Caloric restriction exerts site-, sex-, and duration-dependent effects on skeletal structure and bone marrow adiposity

Bone marrow adipose tissue (BMAT) is a unique fat depot. It has distinct metabolic-endocrine functions and negatively correlates with bone mineral density, increasing with aging and osteoporosis. Intriguingly, BMAT also expands during caloric restriction (CR), a lifespan-extending dietary intervention that also promotes bone loss; however, whether BMAT is a cause or consequence of this remains unclear. To address this, we studied 9-week-old male and female C57BL/6NCrl mice subjected to 30% CR for 1, 2, 4, or 6 weeks. Bone structure and BMAT volume were quantified by micro-computed tomography using a spatial method that precisely maps BMAT within bones. CR-induced BMAT expansion was site-specific, being greater in tibiae than femora and absent in humeri. Surprisingly, CR increased distal tibial BMAT despite prior suggestions that it resists such environmental modulation. Expansion was also duration-dependent, plateauing after 4 weeks; and region-specific, particularly in tibial metaphysis, femoral metaphysis, and the distal tibia. Similarly, CR altered trabecular and cortical bone in time-, region-, and sex-dependent ways, with BMAT levels more tightly associated with trabecular than cortical changes. CR also increased adiponectin, corticosterone, and ketones, while decreasing leptin, IGF-1, and insulin, in a sex- and/or duration-dependent manner. Metabolic phenotyping demonstrated that BMAT expansion was not associated with altered glucose tolerance, body mass, total fat mass, or fat percentage, but correlated with energy deficit and systemic lipid mobilisation. Together, our findings highlight the complex interplay between BMAT expansion, skeletal remodelling and metabolic homeostasis, providing new insights into BMAT formation and function and the health benefits of CR.