At least once, everyone should see how their body operates with as few confounding variables as possible. Your baseline performance, feeling, mood, energy - is very valuable to know.
Elimination Diet - Remove as many variables from your total intake as possible. Ideally choose a single bioavailable food prepared very cleanly for 30 days.
This is important because lots of people don't know what they have normalized as "getting older", or "I've always had that", or "I'm just inflamed". Skin conditions, hair issues, attention, clarity, are often reported to resolve on these type of protocols.
The most famous elimination diet demonstration was with Celiac disease during WWII, wheat/bread shortages created a accidental elimination diet in Denmark and established the link between wheat protein and Celiac.
Metabolism touches every part of the body, including the brain. Metabolism is driven by diet. Food literarily affects every part of our lives.
I think people should be aware this is a very useful tool, and if there is some persistent or difficult to nail down issue - why not try it?
elimination options
Omnivore options - eggs, red meat are good options. Ground meat has higher histamine levels, so it would confound the results.
Pescatarian - fish are ok, but they are not biocomplete, but that shouldn't be a problem for 30 days. The "sardine fast" is a type of elimination diet protocol.
Plant Based - low fodmap diets probably eliminate the most variables, but I'm not very well read on the options
Fasting - DO NOT DO LONG FASTS WITHOUT MEDICAL SUPERVISION. Probably the the most extreme option, total elimination, not exactly your base line, but if something was bothering you in your food you would at least notice it. REFEEDING SYNDROME is a real thing, and needs to be planned for when ending the fast.
Regardless, be aware of confounders - cooking oils and fats can change also be triggers, so be deliberate in your choice. Spices, seasonings, rubs, "electrolyte mixes", marinades introduce more variables.
My Biases
I run the ketogenic and zero carb carnivore (Which is just a elimination diet I decided to live with) communities, I'm all in on that metabolic lifestyle. However, elimination protocols don't have to aligned with my biases to be effective. Even doing something as simple as 30 days without processed foods can be helpful to know for someone.
Judy Cho wrote a great eliminate diet protocol book The Carnivore Cure Which is just eating red meat for 30 days and mapping out symptoms, mood, feelings. Plus guidance on starting, and reintroducing foods to nail down triggers. But, there are many different protocols out there, you can find one that fits your requirements.
Sometimes you can accidentally do a mini-elimination. I know when I've done long business trips where I don't have my comfort food and I just end up eating at one place (or convenience store) again and again... and my health incidentally improves; I wish I'd been more thoughtful when I was younger and used those events to examine the terrible things I was putting into my body.