Sugar (carbohydrate) has 4 kcal per gram.
Protein has 4 kcal per gram.
Alcohol has 7 kcal per gram.
And the winner, fat, has 9 kcal per gram.
Except for the alcohol, none of these can be eliminated from the diet without some pretty severe consequences. None of them are responsible for weight gain. Consuming more calories than one uses is the only factor which has been shown to cause weight gain.
If only losing weight could be so simple as the elimination of a specific food; no one would eat that food, and we’d all be thin.
No, the winner is sugar.
Simple carbohydrates need virtually no digestion, hence the reputation for "instant energy". Sugar and carbs are cheap and are contained in practically all processed foods.
Fat and protein are much slower to digest and don't have the addictive characteristic of sugar. Without the sugar-high/carb-crash syndrome, a person is not as likely to consume as much per day.
Yes, and that "food" is simple carbohydrates. There is no real need for them in your diet. Your body can craft the exact nutrients it needs from meats, seafood, nuts and green vegetables. Like you said, if we were all on a low-carb diet, we'd all be thin.
(See: High-glycemic index foods)
Those who have adopted this concept can tell you that.