Reaction pathways in body are complicated. It will convert all kind of nutrients to needed molecules. To make it more complex there is many different kind of inhibition mechanisms to limit conversions. Chemistry and physics is well understood but it will take time to map and understand all cumulative reactions.
On the other hand human kind is proven in practise that you can survive with crappy diet quite well. I don't think caveman's main reason to die was crappy diet but rather diseases and accidents.
However nowdays it is necessary, not just to survive, but feel good and make optimal procress and have good energy levels trough day.
To my experience eating less calories then you use will make you thiner. But it will also make you hungry if you eat energy rich foods. If you switch low energy foods you can eat more and it will stretch your stomach more wich tells your brain that you are full. Therefor you are not hungry anymore.
Making things even more complicated, people are not clones. Then there is the human mind screwing things and making shit up.