Nutritional homeostasis in locusts: is there a mechanism for increased energy expenditure during carbohydrate overfeeding?

Maintenance of carbohydrate balance via changes in CO2 output volume was investigated in locusts using a flow-through respirometer. The effect of an imbalance in the dietary protein to digestible carbohydrate ratio on expired CO2 levels was measured in locusts fed one of two synthetic diets [7% protein, 21% digestible carbohydrate (7:21) and 21% protein, 7% digestible carbohydrate (21:7)]. Additionally, the effect of dietary dilution was investigated by feeding locusts one of two diets with a close-to-optimal ratio of protein to carbohydrate, one containing 7% protein and 7% digestible carbohydrate (7:7) and the other containing 21% protein and 21% digestible carbohydrate (21:21). For insects fed unbalanced diets, a higher CO2 output volume was measured during feeding on diet 7:21 when compared with insects fed on diet 21:7. Locusts also expired a greater volume of CO2 during the entire 2h observation period. This response is consistent with specific metabolic control of carbohydrate balance via enhanced respiration. For insects fed balanced diets, the total volume of CO2 expired over the duration of a meal was greater for insects fed diet 7:7 than for those fed diet 21:21, although this was due entirely to meals lasting longer on the more dilute diet. However, the basal level of respiration rate was greater for insects fed diet 21:21 and, as a result, over the entire 2h period, CO2 output volume did not differ between locusts fed diet 7:7 or 21:21. A possible mechanism for enhanced CO2 output volume on the nutritionally unbalanced diet was investigated, namely triglyceride/fatty-acid substrate cycling. There was no evidence for the presence of the thermogenic effect of this particular cycle on locusts as a means for dealing with excess ingested carbohydrate.