The episode you are looking for is the trap of 686 being careful to lose weight!

Episode 686 beware of the weight loss trap

On this day Conan accompanied Xiaolan and Zi Yuan to practice yoga to lose weight. At this time, they met Chu Chuan who came to attend the weight-loss class. Chuchuan Houzi deliberately made things difficult, forcing Sun Moon coach to kneel down and make peace. Training in the evening, thick son also leave early, a person back to the house. After training, the steamed stuffed bun came to Li Yue, because she couldn't knock on Chuchuan's door, but when she opened the door, Chuchuan really died of poisoning! After Kenner visited the scene, he discovered the criminal modus operandi of the prisoner, and the prisoner also admitted the fact of killing for revenge.

Below is a screenshot ~

Accurate, I hope I can help you!