BackgroundHealthy and predictable physiologic homeostasis is paramount in animal models for biomedical research. Proper macronutrient intake is an essential and controllable environmental factor for maintaining animal health and promoting experimental reproducibility.Objective and MethodsEvaluate reductions in dietary macronutrient composition on body weight metrics, composition, and gut microbiome in Danio rerio.MethodsD. rerio were fed reference diets deficient in either protein or lipid content for 14 weeks.ResultsDiets of reduced-protein or reduced-fat resulted in lower weight gain than the standard reference diet in male and femaleD. rerio. Females fed the reduced-protein diet had increased total body lipid, suggesting increased adiposity compared with females fed the standard reference diet. In contrast, females fed the reduced-fat diet had decreased total body lipid compared with females fed the standard reference diet. The microbial community in male and femaleD. reriofed the standard reference diet displayed high abundances ofAeromonas, Rhodobacteraceae, andVibrio. In contrast,Vibriospp. were dominant in male and femaleD. reriofed a reduced-protein diet, whereasPseudomonasdisplayed heightened abundance when fed the reduced-fat diet. Predicted functional metagenomics of microbial communities (PICRUSt2) revealed a 3- to 4-fold increase in the KEGG (Kyoto Encyclopedia of Genes and Genomes) functional category of steroid hormone biosynthesis in both male and femaleD. reriofed a reduced-protein diet. In contrast, an upregulation of secondary bile acid biosynthesis and synthesis and degradation of ketone bodies was concomitant with a downregulation in steroid hormone biosynthesis in females fed a reduced-fat diet.ConclusionsThese study outcomes provide insight into future investigations to understand nutrient requirements to optimize growth, reproductive, and health demographics to microbial populations and metabolism in theD. reriogut ecosystem. These evaluations are critical in understanding the maintenance of steady-state physiologic and metabolic homeostasis inD.rerio.Curr Dev Nutr20xx;x:xx.