I could be wrong, but I think OP meant, choose something to eliminate and do it for 30 days. Then choose to introduce it back in if you wish. This allows you to observe the effect it has on you.
I did this with added sugars. I could definitely feel the cravings to not only eat more as you added it back in but also the baseline cravings to just have some amount of sugar in your body. It is very eye opening.
You also become very aware of how much sugar is in everything.
An elimination diet is a protocol where you start with a baseline of "safest" foods and exclude everything for a while, and then.. in an orderly fashion, start to re-introduce foods while noting how you respond or change in light of those additions. It's a protocol, one you can get from a nutritionist or dietician or by searching online. And the food lists can be adapted to people with varying eating modalities.
To clarify, the thing one eliminates is typically a molecule or a food group--the most common are probably (in no order) added sugar, gluten, fried foods, processed foods, dairy, and red meat. One can eliminate any or all of those, typically for a month (gives your gut time to change), and then, if you want to, experiment with adding things back in.
A nice thing about the gut microbiome is that it's self-reinforcing, both psychologically and gustatorily--as in, once one's body realizes a food doesn't help, one tend to both think "why would I make myself feel like that?", and to find the experience of eating/drinking it less pleasurable, alien, or even disgusting.
I'll add my anecdote. Nightshade vegetables, in particular the potato, wreak havoc on my body. Took me years to figure out. I didn't even know what a nightshade was. Every few months I'll start to question my sanity and I'm sure potatoes won't bother me. Without fail, every time, rashes, pain, insomnia, deep depressive thoughts, all within 8 hours of consuming potatoes. It's truly crazy. I still have a hard time believing it. Funny enough, I grew up in Idaho, can't eat potatoes. Sweet potatoes it is!
Sjogrens autoimmune support forums are full of anecdotes from people who discover that nightshades are problematic for them. They will often give a list of good / bad things that work for them.
I find it highly amusing that people who have never heard of AIP diet end up recreating it over and over again.
Rice is often mentioned as another low-inflammation / -irritation food. Though I've heard through a friend of someone with a rice allergy --- by an east-Asian, to boot!
I'd look at minimising complexity whilst achieving nutritional sufficiency (macros, vitamins, essential oils) and take it from there, I suppose.
That is indeed quite obvious, but parent comment was asking about a specific person's situation, not whether food is possible or making somebody sick generally or whether it's bad to be sick.
I'm doing something similar, but I'm using meal replacements. Huel is my choice, but there are a lot of others that seem just as good. I basically switched to Huel only for 2 weeks. Then started re-introducing other foods to see how I felt.
It became very obvious, very quickly that a few foods (mostly dairy) were causing me major issues.
I did this with added sugars. I could definitely feel the cravings to not only eat more as you added it back in but also the baseline cravings to just have some amount of sugar in your body. It is very eye opening.
You also become very aware of how much sugar is in everything.