Healthiest diet?

You're thinking of starchy (carb) foods like potato. Normal vegetables contain carbohydrates but definitely not enough to be classified as "extremely high". They contain just the right amount to be healthy. Like the sugar in fruits are healthy because the fruits contain fiber and whatnot to help digest the sugars properly. But whereas fruits should be eaten in moderation or for strenuous activities, vegetables are so low in kilojoules that you cannot eat too much.