To Fish or not to Fish: Hg is the question?

Fish can play an important part in a well balanced diet, providing important nutrients, some of which are not naturally available in many other foods. However, with continued increases in industrial outputs of mercury, both into the air and waterways, do the risks of exposure now outweigh the nutritional benefits? Organic mercury is a bioacculumator, so once it has entered the food chain, the concentration of mercury in animals increases up the food chain, until it reaches our plates. Once ingested it can take up to 50 days for half the mercury to leave the body, while the half life of methylmercury in blood is 2-3 days. In this article will we examine the relationship between diet type (non-vegetarians, vegetarian and vegan) and blood levels of mercury.

The only survey capable of examining this type of question with statistical confidence is the American NHANES, using data from 2005 to 2010, where blood mercury levels were measured. There are a number of difficulties associated with using surveys to determine diet type. In a previous US study up to two thirds of self-defined vegetarians reported consuming meat, poultry, or fish on dietary recalls (Haddad & Tanzman, 2003), whereas only 3% of self-defined non-vegetarians did not consume meat. The method chosen to determine diet in this study was the examination the dishes consumed over two non-consecutive days. While this should predict excessive numbers of vegans and vegetarians, the results that roughly 0.25% of Americans are vegan and 3% are vegetarian are consistent with a recent poll carried out by the Vegetarian Times.

The US EPA has set a reference dose for mercury of 0.1 µg/kg body weight/day as an exposure at which there are no recognized adverse effects. This corresponds to a blood mercury level of 5.8 µg/L. The graph below shows how the average blood mercury levels varies with diet type, highlighting the increased level in blood the day following consumption.

The (geometric) average for each diet type is well below the US EPA’s guideline, even with the high increase the day following consumption. Very few subjects exceeded the guideline: 0.4% of non-fish consuming non-vegetarians, 2% of vegetarians, and 4% of fish consuming non-vegetarians (excluding those who consumed within a day of the test, but including those who consumed 2-3 days previously). While the level of mercury in those that consume fish is higher than those that don’t, they remain well below levels that would cause any concern, provided fish is consumed in moderation.

References:
Haddad EH, Tanzman JS What do vegetarians in the United States eat? 2003;78:626S–32S