It amazes me the simplistic view that you and jtimon have of any science. So, there is no clear distinction between food and poison... actually there is. There is no clear distrinction between certain foods and certain substances, in particular circumstances, doses, etc... but we generally know what constitutes poison and what not.

Science is not about this or that, black or white. That's philosophical bullshit. Science is about studying the millions of degrees in between things, defining the different conditions, how those play out when you change the variables etc.

According to your reasoning, nutrition is not a science and serves nothing, because there is no distinction between what's good for you and what's not. We might as well drink arsenic instead of water, because we there is no clear distinction between what's good and bad for us, and we can't know anything!

We all know that's bullshit. Certain foods and good, in certain doses, to certain people. To others, it plays out differently. But this is not a deficiency of science, this is its very strength, it has the ability to adapt and be scalable.