Accelerated physiology and increased energy expenditure in animals and humans with mitochondrial defects: A meta-analysis
Mitochondria are key energy transforming organelles in mammalian cells. However, how defects in oxidative phosphorylation (OxPhos) and other mitochondrial functions influence whole-body energy expenditure (EE) has not been rigorously studied. Cellular and organismal responses to OxPhos defects likely involve a combination of functional downregulation to conserve energy and compensatory upregulation of stress responses. If the energy cost of compensatory responses exceeds the potential energy savings of functional downregulation, as recent work suggests, the result would be an increase in total EE. To address the hypothesis that OxPhos defects increase the energetic cost of living, we performed a meta-analysis of available studies reporting EE in animal models with mitochondrial gene defects. Of all reported experimental conditions (n = 91, from 29 studies), 51% reported a >10% elevation in EE relative to control animals, compared to 11% reporting <10% reduction in EE (p<0.0001, Chi-square). Of the experimental conditions where locomotor activity was also quantified, 39% showed that OxPhos-deficient animals had elevated EE despite reduced locomotor activity, which would be expected to decrease EE. To extend this finding in humans, we re-analyzed a high-quality clinical and multi-omics dataset (Sharma et al. 2021) of mitochondrial disease patients with the m.3243A>G mutation. This analysis similarly indicates an upregulation of energetically costly physiological, immune, and metabolic parameters in people with OxPhos deficiency. These results suggest that animals and humans with mitochondrial defects must expend more energy to sustain life, a state clinically called hypermetabolism. High-quality human energetics studies are needed to understand the magnitude, mechanisms, and modifiability of hypermetabolism in mitochondrial disorders.