Spokesperson: The major school lunch vendors recently agreed to stop selling high-calorie beverages in elementary and middle schools because studies show that children of ages 7 to 8 who substitute one low-calorie beverage for one high-calorie soft drink in their daily diets will, on average, weigh 20 pounds less than they would have by the time they reach high school. Since only low-calorie beverages will be sold in schools, within six to eight years, we can expect to see a reduction in the percentage of overweight high-school children.

The spokesperson assumes which of the following to be true?