Even your grandmother could tell you, eating potatoes all the time will make you fat. These days the supermarkets are flooded with foods that spike your blood sugar much stronger than potatoes, such as Special K, which is supposed to be a "healthy" cereal. This came about when the government release the food pyramid and put wheat and grains on the bottom, and fats on the tip.
From Wikipedia:
Food guide pyramid - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Food_guide_pyramid#cite_note-28
I'm still a teenager, so I need not give fucks about sugar content (And there's only about 10g in the Special K I eat). I just finished a bowl of it after doing P90X and my Insanity DVDs... So you're saying I'm going to get fat? Losing weight isn't hard. Just gain muscle and do a mix of cardio/resistance training.
What you need to eat to gain muscle and maintain a healthy lifestyle:
Fats/Carbs/Protein in a 20/40/40 ratio. I've lost about 8 pounds last month (and I've gained a lot of muscle) after a winter of stuffing myself during the holidays and I've been eating Special K all throughout that. Maybe it's because I'm younger, but all you really need to do is
exercise and eat right.
Special K tastes great, contains a lot of fiber and carbs, and is made from real fruit (despite misconceptions by people).
Eat 5-6 small meals a day, or 3 full meals with 2 snacks (I like peanut butter/Nutella and Whole Wheat Bread) -- Eat leafy green vegetables, fish, nuts (almonds and peanuts), chicken, fruit, and whey protein, while staying away from soda and candy. And most importantly, drink lots of water.
EDIT: My friend (jeffrey on here) is a health-freak, and a personal trainer irl. He told me abs are made in the kitchen, and that the key to ultimate abs and muscle is related to this ratio:
60% Diet
30% Exercise
10% Genetics and Daily habits