Hijiki and Arsenic

Hijiki ( Hijiki photos )

Hijiki is an uncommon kind of seaweed harvested mainly from seas off Japan and Korea. It is generally sold in dry and shredded form (short coarse strips) and is almost black in colour. Because of these appearances, dried hijiki seaweed is physically distinct from other varieties of seaweed such as arame, kombu, wakame and nori. It is commonly used as starter or appetiser in Japanese and Korean cuisines. Hijiki can also be used as an ingredient in salad, soup and vegetarian dishes. Unlike nori seaweed which is usually sold in dried, thin sheets, hijiki is not used in rolling sushi. In Hong Kong, hijiki seaweed is usually sold in Japanese supermarkets and restaurants.

Hijiki may contain arsenic naturally at high levels, especially the more toxic inorganic form. On the other hand, overseas studies show that arsenic contents in other seaweed varieties such as kombu are mainly in the less toxic organic form and the levels are much lower than those found in hijiki. Moreover, one of these studies also reveals that none of the other varieties is found containing detectable level of inorganic arsenic. Based on these findings, seaweed other than the hijiki variety is safe to eat with respect to its arsenic content.

Arsenic

Arsenic is a metalloid present naturally in the earth's crust and is found in trace amounts in rock, soil, water and air. It exists in both organic and inorganic forms in foods. Primary route of exposure in humans to arsenic is mainly through ingestion of foods, especially aquatic foods, which contain relatively high levels of arsenic.

In general, inorganic arsenic is more toxic than the organic form. Exposure to high levels of inorganic arsenic has been linked with gastrointestinal effects, anaemia and liver damage. In 2010, the International Agency for Research on Cancer (IARC) concluded that arsenic in drinking-water causes cancers of the urinary bladder, lung and skin in humans and has classified arsenic as Group 1 agent, i.e. carcinogenic to humans.

In 2010, the Joint Food and Agriculture Organization / World Health Organization Expert Committee on Food Additives (JECFA) determined the inorganic arsenic benchmark dose lower confidence limit for a 0.5% increased incidence of lung cancer in human (BMDL_0.5). The BMDL_0.5 was computed to be 3.0 μg/kg body weight/day (2-7 μg/kg body weight/day). Therefore, JECFA withdrew the Provisional Tolerable Weekly Intakes (PTWI) of 15 μg/kg body weight/week to inorganic arsenic established in 1989 since the previous PTWI (15 μg/kg body weight/week = 2.1 μg/kg body weight/day) is in the region of BMDL_0.5 of 2-7 μg/kg body weight/day .

Local situation

Samples of hijiki taken under the Food Surveillance Programme of the Food and Environmental Hygiene Department (FEHD) in November 2004 found that the arsenic levels of the samples exceeded the statutory limits (1.4 ppm (parts per million) of arsenic expressed in As 2O 3) under the Food Adulteration (Metallic Contamination) Regulations made under the Public Health and Municipal Services Ordinance ((Cap.132) . Offenders upon conviction are liable to a fine of $50,000 and to imprisonment for 6 months. All the products in question have been withdrawn from the market.

Due to the high level of inorganic arsenic present, consumption of a relatively small amount of hijiki can significantly increase the dietary exposure to arsenic. Further safety assessment showed that occasional consumption of small amount of hijiki is unlikely to cause adverse health effect. However, people who are fond of hijiki are at greater risk.

Advice to public

• To avoid consumption of hijiki and avoid choosing hijiki as food ingredient.
• To choose foods from markets carefully in particular prepackaged ones by reading their labels to make sure they do not contain hijiki.
• Except the hijiki variety, there is no need to stop eating other varieties of seaweed as they contain high levels of minerals and trace elements such as iodine that are beneficial to health.

Advice to trade

• To avoid sourcing hijiki as food ingredient.
• To check the seaweed supply and its label carefully to ensure it is not hijiki.
• To avoid using hijiki as ingredient in any kind of foods such as starters, appetisers in Japanese or Korean cuisines, etc, and to use other safe food ingredients as substitutes for hijiki.

Risk Assessment Section
Centre for Food Safety
February 2011