Hijiki

Nutrition
Hijiki contains dietary fiber and minerals, such as iron, calcium, and magnesium. Dietary fiber is good for the intestine and iron helps to prevent anemia. Hijiki has a good calcium and magnesium balance. The ratio of calcium to magnesium in hijiki is 2 to 1.

Possible arsenic health risk
Several government food safety agencies advise consumers to avoid consumption of hijiki seaweed. Test results have indicated that levels of inorganic arsenic were significantly higher than in other types of seaweed. These results have been independently verified.
Government food safety agencies that have issued warnings include:
the Canadian Food Inspection Agency (CFIA)
the Food and Environmental Hygiene Department (FEHD) of Hong Kong
the Food Standards Agency (FSA) of the United Kingdom
the New Zealand Food Safety Authority (NZFSA)
The Ministry of Health, Labour and Welfare of Japan has responded with a report pointing out that, while the consumption of more than 4.7 g hijiki seaweed per day could result in an intake of inorganic arsenic that exceeds the tolerable daily intake for this substance, the average daily consumption for Japanese people is estimated at 0.9 g. Several of the reports from other food safety agencies acknowledged that occasional Hijiki consumption was unlikely to cause significant health risks, but advised against all consumption regardless.
Although no known illnesses have been associated with consuming hijiki seaweed to date, inorganic arsenic has been identified as carcinogenic to humans, and exposure to high levels of inorganic arsenic has been linked with gastrointestinal effects, anemia and liver damage. People who follow a macrobiotic diet that often includes large amounts of seaweed may be at greater risk.