Can seaweed help with weight loss?


DOES IT WORK? BACKGROUNDSeaweed has been an important natural resource for coastal communities. In Ireland it was gathered both for food and as a fertiliser.

More recently, seaweed has been harvested to make thickening and stabilising agents for products ranging from paints to foods and cosmetics.

Seaweeds are algae, not plants, and come in many species. Carrageen or Irish Moss is a red algae. One of the most common seaweeds found around Ireland is the brown seaweed, Fucus vesiculosis. It goes by many other names, including bladderwrack, Atlantic kelp and sea oak.

Having been an important source of calories in the past (and currently in Japanese dishes), brown seaweed is getting a lot of attention these days as a way to lose calories.


Early claims about seaweed and weight loss centred on its iodine content. Iodine was first isolated from brown seaweed in 1811. The metal is an essential trace element required for thyroid hormones which regulate basal metabolic rates. Too little iodine leads to a condition called hypothyroidism. Its symptoms include fatigue, weight gain, low body temperature, depression and goitre (swollen thyroid glands).

Since weight gain is a symptom of iodine deficiency, some have recommended iodine supplements to lose weight and brown seaweed as a natural source of iodine. While the recommendation makes sense, it will only work if people lack iodine or have thyroid problems. Controlled studies of seaweed for weight loss have not been conducted.

Some research has examined extracts of brown seaweed for weight loss. One product called alginate is a carbohydrate that forms a thick gel when it encounters stomach acid, leaving people feeling full and eating less. The first controlled study found that people taking 1.5 alginate consumed, on average, 170 fewer calories per day.

Scientists in Japan have identified an ingredient in several brown seaweeds called fucoxanthin. The first reports in 2006 received much media attention. Obese rats and mice fed fucoxanthin lost more weight than others. The substance works by stimulating a protein that breaks down fat, especially around the waist. The scientists cautioned that much more research was needed as the protein effected by fucoxanthin is also involved in cholesterol metabolism. Another study published at the end of 2009 reported that all animals given fucoxanthin had significantly higher cholesterol levels, in some cases, double their prior level.


Seaweed and its extracts have generally been found to be safe. Two cases of serious kidney problems have been reported in people taking Fucus vesiculosis for weight loss. In one case, the tablets were found to contain arsenic, cadmium, mercury and chromium. Seaweeds naturally remove heavy metals like iodine, arsenic and cadmium from the ocean. Seaweeds must be harvested from unpolluted waters and tested for heavy-metal content.


In Japan, general consumption of seaweed remains high, iodine deficiency is very low and they have one of the lowest incidences of obesity (one-quarter that of Ireland, according to OECD 2005 data). However, the reasons for such trends are complex and cannot be simply attributed to one component of the diet. Weight gain can be related to thyroid problems, but if these are suspected they should be examined medically.

Seaweed is a nutritious food and can be safely consumed provided it comes from a reliable source and has been checked for heavy metal content. Little evidence supports its use in weight loss. Some products extracted from seaweed may eventually prove beneficial, but much further research is needed.