I see this as, "What we thought was true actually is, and we still have no real idea why." The fact that this is a longitudinal study lets us say that not only

I see this as, "What we thought was true actually is, and we still have no real idea why." The fact that this is a longitudinal study lets us say that not only is heavier use of low calorie sweeteners tied to being fat, it is correlated with getting fatter faster. That's an important point that only longitudinal studies can verify. However without being able to control people's diet, we don't know which way causation goes. Do low calorie sweeteners cause people to gain weight faster? Or does having a tendency towards gaining weight cause people to use low calorie sweeteners? And if it is a cause, why that would be.