[PoachedSausage]

Did people really believe that dietary fat was stored by the body without any kind of biological processing?

If you examine it from a thermodynamic perspective (I know that is simplistic for human biology), weight for weight fat has double the energy density of sugar, 38 MJ/kg vs 17 MJ/kg according to [0]. For reference, gasoline and diesel fuel are around 45 - 48 MJ/kg.

Also, according to [1], dietary fats are broken down to generate acetyl-CoA which is the precursor for lipogenesis (fat storage), therefore eating fat does not necessarily make you fat but consuming a lot of it and not metabolising it to do work will make it easier for you body to store fat.

[0] https://en.wikipedia.org/wiki/Lipid#Energy_storage

[1] https://en.wikipedia.org/wiki/Lipid#Degradation