So lets see, people make decisions based on what feels right to them (gut feelings), ignore any conflicting information (confirmation bias) then develop philosophies based on these theories. Of course your going to get conflicting ideas, its all based on faulty logic. Garbage in, garbage out.
That said since we don't genetically know what we are designed to eat (like many animals do) we need good science to figure it out (though much of science is also built on false premises). What makes the most sense is we need certain nutrients and the paleo type diet is probably closest to correct, but modern food is processed, factory farmed and monocultured/bred making it not quite the same as our ancestors would have had. Also grains, processed sugar, trans fats and so forth are recent and likely not ideal health foods. Also eating the same things for generations may affect what our bodies are designed for, someone had an interesting premise that we should be eating what our ethnic ancestors have been eating because our bodies have adapted to it. It may be true, it may not be.
We cannot come to conclusions because we lack the necessary information to do so, leaving us to make best guesses and most people do this as stated above.
Its likely most vegetables are good for us, some fruit (but lets not forget selective breeding has given us fruit that never existed before), free range meats, and water. Items to avoid would be grains, excess carbohydrates, sugar, alcohol, artificial sweeteners, trans fats, juices, chocolate (eek). Dairy is hard to say, calcium and other nutrients are required but its hard to envision cavemen having dairy to consume.
Is there a way to determine the ideal diet, its hard to say, we are still figuring out whats in foods, various ingredients have been found to have effects on the body but are not considered vitamins, form Quercetin to Lycopene to Anthocyanins to infinite others.
