for me, the issue of nutrition isn't a huge mystery
It needs to be 
1. Bioavailable/assimilable
2. Nutrient dense
3. Void of additional toxins
If your body is sick, it becomes overburdened, and you want to reduce the workload required in order to assimilate necessary nutrients
how much flexibility one has to play around with nutrition is predicated by a combination of factors, including:

1. Adaptation
2. Resource buffering
so theoretically all sorts of dietary regimens can work fine for most people, most of the time

the moments when you see the real difference is when the body is sick, and constraints need to be applied to ensure the highest degree of assimilation of the most vital nutrients