Pretty simple stuff from basic nutrition chapter in high school biology text, surprised more people haven't given the correct response. Sugar is the biggest cause of weight gain. Excess sugar leads to insulin resistance, which leads to reduced sugar absorption. The extra sugar is converted into fat. In addition, insulin resistance leads to elevated appetite as the body had trouble absorbing the food you are giving it (the REAL "slow metabolism"). Also to clarify for those who don't understand Into to Biology: "sugar" is glucose, but of COURSE most sugar is not ingested as glucose, but as fructose from fruits and corn syrup, or sucrose from table sugar, etc etc. Even starch from potatoes is a sugar. However, as is well known, the more complex the sugar, the more justified it is to eat it. Whole breads, despite still being loaded with simple carbs, are probably ok if you cut out the other items on this list, which are all garbage:
For most people who are overweight or mildly obese, cutting out
>Soda, juice, energy drinks, sport drinks, etc etc
>Limiting alcohol to 1 or 2 times (tons of sugar) a week
>Limiting dessert to an indulgence once a week
>Recognizing that LOTS of the packaged food that is probably sitting in your house is simple carbs/sugar. This means NO packaged crackers, crisps, crunchies, munchies, pretzels, chips, morsels, sugared yogurts (these should be a dessert item!), and the obvious dessert stuff like cookies, cakes, breakfast cereals, etc.
The REAL challenge is realizing just how much sugar you are actually consuming and trying to curtail it as much as possible. They have dumped it everywhere in our diet and it is destroying our health.