They are, if they're designed to be addictively delicious.

McDonalds and their ilk, have created taste formulas that are nearly irresistable. I've seen people from foreign countries come to america who've never tasted anything other than their local cuisines who can't imagine eating anything else. but the second they sink their teeth into a McDon hamburger, they are hooked.

MSG has been shown as a super reliable way to make mice obese: there's even a scientific procedure named after it: "MSG induced Obestity". It's a standard procedure used as a precurser to obesity experiments on rats. If it's that reliable on rats, just imagine what it would do to humans.