Xiaolongxia (crayfish), a popular dish in China, is a good source of selenium.

Selenium is a mineral in our diets, known as a trace element because it is only required in small amounts. In recent years the focus has intensified as scientists have learned more about its importance.

It is now known that selenium helps to create "selenoproteins". These in turn are anti-oxidant enzymes which help to combat the harmful effects of gruesome free radicals in our environment caused by pollution, carbon monoxide emissions and even the sun. Selenoproteins also maintain a healthy metabolism and ensure the immune system is in tip-top condition.

Deficiency of selenium (with an intake of less than 10 micrograms per day) is very unusual but can lead to two rather unpleasant conditions.

The first causes an abnormality of the heart's muscle (cardiomyopathy) which can lead to death and is called Keshan's Disease, so named after the county in Heilongjiang province, Northeast China where it was first recognized in the 1930s.

At that time selenium deficiency was a real problem for many areas in Northeast and Southwest China where people relied upon crops grown in the local selenium-poor soil, or meat from animals which grazed upon these same crops.

A second disease, Kashin-Beck's, affects the joints and bones and has also been seen in similar parts of China.

Currently supplementation programs have reduced the occurrence of both conditions.

Ironically, China is also home to some of the most selenium-rich soil, so much so that people living in these areas have occasionally developed symptoms of selenium toxicity known as selenosis.

Normally this occurs when a person's daily intake exceeds 400 micrograms. Symptoms include garlicky breath, hair loss, fatigue and weak nails, and this condition can even be fatal.

Selenium is found in many different types of plant food - vegetables, fruit and grains - but its concentration really depends on the soil it was grown in. It is also present in animal meat, especially offal, but again, the selenium content depends on the amount in the plants the animal grazed on.

This means it's difficult to precisely advise people what foods they should eat to get an ideal amount of selenium.

Anyone living in a major city in China should not be too concerned, providing they have a balanced diet (meat, grains, fruits, vegetables and dairy products) taken from different areas of the country.

The richest source of selenium is thought to be the brazil nut, which admittedly is neither widely available here nor cheap. A study carried out earlier this year in a Selenium-poor country, New Zealand, showed that selenium deficiency could be corrected with just 1-2 brazil nuts per day. These researchers also warned against having more than two a day because of the potential threat of selenosis.

Other more common rich sources in China include animal off-cuts; kidneys are particular good, as is liver. The Chinese are masters of getting creative with offal - so next time you're at your local restaurant, ask for something with yaozi (kidney) or gan (liver).

For those who prefer something a little more refined, certain fish like longxia (lobster), xiaolongxia (crayfish) and pangxie (crab) contain good amounts of selenium. Pop down to Gui Jie in Beijing to try some spicy crayfish or any of the speciality restaurants in Shanghai serving hairy crabs.

This nutrition-related column is written by Nina Lenton, a qualified dietitian living and working in Beijing. Contact her at nina_lenton@hotmail.com.

(China Daily? July 23,2008)