What are the impacts on learning of allowing students to eat in class?

Most of the answers here suggest limiting or prohibiting eating in the classroom, but I have to disagree.

First, students generally do not control their own schedules. Personally, I often had five or six hours of class back-to-back as an undergrad, and I was not unusual in that regard. Yes, there are breaks between classes, but students need that time to get to the next classroom. In addition, many professors let their class go overtime, reducing the length of the break. If you prohibit eating, hungry students will simply choose to arrive late after getting a snack, which defeats the idea of minimizing disruptions by prohibiting eating.

Second, you don't know what medical issues a particular student has: they may be diabetic or need to eat at regular intervals for other reasons. While a student can tell you about this at the start of the term, I don't like the idea of forcing students to discuss their medical issues with every professor, every term. After all, students who get special classroon accommodations due to a disability are not obliged to tell their professors what the specific disability is.

For these reasons, if the university does not have a specific policy, I would tend to be lenient at first. Most students are reasonable people who won't show up to class with a five course dinner. If a specific concern arises during the term, such as very noisy food or garbage being left behind, you can address it either with the individual student or the class as a whole, as appropriate.

EDITED: As you mention, there are two competing issues here: Running low on carbohydrates and especially hydration leads to decreased cognitive ability; but eating is a distraction from attending the lectures. I have found mostly opinions and little hard data on this, for example

• http://www.ludlowcub.com/opinion/2012/05/01/allowing-students-to-eat-in-class-helps-with-learning-ability/
• http://www.breakfastintheclassroom.org/pdf/BIC-Fact%20Sheet_FINAL_1-07-11.pdf
• http://policy.federation.edu.au/learning_and_teaching/ub_tec/classrooms/ch01.php

The consensus seems to be that water (in sealable bottles) should definitely be allowed, and that students should not go longer than three hours without a chance to snack. This would suggest treating the classroom like a study place in a library: Water (or anything that doesn't leave a mess when spilled) yes, snacks only during breaks.

One thing to keep in mind that diabetic students will have more strict requirements on when (and what) to snack, which should be accommodated, see http://care.diabetesjournals.org/content/6/2/180.full.pdf. Not allowing other students the same opportunity to refuel might be seen as unfair.