PHYSICAL INACTIVITY DIFFERENTIALLY ALTERS DIETARY OLEATE AND PALMITATE TRAFFICKING

Abstract

Objective: Obesity and diabetes are characterized by the incapacity to use fat as fuel. We hypothesized that this reduced fat oxidation is secondary to a sedentary lifestyle.

Research Design and Methods: We investigated the effect of a 2-month bed rest (BR) on the dietary oleate and palmitate trafficking in lean women (control group, n=8) and the effect of concomitant resistance/aerobic exercise training as a countermeasure (exercise group, n=8). Trafficking of stable isotope-labelled dietary fats was combined with muscle gene expression and MRI-derived muscle fat content analyses.

Results: In the control group, BR increased the cumulative [1-¹³C]oleate and [d₃₁]palmitate appearance in TG (37%, p=0.009 and 34%, p=0.016, respectively) and NEFA (37%, p=0.038 and 38%, p=0.002), and decreased muscle LPL (p=0.043) and FAT/CD36 (p=0.043) mRNA expressions. Plasma NEFA/TG ratios for [1-¹³C]oleate and [d₃₁]palmitate remained unchanged, suggesting that the same proportion of tracers enters the peripheral tissues after BR. BR unaffected [1-¹³C]oleate oxidation but decreased [d₃₁]palmitate oxidation by −8.2±4.9% (p<0.0001). Despite a decreased spontaneous energy intake and a reduction of 1.9±0.3kg (p=0.001) in fat mass, exercise training did not mitigate these alterations, but partially maintained fat-free mass, insulin sensitivity and total lipid oxidation in fasting and fed states. In both groups, muscle fat content increased by 2.7% after BR and negatively correlated with the reduction in [d₃₁]palmitate oxidation (r²=0.48; p=0.003).

Conclusion: While saturated and monounsaturated fats have similar plasma trafficking and clearance, physical inactivity impacts the partitioning of saturated fats towards storage likely leading to an accumulation of palmitate in muscle fat.