The rules are simple, eat the second bloc of the tuna and not the first. Since eating sushi in Japan now for more than 30 years, I have enough experience to say what I feel is the best bloc. Tuna fish at Tsukijii is divided into blocs, those blocs that are reserved for Michelin chefs and others without stars.
The first bloc of tuna is most oily, this is what you see at numerous sushi counters where people “chomp at the bit” for some more fat. We understand that human’s perceive and taste fat more than we think. The taste receptors in your mouth crave fat, and the amount of this fat-tasting protein on the tongue varies. If you’ve inherited a tendency to have fewer of these receptors, then your response to fat is muted and you are more likely to be obese.