Heavy Metals: Mercury, like zinc, iron and lead are heavy metals. But mercury is the only one of these 4 heavy metals that has no useful function in the body. The only thing mercury does in the body is to negatively affect the brain and kidneys.
What happens to the tuna you eat?? Well, first the tuna is absorbed by the small intestine and then it's shipped over to the liver. After that the mercury either enters the bile for discharge or the blood. If it reaches the blood from there it can enter the brain or kidneys, not good. However if the dosage of mercury is not excessive and doesn't overwhelm the system the kidneys will do their job. But if it finds to the brain via the blood brain barrier it gets stored. That storage option is what leads to mercury damage to the brain and kidneys.
So how much mercury is in tuna?
As a general rule canned tuna has less mercury than a tuna steak and chunk light has less (Boo!) than chunk white tuna.
Of course it also depends in which ocean the tuna was caught. An Atlantic tuna would be my first choice. But it's hard to find that information and will become even harder when TPP allows food products not to be labeled with their place of origin. Trying to eat healthy just gets harder. Also there is a lot of contradictory information floating around about mercury in tuna.
The CDC states that a person can chronically (all 365 days) ingest .0003mg/kg of mercury per day with "no observed adverse effect." but that doesn't mean it's healthy in my book.
I have never eaten much seafood as I have an iodine intolerance but I do eat tuna on occasion. I suppose if I was eating seafood on a regular basis I would need to rethink the amount of tuna I would eat each week.