Hormesis
Hormesis describes a process where the effect of a chemical, substance, force, or field is beneficial to an organism through one range of doses, and harmful through another range. Usually lower doses are increasingly beneficial to a threshold where they become harmful. An example is the sun, where lower doses are beneficial, and very high doses harmful. Salt is another example.
Hormesis is an interactive process, and so the optimal level is always going to vary according to the person. Because of this, determing the safe dose objectively for all people with certainty is difficult. Now, people should be able to regulate themselves and their intakes by how they feel. Our culture however, insists that people should look outside themselves for knowledge of what is good for them. This results in a finding of 'no safe dose' and the demonization of such traditional adjuncts to health as sun, salt, sauna, etc..