[toasterlovin]

The person you're responding to is not saying that avoiding fat led to obesity. They're simply saying that the obesity epidemic is evidence against the hypothesis that consuming fats leads to becoming fat. Disconfirming a hypothesis is not the same thing as as proving a competing hypothesis.

[sykhotic]

Without further evidence/study one can’t conclude much. We have two facts:

1. There was a campaign against eating fats.

2. There was an obesity epidemic that started shortly after the campaign against eating fats.

Fact 2 is seductive because it’s natural to assume that 1) is the cause. But without further analysis and study one can’t conclude this. In a post decrying causal links without proper statistical evidence it’s ironic to fall victim to this seduction without proper statistical evidence.

[toasterlovin]

There was an intervention (telling people not to eat fat) based on the "eating fats = bad" hypothesis and that intervention failed to have the effect that the hypothesis would predict. That is disconfirming evidence. You are correct that it's not conclusive, but it is disconfirming nonetheless.

That's all I am saying.

[sykhotic]

It’s not disconfirming since it’s possible that eating fats is bad but also that replacing fats with sugar is bad too or possibly even worse than eating fats. There are also other possibilities to explain why eating too many fats is bad even though when they were targeted for reduction health outcomes did not improve.

[toasterlovin]

It disconfirms the original hypothesis. You have mentioned other, more complicated hypotheses which it does not disconfirm.