While "Million Dollar Baby" earned critical acclaim and a Best Actress Oscar for Hilary Swank, it also took a toll on her health.

Back in '09, she puzzled the vegetarian community after telling Marie Claire magazine that she was dropping the green diet after 18 years because it was no longer healthy for her. “I just started withering away," she said. "I wasn’t getting enough vitamins or protein. (It was) very emotional.”

Turns out Swank's real reason for the shift has nothing to do with plants but fish. In the new issue of You magazine, the 36-year-old says she contracted mercury poisoning while filming "Million Dollar Baby".

“I put on 19 pounds of muscle. I was 29, I was a vegetarian and suddenly I was eating so much fish that I got elevated mercury problems. It plagued me for a couple of years after that movie.  When I discovered that my health was suffering, I had to change my diet. So now I eat pork and beef.”

Curious as to why Swank didn't just drop the fish and shift back to her vegetarian diet? One commenter over on Ecorazzi offers a possible reason: 

"Yes, I recovered from mercury toxicity and high levels of mercury can do several things that make a vegetarian diet very difficult. First it can cause intestinal dysbiosis – a lack of healthy bacteria and an overgrowth of unhealthy bacteria and yeast. The best diet for restoring balance is very limited carbs – no fruit, low grains. The other issue is you can develop allergies to so many foods (due to intestinal imbalance and immune dysfunction) that it is hard to find foods that don’t make you sick – allergies to gluten, casein, wheat, soy, egg, etc. are very common."

Swank is yet another example of the dangers associated with eating too much fish. Make sure you know which species are safe to consume and those that should be avoided by jumping here. 

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.