I don't know that I'd say that it's COMPLETELY impossible - the only way that I could see it being a thing would be the people that will straight up devour high calorie natural foods like nuts by the bucketload, but even then, overindulging in nuts or grains may make you overweight, but not deathfat level overweight.  Meanwhile, it is insanely easy to gain weight stuffing your face with oreos every fucking day...