To start with, 1/2 cup of potato has 14g of carbs in it. This is more than pretty much any other vegetable on the planet. While other veggies like tomatoes and zucchini are loaded with nutrients, the potato really has very little in comparison.
So why do people eat a ton of potatoes? First, they're cheap. You can grow a ton of potatoes very easily. Remember the Irish being forced to grow giant batches of potatoes? It's an easy-grow food. It grows underground, safe from deer, rabbits and such. Also, it is full of calories and carbs. While this may seem BAD to us in modern times, to hard working farmors and laborers, this was great. It gave you the energy to keep working.
Sweet Potatoes are, as you might guess, even worse for you. They are a high sugar version of the potato. They are a full 22g of carbs per half cup.
How about McDonalds French Fries? Be ready for this one - a large fries has 70g of carbs - and that's only 6oz of fries!! A medium order, at 4oz, is 47g. Never mind that they fry the potatoes in trans fatty acids. Talk about a double dose of bad.
I really recommend you wean yourself off potatoes - and find some of the other delicious veggies that are good for you!
Vegetable Carb Count
Lisa Shea's Library of Low Carb Books
