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Volume 14 Issue 84

There are many varieties of tuna fish, but the level of mercury in some varieties is far higher than in others.

Yet food regulators, retailers, and especially consumers, have little or no idea how to tell what tuna they are looking at or eating because some varieties are almost impossible to differentiate from others.

To address this situation, a group of researchers from Columbia University and other institutions gathered and tested over a hundred samples of tuna fish, identified their species using mitochondrial DNA, and tested each for concentrations of mercury. Some varieties, commonly found in restaurants or markets, had never been tested for mercury content even by the federal agencies responsible for monitoring mercury risks.

The mercury tests were surprisingly different among species: Akami is the Japanese word for lean red tuna, and toro for fatty tuna. Bluefin akami and Bigeye akami and toro were significantly higher than Bluefin toro and Yellowfin akami, which was the lowest of all.

The mercury concentrations of all samples tested exceeded Japanese regulations and the daily safe level of the US Environmental Protection Agency. Levels for bluefin akami exceeded the US Food and Drug Administration, Health Canada and the European Commission levels. On average, one order of Bigeye sushi-the species used most often for sushi-exceeded the safe daily maximum of Health Canada, and the safe limit of the World Health Organization for women of childbearing age.

The team developed DNA Barcodes - short genetic markers - that can be used by food regulators to quickly identify each variety, and from that, estimate mercury levels.

And why is this important? 

First, there is considerable fraud (called "substitution") in the market. Second, mercury can damage the brain, kidneys, lungs and nerve tissue. Mercury poisoning does occur from eating too much sushi. The result is a whole spectrum of possible conditions:

Vision, hearing and speech impairment
Nerve damage
Heart problems
High blood pressure
Loss of coordination
 Emotional "rollercoaster"
Memory impairment
…and many other symptoms.

The bottom line for consumers: Make tuna sushi a rare occasional treat, not a regular thing.

SOURCE: Biology Letters, "DNA barcodes reveal species-specific mercury levels in tuna sushi that pose a health risk to consumers", http://rsbl.royalsocietypublishing.org/content/early/2010/04/13/rsbl.2010.0156.full

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