My guess & understanding..
in the fundamental sense, assuming metabolism is perfectly fresh, healed, in balance & running pure & healthy, calories in/calories out are the only thing that matters.. My concern is, I don't know 1 person (with the exception of the immune system & digestive system of an infant) that has a metabolism that doesn't need to be healed, or consistently in the repair shop.. As you age, my guess is, it becomes impossible to run a constantly "healthy" metabolism..
The best diet is one built to sync with your particular body, genetics, & immune system, @ that particular time.. "Listen to your body".. "Understand your body & its reactions to certain foods".. No 2 bodies are the same, we are all unique snowflakes, no absolute principle applies to the same body, all the time.. & as we eat everyday, your body adapts, inflames, changes, reacts, etc. to what you've eaten from years past..
Do you have high blood pressure? Is there anything in your gene pool that exposes you to threats others may not be threatened by? How stressed do you get @ work? Do you have a sugar sensitivity? Do you a severe gluten intolerance & if you do, how many years of eating gluten did you endure before you realizing certain foods were creating leaky gut? How much alcohol are you drinking & how long have you drank? We all have damage, I don't know anyone who doesn't.. The question is, to what extent is your damage & how you can repair & "heal", with what foods.. All these questions affect the calories you ingest.. We are all damaged in some form or fashion.. Calories in / Calories out is so far down the list of things a "HEALTH" conscious body should concerned with.. Biggest concern: what type of foods work with my body, genetic, & lifestyle.. Then, we'll approach the "amounts" of those foods to consume..
My advice: Learn to LISTEN to your body.. HEAL your body first (which could take years of discipline, & clean eating for your genetic) or the fundamentals mean nothing.. I have celiac & mega sugar sensitivity.. To me, sugar & gluten are the most toxic things I could put in my body bc I react horribly to it, even when consuming some fruit.. I don't think I'm alone in this, & I probably pay more attention to how my body reacts to certain foods than most.. I know the way I'm supposed to "feel" now after I eat, so I can now begin eliminating or limiting foods that don't correspond with that feeling.. Food allergies are very common but most aren't paying attention to the little red flags.. I personally battle digesting starches well.. They give me gas, bloating, & sometimes skin reaction.. Sweet potato is totally primal.. BUT, sweet potatoes do not work for me.. Higher Fat diet calms my body, I'm now realizing this after years of experimenting.. Food allergies are very common, but many think bc someone in this forum said potatoes are a great starch & eating 2lbs of them, post workout, is a great carb refeed, so it must be good for me.. WRONG.. Try it out, your digestive system may not be healed enough to digest this correctly..
If there's anything thats constant in this life, its change..
Learn to roll with the nutritional punches.. Your body changes, it adapts, it grows, it gets sick, stressed out, it aches, it ages, & on & on.. My bodys nutrtional needs even seem to change with weather.. this is normal, & will never change.. Everything is connected, learn to feed it what it needs in different situations.. (the stoner in me is coming out
Calories in, Calories out makes sense in a fundmental manner, but the question we should be asking is, (IMO) how can I "heal" my body? what foods "work" with my body? What foods make me healthier, digest correctly, & nourish me.. Afer we figure that out, we can then begin playing with caloric #'s & let our weight shift up & down in a healthy manner.. I've seen a lot of skinny UNHEALTHY people.. I've also seen a lot of heavier, very healthy people..
Not all "healthy" foods, make every immune system "healthier"..
Become a master of your immune system.. Listen to your body, nail down the foods that sync with your digestive system, then you can play with the numbers, using those foods, to moderate the scale..