http://revach.net/article.php?id=3450

Section: Hanhagos   Category: Ma'aseh Rav
Rav Zemele Volozhin Follows The Doctors Orders
Rav Zemele Volozhin the beloved talmid of the Vilna Gaon and the brother of Rav Chaim Volozhin once went with another Rov to the house of a very poor person.  The host was in middle of eating and begged them to sit down and join him.  Rav Zemele knew that the host did not have enough food for them so he refused claiming the doctor told him he cannot eat.

When they left the Rov that accompanied him, asked him if it is true that he is sick.  He said no he is not.  "Then how could you lie?  What happened to Midvar Sheker Tirchok?"

He answered that the great doctor, the Rambam wrote that it is assur to eat from a meal where there is not enough food for the host.  He indeed was just following the doctor's orders.