Seafood revealed as biggest source of dangerous toxins

PUBLISHED : Wednesday, 14 December, 2011, 12:00am
UPDATED : Wednesday, 14 December, 2011, 12:00am


Seafood is the largest source of potentially harmful dioxin-based toxins in the diet of Hongkongers, according to a major food study conducted by the Centre for Food Safety.

The highest levels were found in mandarin fish, followed by oysters and pomfret.

However, the toxins are not consumed at dangerous levels based on Hongkongers' normal eating habits, so the findings were no cause for alarm, said Dr Xiao Ying, a food safety officer at the centre.

'People should not stop eating fish, because fish contains many essential nutrients,' Xiao said yesterday. Dr Ho Yuk-yin, a consultant in community medicine at the centre, recommended consuming fish three times a week.

In the first major study of city residents' diets, researchers tested 142 food samples over the past year, focusing on two powerful classes of toxins - dioxin and dioxin-like PCBs (polychlorinated biphenyls). The samples were drawn from 71 types of food purchased from markets and prepared as they would be served on the table.

The results were compared to a survey that interviewed more than 5,000 people on their dietary habits.

To lessen the intake of dioxins, the centre advised Hongkongers to trim the fat from meat and opt for low-fat dairy products, since dioxins are fat-soluble and accumulate in the fatty tissues of meat and seafood. It recommended a balanced diet with a rich variety of fruit and vegetables.

The study found that the average Hongkonger's monthly diet contained just 31 per cent of the maximum safe amount allowed for the two toxins. The top 5 per cent of residents consumed 85 per cent of the tolerable limit.

The safety standard for both toxins is 70 picograms of toxicity equivalents (pg TEQ) per kilogram of body weight per month, set in 2001 by the Food and Agriculture Organisation and the World Health Organisation.

The test results showed that mandarin fish, oysters and pomfret contained 1.056, 0.926 and 0.885 pg TEQ per gram respectively.

Dioxins occurred naturally and were by-products of combustion and industrial processes, Ho said. Animals that ingest contaminated plants will accumulate dioxins in their bodies. Some dioxins are carcinogens; long-term exposure may also affect the immune, nervous and reproductive systems.

The report by the centre's First Hong Kong Total Diet Study (TDS) is the first in a series that will examine more than 130 contaminants and nutrients in Hongkongers' food, issuing reports in phases until 2014.

The TDS approach is widely considered the most cost-effective way to estimate dietary exposure to a range of chemicals and nutrients. Unlike food surveillance programmes, it considers the impact of cooking rather than concentrations of substances in raw food.