It is really not an increased IQ level. People who eat fresh fruits constantly always are slimmer as they consume 2-4 fruit servings in a day, which are essential for a beautiful shape.
Such people simply take care of their health. They eat less high-calorie foods. Fruits are low in calories but they are nutritious. For example, just one apple can remove hunger for an hour. You don't need to eat something high-calorie.
For beginning you can eat fresh fruit for snacking. And then include some fruit in basic food.
Do not compare or replace fresh fruit with juices or different canning fruits. Juices and canning fruits contain high levels of sugar and calories. They contribute to the contrary appetite and because of this you start to eat more than you usually do.