I have no idea what you mean.
Are you saying that one can never study the impact of a glass of wine a day on people that drink once a week?
Or we can never find out what amount of drinking water supports a heatly life style because people have to drink some water to subsist and therefore you can never find a control group?
I'll try to explain this one more time. No circles of reasoning here. Usual background radiation results in normal life. So-called "Low level" radiation (which is higher than background) results in lower cancer rates than "normal life". High levels of radiation result in higher cancer rates than "normal life".
No, that’s not what I’m saying. What I’m saying is, it’s misleading to state “low levels of radiation don’t cause cancer” as a hypothesis, when you have defined “low levels” to be those that do not cause cancer. The hypothesis is essentially meaningless.
For the glass of wine example, the amount you are studying is not dependent on the conclusion you will reach, so you could fine that a glass of wine a day is fine, or you could find out it is not. However, if you said “low levels of wine”, instead of a glass a day, and then defined “low levels” to be whatever is not harmful, then you will reach a foregone conclusion, due to circular reasoning.