Comment by lithocarpus

There are tons of possible symptoms because systemic inflammation including from dietary causes basically damages the entire body. But here's a list of some common symptoms from one source:

persistent pain, chronic fatigue or insomnia, joint stiffness, skin problems, elevated blood markers (such as C-reactive protein), gastrointestinal issues (constipation, diarrhoea, acid reflux), depression, anxiety and mood disorders, unintended weight gain or loss, frequent colds or flu.

https://stories.uq.edu.au/research/2023/9-signs-you-have-inf...

For myself, when I eat processed food I mostly feel fatigue and low energy and gastrointestinal pain. But it's been extremely rare for me to eat these things in the last 10 years.

Most of the US population are systemically inflamed including from diet basically all the time, and most of the US population are suffering from several of the above symptoms chronically. (Not to claim that it's definitely or only because of food, but I'd bet it's a big part of it.)