The interplay between voluntary food intake, dietary carbohydrate-lipid ratio and nutrient metabolism in an amphibian, (Xenopus laevis)

Abstract

Digestion of food and metabolism of frogs are little studied at the moment, and such processes could be very particular in the case of amphibians, given their ectothermic and carnivorous nature which may lead them to use nutrients through specific biochemical pathways. In the present study, 24 adult Xenopus laevis (six replicates with two frogs per treatment) were randomly assigned to two diets with different carbohydrate:fat ratio (4.5:1 and 2.1:1), changing the dietary glucogenic and lipogenic proportions. Food intake (FI) per unit metabolic body weight (MBW) as well as macronutrient digestibility were calculated, and circulating blood acylcarnitines and amino acids measured, in order to evaluate the effect of the diet treatments. Results demonstrated that food intake regulated most of the changes in the parameters evaluated; significant differences were obtained in crude protein and fat digestibilities through the effect of FI/MBW (p<0.05), whereas diet treatment had a significant effect on the levels of malonyl-CoA. Food intake also significantly impacted malonyl, isovaleryl, hydroxyisovaleryl and long chain fatty acid concentrations; significant (p<0.05) interactions between those metabolites were detected owing to diet. The findings obtained suggest that food intake was the main factor controlling digestion and metabolism in X. laevis, with frogs preferring to utilise protein and fat as primary sources for energy production in the citric acid cycle, reflecting characteristics of a strict carnivore physiological model.

Current Feeding Practice of Xenopus laevis in a Laboratory Setting

“…This is in accordance with literature on feeding practice and recommendations for Xenopus laevis [4,10,11,31]. Protein utilization is known to be the major energy source in this species [30]. Further research on species-specific nutrient requirements [32] is certainly necessary.…”

“…Because of the capacity of many amphibians to overfeed [29], the voluntary intake may not be the ideal marker for an adequate amount of feed. There is also the possibility that the actually consumed amount of feed differs from the average amounts given in the questionnaire, i.e., higher amounts fed in ration A or frogs not consuming the whole amount in rations D and E. There is evidence that feed intake influences digestibility in Xenopus frogs [30], so that the adequate amount of feed will affect the phenotype. Systematic studies are needed to determine the adequate energy supply for Xenopus frogs depending on water temperature and other factors.…”

A Novel Animal Model in Diabetes: Regeneration of β cells in the Axolotl Salamander?

“…In metabolic studies of amphibians, it is worth noting that amphibians are carnivores, and their macronutrient requirements are different from species such as rodents because they rely more on lipid/protein uptake than carbohydrates [26]. In other carnivores, such as the domestic cat, protein has a more prominent role as a gluconeogenic substrate than carbohydrates [27].…”

No longer a leap in the dark: the importance of protein as an energy source in amphibians

“…Clearly, protein and fat would become the major sources of energy in animals eating these items. From experimental work with Xenopus laevis, protein and lipids have been identiied as preferred energy sources, and their use in metabolism is associated with voluntary food intake (Brenes-Soto et al 2018), denoting anurans as 'true carnivores' with a unique energy and glucose metabolism, determined through both amino and fatty acid anabolic and catabolic routes (Case et al 2000). Yet, no substantial data are available on nutrient metabolism of amphibians in the wild.…”