High carbohydrate diets increase respiratory quotients above 1 due to lipid synthesis

Abstract

The respiratory quotient (RQ, carbon dioxide emission/oxygen consumption) is believed to range from 0.7–1 depending on the fuels used for catabolism. However, RQ can rise above 1 when CO2 is released without oxygen consumption. This can occur when hexose sugars are catabolized to pentose sugars via the pentose phosphate pathway to produce NADPH for glutathione regeneration. It is also hypothesized RQs > 1 can occur under lipid synthesis when glucose is catabolized by glycolysis to acetylCOA. However, to our knowledge, no prior studies have assessed animal RQ during periods of carbohydrate conversion to lipid. We used stop‐flow respirometry to measure RQ values for nymphal South American locusts (Schistocerca cancellata) that had been consuming artificial diets varying in carbohydrate:protein ratio for 3–5 days. For locusts measured during the hour after a meal, RQ values increased as the carbohydrate:protein content of the previously‐consumed diet increased, to a maximal value of 1.2. Locusts consuming diets high in carbohydrate:protein ratio synthesized fat at high rates, consistent with the hypothesis that an RQ above 1 is an indicator of synthesis of lipid from carbohydrate.Support or Funding InformationThis research was supported by NSF IOS‐1826848 and BARD FI‐575‐18.