Eating kombu daily enhances gut health and heart markers in men

By Priyanjana Pramanik, MSc.May 20 2025

New research reveals that eating kombu seaweed daily may help Japanese men lose body fat and improve their gut bacteria, without affecting thyroid health or causing adverse effects.

Effects of Daily Kombu (Laminaria japonica) Intake on Body Composition, Blood Pressure, and Fecal Microbiota in Healthy Adult Japanese: A Randomized, Double-Blind Study. Image Credit: boommaval / Shutterstock

In a recent article published in the journal Food Science & Nutrition, researchers evaluated whether consuming boiled kombu powder causes changes in gut microbiota and body fat reductions for Japanese adults. Their findings indicate that kombu intake significantly reduced the percentage of body fat and increased serum adiponectin in males, while lowering systolic blood pressure in both sexes; it also caused positive changes in the gut microbiota without adversely affecting thyroid function, despite a daily iodine intake of 420µg.

Background

Metabolic syndrome increases the risk of cardiovascular diseases and is linked with issues such as insulin resistance, high blood pressure, and abnormal lipid levels. Accumulation of visceral fat is believed to be a key contributor to this condition.

Diets that limit the accumulation of visceral fat could help prevent metabolic syndrome. Kombu, a type of edible kelp, contains high levels of dietary fiber, particularly sodium alginate, which has been associated with health benefits like regulating blood sugar and reducing blood cholesterol. One proposed mechanism is that studies suggest alginate forms a stomach gel that hinders glucose and cholesterol uptake.

These effects are also thought to occur through increased excretion of bile acid, changes in liver cholesterol synthesis due to fermentation products, and reductions in insulin secretion.

Kombu also slows digestion and nutrient absorption, which may help combat obesity. It also contains other fibers like fucoidan and laminarin, which might uniquely impact the gut microbiota.

However, no studies have explored kombu’s long-term effects on fat accumulation and gut microbiota among Japanese individuals through dietary consumption (rather than supplements).

About the study

This 12-week, randomized, double-blind, placebo-controlled study aimed to assess the effects of boiled kombu powder on gut microbiota and body fat among Japanese adults with a body mass index (BMI) of 23 kg/m² or more.

Participants, between the ages of 20 and 65, were required to have regular eating habits and were excluded if they used functional health foods, certain medications, disliked kombu, consumed excessive alcohol, had iodine restrictions, had serious medical conditions, or had recently donated blood.

A total of 44 participants were randomized into kombu or placebo groups, balanced by BMI, age, sex, visceral fat area, and low-density lipoprotein (LDL) cholesterol levels.

The intervention involved consuming six cookies daily (replacing a staple meal) that contained either boiled kombu powder or microcrystalline cellulose (placebo). Each test cookie included 0.6 g of alginate and 0.07 mg iodine (contributing to a total daily intake of 0.42 mg or 420µg from the cookies).

The kombu was processed to reduce iodine before being dried and powdered. Throughout the study, participants recorded their daily intake and diet. Dietary records were analyzed using standardized Japanese food composition tables.

Anthropometric data, blood, and stool samples were collected at baseline and week 12. Visceral fat was measured using tomographic scanning, and blood samples were analyzed for hormones and metabolic markers.

Stool samples were used for microbiota analysis. Genetic material was extracted from fecal samples and sequenced to identify microbial composition using gene sequencing, after which bioinformatic tools were used to classify and analyze microbial taxa and diversity.

Key findings

Of 44 participants, 41 completed the trial of kombu (boiled seaweed) cookies or a placebo. Baseline characteristics and nutrient intake were similar across groups. LDL- and high-density lipoprotein (HDL)-cholesterol levels decreased in both groups, with no between-group differences. Serum uric acid increased significantly only in the placebo group.

Among males, kombu intake led to significantly greater reductions in body fat content and percentage compared to placebo. In females, such reductions were not observed; in fact, at 12 weeks, the body fat percentage was significantly higher in the female kombu group compared to the female placebo group, though temporary increases in weight and fat measurements had also been observed in females in the kombu group at week 6. Systolic blood pressure decreased significantly at weeks 6 and 12 in both sexes in the kombu group, and diastolic pressure decreased at week 6.

Serum adiponectin increased significantly in males consuming kombu, but no changes were observed in females. Serum leptin levels and thyroid hormone levels remained unchanged, confirming that the iodine intake from kombu did not negatively impact thyroid function. No adverse effects related to kombu were reported, except for one case of loose stools.

Visceral and subcutaneous fat areas did not significantly differ between groups.

Fecal microbiota in the group consuming kombu showed higher levels of Agathobacter, Alistipes, Bacteroides, and Faecalibacterium; Agathobacter abundance in particular showed a negative correlation with fat mass and body fat percentage in male participants. Furthermore, the Simpson's alpha diversity index, a measure of gut microbiota diversity, was significantly increased in the kombu group compared to the placebo group. However, there were no significant differences in fecal short-chain fatty acid (SCFA) concentrations between the groups, which the researchers suggest might be due to rapid absorption and utilization of SCFAs in the large intestine.

Conclusions

The study suggests kombu intake may reduce body fat in men and blood pressure in both sexes, without adverse effects or disrupting thyroid function despite the specified iodine intake. Its high compliance and safety support its potential as a dietary intervention.

A major strength was the controlled design and gender-stratified analysis, revealing sex-specific effects. However, the small sample size and short duration limit generalizability. The differing results in females, including the temporary changes and the comparative increase in body fat percentage at the end of the study, and lack of significant effects on fat distribution or leptin, suggest that responses may differ by gender and warrant further exploration. The lack of change in fecal SCFA concentrations, despite positive microbiota shifts, also merits further investigation, potentially through in vitro fermentation studies as suggested by the authors.

Future studies should involve longer durations, larger samples, and a focus on underlying mechanisms, including hormonal and microbiome interactions.