March 22, 2004: How much tuna is safe for pregnant women, nursing mothers, and young children?

The FDA has just released an Advisory recommending that women planning to become pregnant, pregnant women, nursing mothers, and young children may eat up to 12 ounces a week of a variety of fish and shell fish low in mercury.  This group should not eat shark swordfish, King Mackerel or Tilefish, and not more than 6 ounces of albacore tuna per week.  The reason is the concern about methymercury.

Mercury vapor is emitted from both natural and anthropogenic sources and is returned as a water-soluble form in precipitation into bodies of water.  Inorganic mercury is converted by bacteria to methylmercury compounds that bioaccululate in the aquatic food chain and reach the highest concentration in predatory fish.  Human exposure is primarily from consumption of the fish.

History demonstrates the toxicity of methylmercury.  In Minimatta in the 1950s, more than 100 people died following methyl mercury exposure from a factory dumping mercury runoff into the water.  In Iraq in the 1970s, bread was made from grain that had been treated with a fungicide containing methylmercury.  More than 4000 people were hospitalized and 600 people died.  Children born following the exposure had ataxia, loss of peripheral vision, decreased hearing, developmental delay and mental retardation.

The problem is that we do not know how much methylmercury is safe.   Most of the data comes from the Iraqi exposure, an acute exposure, and we are trying to make statements about chronic low level exposure from eating fish.  To complicate matters, a couple of studies in people from fish-eating islands who have elevated methylmercury levels do not demonstrate any ill-effects.   However there are significant study design problems and the groups are very small.  

The FDA Advisory is a guideline as a result of a committee of multiple experts.  This Advisory is actually a joint statement from the EPA and FDA, two groups that have not always seen eye to eye.  The guideline is based on the best available data.  

Concerns may be raised by patients who have eaten more than the recommended amount of tuna.  Hair analysis is the best estimate of methylmercury body burden.  However, there is no correlation between hair concentrations and toxicity.  Obtaining hair samples would offer little information. There is no known treatment.  Studies in animals are suggestive that DMSA may chelate methylmercury, but there is no human data.  

(Remember this is methyl mercury, not ethyl mercury which is found in thimerosol in vaccines.  They are two different agents.)

As always, if there are any questions, call the TPC.

I am interested in any questions that you would like answered in “Question of the Week.”  Please e-mail me with any suggestions at

Donna Seger, M.D.
Medical Director, Tennessee Poison Center