I spent a few weeks eating only potatoes and vegetable oil, several years ago. It wasn't for weight loss, it was because I was breastfeeding, and my baby had some kind of protein sensitivity we couldn't nail down. Potatoes turned out to be a safe food for him, so that's all I ate for a while. As it happens potatoes are my favorite food, and I had vegetable oil available so I could eat fries/chips/crisps, but even then I can't imagine doing it without a similarly serious motivation. When my choices were "listen to the baby cry in pain every time he eats" or "eat potatoes until the allergist appointment," it was an easy choice. Otherwise I wouldn't last long on the potato diet.