[_heimdall]

Once you consider seed oils (many people still vilify saturated fats) the fast food timing gets even murkier. Fast food restaurants originally cooked in beef tallow where as its all seed oils today. If the oil type makes a difference and it wasn't necessarily the design of a fast food restaurant, the clock would have to start when chains got rid of tallow.

[trealira]

Why are seed oils unhealthy, though? I can't find anything that's bad about them in particular. I've only seen people commenting online saying to avoid them.

[_heimdall]

There's too much there for me to give a great answer here, but I can recommend Dr Michael Eades as a starting point. He wrote a couple books over the years on heart health and has been writing a newsletter for the last few years where he discusses various related topics in detail.

At a super high level, it has to do with how seed oils ("vegetable" oils, etc) are produced and how oxidative they are in the body.