatively affected femur bending strength, with 18% lower yield load than CFD. Although HFD negatively altered cancellous microstructure in the distal femur, with 32% lower bone volume fraction than CFD, it did not affect cortical bone geometry in the femoral metaphysis or diaphysis. HFD caused increased carbonate substitution but had no effect on other composition metrics or apparent- or tissue-level material properties in the femoral diaphysis. Exercise did not affect bone strength or microstructure but increased endosteal mineralizing surface in the tibial diaphysis, mineral crystallinity and mineral-to-matrix ratio in the femur, and blood supply to the proximal tibial metaphysis. HFD did not affect blood supply in the tibia or 2D osteovascular structure in the distal femoral metaphysis, indicating that HFD negatively affects cancellous bone without affecting osteovasculature. This study reveals that HFD negatively affected cancellous microstructure without affecting osteovascular structure, and whole-bone strength without altering cortical geometry or material properties.