Table 1.

Summarized studies investigating the omega-3 influence on human gut microbiota.

Human Studies	Studied Population	Diets	Method	Main Outcomes
Rajkumar et al. (2014) [12]	60 overweight healthy people	Commercial prebiotic, named VSL#3, vs. VSL#3 + 180 mg EPA and 120 mg of DHA for 6 weeks	Colony counting on anaerobic or aerobic selective media	No difference between groups.
Watson et al. (2017) [13]	20 middle-aged healthy individuals	4 g of mixed DHA/EPA supplement (as capsules and functional drink) for 8 weeks	Sequencing by NGS (Illumina) of 16S rRNA gene, V4 region	No difference for Firmicutes/Bacteroidetes phyla ratio. Increases in the Clostridiaceae, Sutterellaceae, and Akkermansiaceae families in both experimental groups. Increased abundance of Bifidobacterium, Oscillospira, associated with a reduction of Coprococcus and Faecalibacterium genera in both experimental groups. Increased abundance of Lachnospira and Roseburia genera only in functional drink group.
Pu et al. (2016) COMIT study [16]	25 volunteers with risk of metabolic syndrome	60 g of five different unsaturated oil blends for 30 days: conventional canola oil (35.17 g oleic acid), DHA-enriched high oleic canola oil (37.95 g oleic acid and 3.48 g DHA), high oleic canola oil (42.88 g oleic acid), a blend of 25:75 corn/safflower oil (41.61 g linolenic acid), and a blend of 60:40 flax/safflower (22.48 g linolenic acid and 19.19 g ALA)	Sequencing by pyrosequencing of 16S rRNA gene, V1–V3 regions	No difference between groups at phylum level. Highest level of Faecalibacterium genus in high oleic canola oil, and lowest in DHA-enriched high oleic canola oil. Conventional canola was correlated with Coprobacillus and Blautia genera, whereas canola/DHA was associated with the family Lachnospiraceae of the phylum Firmicutes. All the canola oils are correlated with Parabacteroidetes, Prevotella, and Turicibacter genera, and with Enterobacteriaceae family versus the PUFA-rich oils (i.e., corn/safflower and flax/safflower) correlated with the genus Isobaculum.
Balfego et al. (2016) Pilchardus Study [18]	32 patients diagnosed with type 2 diabetes	Standard diet for diabetes supplemented with 100 g of sardines 5 days a week for 6 months (n = 17) (~3 g of EPA + DHA)	qPCR on target bacterial indicators	Firmicutes/Bacteroidetes phyla ratio decrease, while Prevotella genus increase in the omega-3 group.
Noriega et al. (2016) [19]	One healthy 45-year-old man	Daily supplementation of 600 mg of omega-3 PUFAs by fish protein diet, for 2 weeks	Sequencing by NGS (Ion Torrent) of 16S rRNA gene, V4 region	Increase of the phylum Firmicutes and a decrease of Bacteroidetes and Actinobacteria phyla. Reduction in Faecalibacterium genus versus an increase in Blautia, Roseburia, Coprococcus, Ruminococcus and Subdoligranulum genera.
Menni et al. (2017) [20]	Cohort of 876 middle-aged and elderly women	DHA intake of 350 mg/day with a serum concentration of 0.14 mmol/L. (DHA dietary intake determined by Food Frequency Questionnaire)	Sequencing by NGS (Illumina) of 16S rRNA gene, V4 region	This intake is correlated with 21 OTUs belonging to Lachnospiraceae family, 7 OTUs to the Ruminococcaceae family, and 5 to the Bacteroidetes phylum.
Nielsen et al. (2007) [23]	One hundred and fourteen 9-month-old infants	Cow’s milk or infant formula with or without 5 mL/day of fish oil until the 12th month	Fingerprint profiles generated by PCR-DGGE of 16S rRNA gene, V6-8 and V3 regions	Fish oil in cow’s milk groups has a differential fingerprint profile, and this difference was not found in infant formula groups.
Andersen et al. (2011) [24]	One hundred and thirty-two 9-month-old infants	Daily supplementation of 5 mL fish oil (1.6 g EPA + DHA) or sunflower oil (3.1 g linolenic acid, omega-6) for 9 months	Fingerprint profiles generated by T-RFLP of 16S rRNA gene, whole gene	Fish oil gave significant changes in microbiota in comparison with sunflower oil, but only among children who had stopped breast-feeding before the study.
Younge et al. (2017) [25]	32 premature infants with enterostomy	Usual nutritional therapy and an enteral supplementation of a fish and safflower blend oil for a maximum of 10 weeks	Sequencing by NGS (Illumina) of 16S rRNA gene, V4 region	Lower abundance of some pathogenic bacteria as Streptococcus, Clostridium, Escherichia, Pantoea, Serratia, and Citrobacter genera.