Changes in cortical bone response to high-fat diet from adolescence to adulthood in mice

Abstract

Diabetic obesity is associated with increased fracture risk in adults and adolescents. We find in both adolescent and adult mice dramatically inferior mechanical properties and structural quality of cortical bone, in agreement with the human fracture data, although some aspects of the response to obesity appear to differ by age.

Introduction: The association of obesity with bone is complex and varies with age. Diabetic obese adolescents and adult humans have increased fracture risk. Prior studies have shown reduced mechanical properties as a result of high-fat diet (HFD) but do not fully address size-independent mechanical properties or structural quality, which are important to understand material behavior.

Methods: Cortical bone from femurs and tibiae from two age groups of C57BL/6 mice fed either HFD or low-fat diet (LFD) were evaluated for structural and bone turnover changes (SEM and histomorphometry) and tested for bending strength, bending stiffness, and fracture toughness. Leptin, IGF-I, and non-enzymatic glycation measurements were also collected.

Results: In both young and adult mice fed on HFD, femoral strength, stiffness, and toughness are all dramatically lower than controls. Inferior lamellar and osteocyte alignment also point to reduced structural quality in both age groups. Bone size was largely unaffected by HFD, although there was a shift from increasing bone size in obese adolescents to decreasing in adults. IGF-I levels were lower in young obese mice only.

Conclusions: While the response to obesity of murine cortical bone mass, bone formation, and hormonal changes appear to differ by age, the bone mechanical properties for young and adult groups are similar. In agreement with human fracture trends, adult mice may be similarly susceptible to bone fracture to the young group, although cortical bone in the two age groups responds to diabetic obesity differently.

Fig. 1

Body composition, serum leptin concentration, and IGF-I concentration. a Average weekly weights of LFD and HFD groups. Horizontal axis is progression of study in weeks; b young and f adult lean body mass; c young and g adult fat body mass for LFD and HFD groups at conclusion of study; d young and h adult serum leptin concentration (mean ± SE) at conclusion of study; e young and i adult serum IGF-I concentrations at the conclusion of study. Both lean body mass and fat body mass increased, but signficant increase in IGF-I concentration are only observed for the yHFD group. yLFD n = 15, yHFD n = 15, aLFD n = 13, aHFD n = 14 (^** p < 0.01, ^*** p < 0.001)

Fig. 2

Bone mineral. a Young and e adult whole-body bone mineral density (aBMD) is unchanged in HFD; b young and f adult whole-body areal bone mineral content (BMC) is lower for the yHFD vs. yLFD, which is likely due to reduced spinal aBMD. c Young and g adult areal bone mineral density of the femora are unchanged; d young and h adult areal bone mineral density of the spine are reduced for HFD despite increasing weight, leptin, and IGF-I. yLFD n = 15, yHFD n = 15, aLFD n = 13, aHFD n = 14 (^*** p < 0.001)

Fig. 3

Cortical bone size. a Young and d adult cortical thickness is reduced in adults only; b young and e adult femoral diameters are increased in yHFD vs. yLFD; c young and f adult femoral lengths are unchanged. g Histomorphometry results: Ma.Ar. marrow area (mm²), T.Ar. total cros-sectional area (mm²), Mean Ct.Wi. mean cortical width (μm), Ps.BFR and Ec.BFR periosteal and endocortical bone formation rate (μm³/μm²/γ). The general trend in the bone size data points to decreasing bone size in adults and increasing bone size in young obese mice compared to LFD, as well as a shift from periosteal activity to endosteal activity with age. yLFD n = 15, yHFD n = 15, aLFD n = 13, aHFD n = 14 (^* p < 0.05, ^** p < 0.01, ^*** p < 0.001)

Fig. 4

Cortical bone quality: whole-bone and tissue-level mechanical property measurements. a Young and f adult bending modulus; b young and g adult maximum load; c young and h adult yield stress; d young and i adult max stress; e young and j adult fracture toughness. Measured size-independent mechanical properties were significantly decreased for HFD group vs. LFD groups (modulus, yield and maximum stress, and fracture toughness); these parameters are an indication of bone tissue quality. Size-dependent measures which address whole-bone behavior (specifically, load) also declined for HFD at both ages, likely due in part to modest bone size changes, as bone size was not able to compensate for poor mechanical quality. yLFD n = 15, yHFD n = 15, aLFD n = 13, aHFD n = 14 (^* p < 0.05; ^** p < 0.01)

Fig. 5

SEM images of the fracture region showing cortical bone tissue structure changes at the posterior region. a yLFD group; b yHFD; c aLFD; d aHFD. The scale bar indicates 20 μm. The posterior cortex in HFD bone in (b) and (d) shows reduced alignment of osteocyte lacunae and reduction in lamellar alignment at the tissue level. These images are representative of three samples each of aHFD, yHFD, aLFD, and yLFD. Medial, lateral, and anterior portions of the bone sections appeared similar for HFD and LFD in both age groups