Kelp powder, body fat and thyroid hormones in overweight Japanese adults
What it found
In overweight Japanese men, 8 weeks of kelp powder tablets lowered body fat percentage compared with a placebo.
In women, no clear changes were seen, and thyroid hormone levels did not rise.
What they found
Studies in people
Body fat in men
In men, body fat percentage dropped significantly more in the kelp group than in the placebo group. Body weight and BMI also tended to drop, but these differences were not statistically significant.
No clear change in women
In women, body fat percentage, visceral fat area and blood pressure showed no significant changes between the groups. Body weight and BMI rose slightly in the kelp group and fell slightly in the placebo group, but the difference was not statistically significant.
Blood fats overall
Looking at all subjects together, there were no significant differences in blood fat levels between the kelp and placebo groups.
LDL cholesterol subgroup
In people who did not have high blood fats, LDL cholesterol fell in the kelp group and rose in the placebo group after 8 weeks. Total cholesterol showed a similar pattern. No significant differences were seen in people who already had high blood fats.
Thyroid hormones
Thyroid hormone levels (TSH, FT3, FT4) did not increase after the extra iodine from kelp, which added 1.03 mg of iodine a day. The authors note this may be specific to Japanese people who already eat a lot of iodine from seaweed.
Side effects
One person in the kelp group had allergic symptoms, but these were not related to kelp allergy. No other side effects such as stomach problems were seen during the trial.
What the authors conclude
“The intake of iodine-reduced kelp powder led to significant and safe reductions in body fat percentage in overweight male subjects.”
Also in their conclusions
- They conclude that boiled kelp powder enhanced weight loss and improved body fat percentage in overweight male subjects after 8 weeks of supplementation.
- They say boiled kelp powder may reduce serum LDL cholesterol concentrations in non-hyperlipidemic subjects.
- They state that additional iodine from kelp did not cause serum TSH elevation in Japanese subjects with relatively high intake of iodine from seaweed.
How it was done
The authors wanted to see whether iodine-reduced kelp powder could help overweight Japanese people lose body fat, and whether it would affect blood fats or thyroid hormones.
They randomly assigned 50 Japanese adults with a BMI of 25 to under 30 to take either kelp powder tablets (6 g kelp powder, about 3 g alginate a day) or placebo tablets for 8 weeks. Body measurements, blood fats and thyroid hormones were checked before and after.
What it can’t tell you
- The study was small, with only 24 people in each group, so the results may not apply to everyone.
- The findings may not apply to people outside Japan, who often eat much less iodine from seaweed.
- The study lasted only 8 weeks, so it cannot show what happens over a longer period.
Who paid
- Conflicts
- The authors declare no conflict of interest. H.O. is an employee of Maruyanagi Foods Inc., which is a kelp processing manufacturer. F.N. and S.F. are employees of CPCC Company Limited, which is the Contract Research Organization. These companies had no control over the interpretation, writing, or publication of this work.
- Authors work at
- Otsuma Women's University, Japan
The paper
- Title
- Effects of Daily Kelp ( Laminaria japonica ) Intake on Body Composition, Serum Lipid Levels, and Thyroid Hormone Levels in Healthy Japanese Adults: A Randomized, Double-Blind Study
- Type
- Randomised controlled trial
- Evidence
- Studies in people
- Summarised from
- Full text
- Licence
- CC BY. Words in quotation marks are the authors’; the rest is our summary.
- Cite
- Aoe S, Yamanaka C, Ohtoshi H, Nakamura F, Fujiwara S (2021). Effects of Daily Kelp ( Laminaria japonica ) Intake on Body Composition, Serum Lipid Levels, and Thyroid Hormone Levels in Healthy Japanese Adults: A Randomized, Double-Blind Study. Marine drugs. doi:10.3390/md19070352Free full textPubMed 34206160DOI
Summary written 30 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error