Dietary inclusions of the brown macroalgae, Ascophyllum nodosum and Fucus vesiculosus, favourably modulate fat deposition and the caecal microbiome of obese mice

Effective preventive measures are needed to lower the global incidence of obesity in humans. We investigated whether the dietary inclusions of brown macroalgae Ascophyllum nodosum (AN) or Fucus vesiculosus (FV) prevent diet-associated obesity development in mice by modulating the gut microbiome and body composition. Female C57BL/6J mice were fed a low-fat control, high-fat (HF), or HF diet with 5 % inclusion of whole AN or FV biomass for 12 weeks. Body composition, whole-body energy metabolism, caecal microbiome, and liver transcriptomics were evaluated. Mice fed AN and FV-added HF diets had lower gonadal (∼30 %), subcutaneous (∼25 %), and total (∼40 %) fat mass compared to the mice fed the HF diet, although the differences were statistically non-significant. Macroalgae-supplemented diet groups had reduced Firmicutes: Bacteroidota ratios and lower abundances of obesity-associated bacterial genera, including Lachnoclostridium, Lachnospiraceae , Lactococcus, Romboutsia , and Tuzzerella in the caecal contents, compared to the HF diet group (more prominently with FV-supplementation). These bacteria were found to be positively associated with fat mass gain. On the other hand, macroalgal addition promoted certain leanness-associated and short-chain fatty acids-producing bacteria, where FV-addition most prominently enriched Alistipes, Akkermansia, Muribaculum, and Oscillospiraceae NK4A214 group, while AN-addition highly promoted the Bacteroides . This study shows that dietary inclusion of AN or FV in the HF diet promotes a unique and healthier gut microbiota and lowers the HF diet-associated fat mass gain in mice. • The effects of dietary inclusions of brown macroalgae on diet-induced obesity and caecal microbiome were studied in mice. • Macroalgae inclusion in the diet led to a ∼40 % reduction in the total fat mass gain. • Brown macroalgae reduced the relative abundance of obesity-associated gut microbial communities during high-fat exposure. • Fucus vesiculosus was more effective than Ascophyllum nodosum in preventing high-fat diet associated gut microbiome changes.