I’m not sure how complex this is really. What we know it is, is deeply counter-cultural and against common sense as we experience it.
First is the question of timescale. I was an athlete, used to run up mountains, those things, and I ate good food: homegrown veg, very little sugar or sweet things, very little processed food. But over decades, my body gave up being able to manage that and I didn’t know how to pay attention to glacially slow weight gain. I am sure given half a chance that our amazing digestive and metabolic systems can cope with almost everything, but not for ever and not pushed to stupid limits by our culture.
The cognitive step I had to take was to go from being fat to ingesting mainly fat. If there are three macronutrients and we want to limit protein to about 15% of our diet, then we have to eat an awful lot of fat. A diet that claims to be low carb but isn’t 75% fat is missing some basic arithmetic.
The polarisation is interesting. I think that it is based on misconceptions about metabolism. The medical establishment is afraid of and misinformed about nutritional ketosis. If people want to be veggie then I think they end up needing to limit calories in order to be in ketosis which seems to be necessary for health, at least intermittently. But for champion athletes like Ronaldo that is no use. I have a brother-in-law, a bit OCD, who eats mountains of carbs but it turns out he runs 15 miles first thing before eating, so almost certainly he is fat-adapted though I assumed he wasn’t.
Between the timescale factors (not clinical trial was ever done over those timescales) and the mis-classification of mechanisms, I suspect the confusion and the complexity can be largely understood. Having said which I agree that because of all sorts of gut biome and associated epigenetic effects, dietary advice had better be about the person not about the theory, let alone the dogma.
The depth of passion and antagonism attached to the diet wars is just amazing. And the scope for confirmation bias is unprecedented too. But I think sanity is available. If you would like to monitor at a distance my next experiment it is to sort out my wife’s arthritis of 20 years once she is clear of the stress of teaching.