As a scientist, I can't say this will work for everyone as I only have myself as evidence.
I was vegan and loving it for a few months prior to my wedding in October. Afterward, I "went down" to vegetarian, and kept the dairy minimal. Then, the dreaded time for every college student, finals week! Once returning to a SAD (Standard American Diet and puny acronym) way of eating for the past few weeks, I now want nothing more than to get back on my vegan track! Cheese, my Achilles heel, now just makes me feel like a big slug and meat makes me sleepy. I sincerely miss the satisfied, energized fullness after a delightful vegan meal!
Also, I am not a fan of food staying inside of me the way (nor as long as) animal products do. If anything made my bowels work like a fine tuned conveyer belt, it's vegan nutrition. A study published in the British Journal of Nutrition (ref. 2) reports that the average vegetarian digestion takes between 27 and 54 hours, while non-veggies take 31 and 96 hours. The median for vegetarians is 40.5 hours while for non-vegetarians it's 63.5. That's a whole day longer! (Which I find gross.)
In 2005, reports Door to Door Organics(ref. 1), Americans ate 12 oz of meat per day. That's just under twice as much as the highest USDA recommended amount! This means that your body is having to process twice the amount of protein it needs and when that protein is meat, it can take days to be completely digested. So hypothetically, if you eat 6 more ounces a day of meat than what your body needs (12 oz is average consumption and 6 oz is average requirement) then in just one week you're carrying around 2.5 extra pounds of meat that your body is taking days to digest and move through you since you don't actually need it. So by simply eating less animal protein and switching to plant protein, you could theoretically lose 2.5 pounds in a week!
Sugars, salt and fats are another story... ;-)