If y = f(x) and x = f(y), then x and y are entangled variables.
Nutritional requirements for selenium and arsenic are likely entangled variables if:
1. The more selenium one consumes, the more the selenium interferes with arsenic metabolism.
2. The more arsenic one consumes, the more one depletes selenium pools.
3. The more arsenic one consumes, the more selenium one needs to consume.
4. The more selenium one consumes, the more arsenic one needs to consume.
If 1-4 are true, then the folly of adding selenium to multivitamins without adding the appropriate level of arsenic becomes obvious.
In general, current supplementation regimens fail because they un-balance key nutrients, whose requirements are entangled variables. What an entangled mess our current system is!