Modulation of Gut Microbiota by Whole Encapsulated Brown Seaweed (Himanthalia elongata) in Overweight Subjects: A Randomized Double-Blind Placebo-Controlled Trial

<b>Background/Objectives</b>: Brown seaweeds, such as <i>Himanthalia elongata</i>, are a promising source of dietary fiber. However, in vivo evidence regarding the effects of <i>H. elongata</i> intake on the human gut microbiota remains limited. This study aimed to evaluate the impact of daily <i>H. elongata</i> consumption on the gut microbiota composition and short-chain fatty acid production in overweight adults. <b>Methods</b>: A randomized, double-blind, placebo-controlled trial was conducted in which 10 overweight adult participants consumed 2 g/day of whole <i>H. elongata</i> for 30 days. Fecal samples were collected before and after the intervention for 16S rRNA sequencing and short-chain fatty acid analysis. Dietary intake was evaluated using a 24 h recall and a 3-day dietary record. Nutritional assessment was performed to determine habitual macronutrient consumption. <b>Results</b>: Baseline dietary analysis revealed an imbalanced macronutrient profile characterized by high intakes of total and saturated fats and protein, along with low carbohydrate and fiber consumption. In addition, 50% of the participants were obese, and 50% were overweight based on the BMI. Notable changes in the gut microbiota composition were observed after the intervention, including increases in short-chain fatty acid-producing species, such as <i>Parabacteroides distasonis</i>, <i>Bacteroides eggerthii</i>, <i>Bacteroides uniformis</i>, and <i>Bacteroides obeum</i>. <b>Conclusions</b>: This study provides the first clinical evidence in humans that whole <i>H. elongata</i> can beneficially modulate the gut microbiota composition. These results support the potential use of this seaweed as a functional prebiotic ingredient in dietary strategies aimed at enhancing gut health.