It can be thought of like this:Shouldn't it be "result" since it should refer to bananas?
Same for this:And the second sentence, plural as well since they are two seaprate activities?
It can be thought of like this:
Taking (bananas plus water) results in faster weight loss.
Same for this:
Doing this – (laying off exercise and allowing afternoon sweets) – reduces stress.
In both cases, what is described can be thought of a single plan or a single approach to achieving a goal, even though the "plan" consists of multiple foods or activities.
Just as you can say "Ham and eggs is a good breakfast." The "ham and eggs" can be thought of as a description of one meal, like "cereal and bananas" or "pancakes and maple syrup."