Just for the record, the statement "people start eating less, because the pain associated with eating negatively reinforces it" is incorrect. Pain "punishes," it is not a negative reinforcer. Eating is a negative reinforcer for hunger. That is, eating reduces hunger, so it increases the future likelihood of eating when hunger is present.
When taking something away increases the frequency of a behavior in the future, it is called a "negative reinforcer." When it reduces the frequency of a behavior, it is called a "punisher."