Dietary intake can influence immune function indirectly by affecting the gut microbiota composition and metabolism(1). Fish consumption has been shown to positively regulate the gut microbiota in humans(2,3);albeit in those studies fish was consumed in high amounts (500-750g/week) and immune function was not investigated. This study investigated the effect of consuming the UK dietary recommendation for fish(4) (2 portions [140-280g/week], one of which is oily) on the gut microbiota alpha diversity. Further, we examined if changes (pre- to post-intervention) in the gut microbiota composition were associated with changes in immune cytokine concentrations.

An 8-week randomised controlled trial in low fish consuming women of childbearing age (n = 41; median age 23y) investigated the effect of consuming 1 or 2 portions of fish (tuna or sardines)/week compared to not consuming fish. A blood sample was collected to measure inflammatory cytokines (tumour necrosis factor-a, interleukin [IL]-1b, IL-5, IL-6, IL-17A and IL-22) pre-and post-intervention. Faecal samples were collected at both timepoints and extracted DNA was used to determine gut microbiota compositional profiles using 16S metagenomic sequencing (Illumina, USA). Statistical analysis investigated significant differences in changes in gut microbiota alpha diversity and compositional relative abundances between fish intervention (N = 26) and control (N = 15), then secondary analysis stratified by portion size (1 vs 2 portions) and type of fish (tuna vs sardines). Differences in cytokines between fish intervention and control were assessed by Mann-Whitney U. Spearman rank coefficient assessed associations between the changes in gut microbiota relative abundances with cytokine changes in fish and control groups.

Fish consumption increased gut microbiota alpha diversity indices (Chao1 [7.37±41.23], Simpson [0.003±0.163], Shannon [0.07±0.33], phylogenetic diversity [0.35±2.59], observed species [9.00±40.06]), albeit this was not significant compared to the control group (p>0.05). Consumption of fish, specifically sardines, for 8 weeks significantly reduced Bacteroidetes (-4.77±4.88%) when compared to control (+4.15±7.58%) (p<0.01). No significant differences were observed between the change in relative abundances of gut microbiota at genus-level taxa or inflammatory cytokines between the fish intervention and control. In the fish intervention group, increases in IL-17A, IL-22 and IL-6 concentrations were positively correlated with changes in Alistipes, Rhodococcus, Haemophilus, Barnesiella and Akkermansia relative abundances (p<0.05).

Although not statistically significant, consistent findings suggest that fish intake, in line with dietary guidelines, may have favourable impact on gut microbiota. Sardines, oily fish rich in n-3 polyunsaturated fatty acids, may have health benefits in disease states where Bacteroidetes is elevated; nevertheless, further research is required in a larger cohort over a longer period.