The researchers examined the relationship between the children's milk intake between 1996 and 1999 and their weight over a one-year period. Those who drank more than three eight-ounce servings of milk a day gained the most weight, even after the researchers took into consideration factors such as physical activity, other dietary factors and growth. The association held, even though most of the children were drinking low-fat milk.They could not have controlled for both calories and exercise, because if they did, they would have discovered that nothing else would have found that nothing else affects weight gain.
I think that their mistake was to single out drinkers of 3 glasses of low-fat milk. Low-fat milk has a higher percentage of its calories in carbohydrates, and may be fattening like sugary sodas. A better test might be to look at kids who only drink whole milk and low-carb drinks.
Update: According to the long-term San Antonio Heart Study:
Researchers analyzed patient information spanning 26 years. They found that those who drink diet sodas were more likely to become overweight. The risk of being overweight or obese increased 65 percent more with each diet drink per day.Go figure.