Exercise Degrades Bone in Caloric Restriction, Despite Suppression of Marrow Adipose Tissue (MAT)

Abstract

Marrow adipose tissue (MAT) and its relevance to skeletal health during caloric restriction (CR) is unknown: It remains unclear whether exercise, which is anabolic to bone in a calorie-replete state, alters bone or MAT in CR. We hypothesized that response of bone and MAT to exercise in CR differs from the calorie-replete state. Ten-week-old female B6 mice fed a regular diet (RD) or 30% CR diet were allocated to sedentary (RD, CR, n = 10/group) or running exercise (RD-E, CR-E, n = 7/group). After 6 weeks, CR mice weighed 20% less than RD, p < 0.001; exercise did not affect weight. Femoral bone volume (BV) via 3D MRI was 20% lower in CR versus RD (p < 0.0001). CR was associated with decreased bone by μCT: Tb.Th was 16% less in CR versus RD, p < 0.003, Ct.Th was 5% less, p < 0.07. In CR-E, Tb.Th was 40% less than RD-E, p < 0.0001. Exercise increased Tb.Th in RD (+23% RD-E versus RD, p < 0.003) but failed to do so in CR. Cortical porosity increased after exercise in CR (+28%, p = 0.04), suggesting exercise during CR is deleterious to bone. In terms of bone fat, metaphyseal MAT/ BV rose 159% in CR versus RD, p = 0.003 via 3D MRI. Exercise decreased MAT/BV by 52% in RD, p < 0.05, and also suppressed MAT in CR (-121%, p = 0.047). Histomorphometric analysis of adipocyte area correlated with MAT by MRI (R² = 0.6233, p < 0.0001). With respect to bone, TRAP and Sost mRNA were reduced in CR. Intriguingly, the repressed Sost in CR rose with exercise and may underlie the failure of CR-bone quantity to increase in response to exercise. Notably, CD36, a marker of fatty acid uptake, rose 4088% in CR (p < 0.01 versus RD), suggesting that basal increases in MAT during calorie restriction serve to supply local energy needs and are depleted during exercise with a negative impact on bone. © 2019 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Keywords: BONE-FAT INTERACTIONS; EXERCISE; MARROW ADIPOSE TISSUE (MAT).

Figure 1

Caloric restriction reduces body and perigonadal fat pad weight. B6 mice after 6 weeks of 30% caloric restriction (CR) versus regular diet (RD) +/− running exercise (E) (4 groups, n = 7–10/group). (A) Body weight. (B) Food intake (g/d). (C) Running distance (km/d) and average running velocity (km/hr). (D) Fat pad weight. Mean ± SD. Significance by 2‐way ANOVA or t test.

Figure 2

Bone quantity was degraded by exercise in caloric restriction. Tibial bone microarchitecture via μCT in B6 mice after 6 weeks of 30% caloric restriction (CR) or regular diet (RD) +/−running exercise (E) (n = 7/group). (A) Trabecular parameters. (B) Cortical parameters. Plots represent means ± SD. Significance by 2‐way ANOVA. For multiple comparisons, *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001****p ≤ 0.0001.

Figure 3

Exercise associated with reduced marrow adipose tissue (MAT), even in the setting of caloric restriction. Average MRI group images (n = 6–9/experimental group) in sagittal (A), coronal (B) planes color labeled for quantity of lipid. B6 mouse femurs analyzed 6 weeks after 30% caloric restriction (CR) or regular diet (RD) +/− running exercise (E) via MRI with advanced image processing. (C) Bone volume (BV) and marrow adipose tissue (MAT) quantification via MRI. Mean ± SD. Significance by 2‐way ANOVA. For multiple comparisons, *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001.

Figure 4

Exercise, in the setting of caloric restriction, attenuates markers of bone resorption. (A) TRAP (red) stain of osteoclasts in representative histologic sections of tibias after 6 weeks of 30% CR +/− exercise. (B) Osteoclasts quantified via semi‐automated histomorphometry in mouse femurs (n = 4–6/ group) with individual mice plotted. (C) Tibial mRNA via qPCR (n = 4/group). Means ± SD. Significance by 2‐way ANOVA between experimental groups. For multiple comparisons, *p ≤ 0.05.

Figure 5

Caloric restriction, exercise attenuation of marrow adipocyte area, marker of fatty acid metabolism. Histomorphometric analysis performed on 3 sections per mouse (n = 5–6) with a minimum of 300 cells analyzed for each experimental group. (A) Marrow adipocyte area histogram. (B) Marrow adipocyte area. (C) qPCR on mRNA from tibias (n = 4). (D) Linear correlation plot of adipocyte area versus MAT/BV via MRI analysis; both performed in distal femoral metaphysis. Plots demonstrate means ± SD. Significance by 2‐way ANOVA between experimental groups. For multiple comparisons, *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001, ****p ≤ 0.0001.