Most of us are passionate about what we do, and have a strong desire to accomplish as much as possible within the time we have for our work.  This is true for meetings as well.  Meetings use people's most precious resource; time.  Many meetings also involve professionals traveling many miles just to be there.  This has lead to the practice of having a "working" lunch where attendees stay in the room and attempt to eat while still staying engaged in the discussion and moving forward on the agenda.
 
It may seem like keeping people in the room and continuing to work is a necessary and effective way to maximize the time you have.  However, it is not an effective way to accomplish the true objectives of your meeting, which is to make informed, creative decisions about what are usually some big projects.  Surprisingly, letting people leave the room to have lunch will jump start you towards your goals when they return.  
 

People have different ways of grappling with tough decisions.  Some people need to talk through them in small groups, some people need to take a walk while thinking through them, and some people need to completely step away from them and just have a good lunch!  A lunch break allows these different styles of processing information to happen so that your attendees can re engage effectively during the second half of your meeting.  So before you order your sandwiches to be brought in, reconsider your meeting objectives.