The present study tested the hypothesis that the liver lipid-lowering effect of insect meal (IM) is caused by its low methionine concentration. A total of fifty, male obese Zucker rats were randomly assigned to five groups of 10 rats each (casein (C), IM, IM + Met, IM + Cys, and IM + EAA). While group C received a diet with casein, the IM-fed groups received a diet with IM as the protein source. In groups IM + Met, IM + Cys and IM + EAA, the diets were additionally supplemented with methionine, cysteine and essential amino acids (EAA), respectively. Hepatic concentrations of triacylglycerols and cholesterol, and hepatic mRNA levels and activities of lipogenic and cholesterogenic enzymes were markedly lower in the IM-fed groups than in group C (p < 0.05). All of these parameters either did not differ across the IM-fed groups or were only slightly higher in groups IM + Met, IM + Cys and IM+EAA than in the group IM. In conclusion, the results indicate that a difference in the amino acid composition between IM and casein, a low concentration of methionine in IM and a reduced cysteine synthesis secondary to a decreased methionine availability resulting from feeding IM are not causative for the lipid-lowering effect of IM.

Background In contrast to protein-rich insect meal, the feed potential of insect fat is generally less explored and knowledge about the suitability of insect fat as a fat source specifically in broiler diets is still limited. In view of this, the present study aimed to comprehensively investigate the effect of partial (50%) and complete replacement of soybean oil with insect fat from Hermetia illucens (HI) larvae in broiler diets on performance, fat digestibility, cecal microbiome, liver transcriptome and liver and plasma lipidomes. Thus, 100 male, 1-day-old Cobb 500 broilers were randomly assigned to three groups and fed three different diets with either 0 (group HI-0, n = 30), 2.5% (group HI-2.5, n = 35) or 5.0% (HI-5.0, n = 35) Hermetia illucens (HI) larvae fat for 35 d. Results Body weight gain, final body weight, feed intake, and feed:gain ratio during the whole period and apparent ileal digestibility coefficient for ether extract were not different between groups. Cecal microbial diversity did not differ between groups and taxonomic analysis revealed differences in the abundance of only four low-abundance bacterial taxa among groups; the abundances of phylum Actinobacteriota, class Coriobacteriia, order Coriobacteriales and family Eggerthellaceae were lower in group HI-5.0 compared to group HI-2.5 (P < 0.05). Concentrations of total and individual short-chain fatty acids in the cecal digesta were not different between the three groups. Liver transcriptomics revealed a total of 55 and 25 transcripts to be differentially expressed between groups HI-5.0 vs. HI-0 and groups HI-2.5 vs. HI-0, respectively (P < 0.05). The concentrations of most lipid classes, with the exception of phosphatidylethanolamine, phosphatidylglycerol and lysophosphatidylcholine in the liver and cholesterylester and ceramide in plasma (P < 0.05), and of the sum of all lipid classes were not different between groups. Conclusions Partial and complete replacement of soybean oil with HI larvae fat in broiler diets had no effect on growth performance and only modest, but no adverse effects on the cecal microbiome and the metabolic health of broilers. This suggests that HI larvae fat can be used as an alternative fat source in broiler diets, thereby, making broiler production more sustainable.

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