Herb library · Seaweed · Fucus vesiculosus
Bladderwrack
A brown seaweed of Atlantic shores with paired air bladders, sold today mainly for weight loss and blood sugar. Small trials of its extracts found no effect on weight, blood sugar or most heart-risk markers. It contains iodine, as King's Dispensatory already noted in 1898.
Traditional use
A common seaweed of European and American coasts. In the 1800s it was recommended to reduce obesity, and its charcoal ("vegetable ethiops") was used for its iodine. Modern trials have tested its extracts for weight, blood sugar and cholesterol.
What studies show
- Limited· shows no benefit
Weight and blood sugar: in a 12-week trial of 56 overweight adults with prediabetes, a brown seaweed extract (bladderwrack with Ascophyllum) added to a weight-loss diet did not change body weight or blood sugar compared with placebo.
- Limited· shows no benefit
Cholesterol and heart risk: in a 12-week trial of 34 overweight adults with high LDL, a polyphenol-rich bladderwrack extract raised HDL slightly but did not change other heart-risk markers, mood or thinking. The authors call for a larger study.
Be careful of
Cautions
- AmountBrown seaweeds contain iodine in highly variable amounts. The adult upper limit for iodine is 1,100 mcg a day (ODS); long-term intake above it raises the risk of thyroid problems.
- ConditionToo much iodine can cause goiter, an underactive or overactive thyroid, and thyroid inflammation. People with autoimmune thyroid disease may react to amounts safe for others.
- InteractionHigh iodine intake with antithyroid drugs such as methimazole can cause hypothyroidism (ODS).
- Ask a clinicianTalk with a clinician or pharmacist before using this if you take prescription medicines, are pregnant or breastfeeding, or are giving it to a child.
From the old books
What herbals and dispensaries of the past printed. History, in their words.
“Later experiments by numerous good observers do not seem to indicate that sea-wrack has any power as a fat-reducer, but that the reduction which takes place in some instances is undoubtedly due to the dietary measures which are a part of the treatment.”
Sources
- Twelve weeks' treatment with a polyphenol-rich seaweed extract increased HDL cholesterol with no change in other biomarkers (2021) (opens in a new tab) · NIH · read September 25, 2026
- Marginal Impact of Brown Seaweed Ascophyllum nodosum and Fucus vesiculosus Extract on Metabolic and Inflammatory Response in Overweight and Obese Prediabetic Subjects (2022) (opens in a new tab) · NIH · read September 25, 2026
- Iodine: Fact Sheet for Health Professionals (opens in a new tab) · NIH ODS · read September 25, 2026
Reviewed September 25, 2026.