Escolar is a type of snake mackerel that cannot metabolize the wax esters naturally found in its diet. These esters are called gempylotoxin, and are very similar to castor or mineral oil. This is what gives the flesh of escolar its oily texture. As a result, when full portions of escolar are consumed, these wax esters cause gastrointestinal symptoms.
To be frankly and bluntly specific - and I'm sorry for this - consumption of escolar causes explosive, oily, orange diarrhea. People have reported that the discharges are often difficult to control and accidents can happen while passing gas. I personally know someone who ate an escolar steak one night, unaware of its side effects. The next day he was riding the elevator to his office when out of nowhere his bowels unleashed a surprise attack on his pants. As he said later, "Thank God I had my gym bag with me, which had a clean pair of underwear in it." This explains why escolar is also called the "olestra fish" and the "ex-lax fish."
The Japanese and Italian governments have banned the importation and sale of escolar for these reasons. The governments of Canada, Sweden and Denmark require that all escolar come with warning labels. The FDA lifted the escolar ban in 1992 because the fish is nontoxic - sure, it causes embarrassing things to happen in your pants, but it won't hurt or kill you.
In spite of all this, escolar is indeed very buttery and delicious, and should be enjoyed, but never in portions larger than six ounces. Portions below six ounces will not cause gastrointestinal distress. Don't say we didn't warn you.
(Image: Kathryn Hill)