[esseph]

> evolution doesn't work over a span of a few generations.

Yes it can

> Over the past two decades, it has become clear that evolutionary change can be fast enough to be observed in present-day populations (Hendry and Kinnison 1999; Kinnison and Hendry 2001; Hendry et al. 2008; Gingerich 2009) and that it can directly affect the dynamics of populations and communities (Hairston et al. 2005; Saccheri and Hanski 2006; Kinnison and Hairston 2007; Pelletier et al. 2009). Much recent interest has focused on the possibility that so-called rapid or contemporary evolution leads to ‘evolutionary rescue’, whereby threatened populations avoid extinction by adapting to an altered environment (Barrett and Hendry 2012; Gonzalez et al. 2013).

https://pmc.ncbi.nlm.nih.gov/articles/PMC3894905/

[CWIZO]

It'd be surprising if that applies in this context. In the case of the individuals OP mentions, their parents would not have been exposed to ultra processed food (or barely, perhaps only after they've reproduced), so ehatever gens they passed on would not have been adapted. There's simply not enough generations in this case. Especially not for such significant changes.

In any case, it's moot as by and large the westeren diet is not good for the population, exceptions are simply that.

[esseph]

My great grandparents in the US were eating diets of ultra processed foods. Soda Shopes, hot dogs, sausages, hamburgers, Spam, boxed spaghetti, hamburger helper, Jello molds with canned fruit in them, etc.

My great grandfather in particular used to smoke a box of King Edward cigars a week, and lived mostly on a diet of plain bologna sandwiches on plain white bread, and candy corn.