I'm not trying to imply that nicotine makes people move on to harder stuff, I think the effect is mild enough that most peoples pleasure/reward responses wouldn't be altered too much by it. I also don't buy substance a (that stuff being legalized in more and more states now) being truly a gateway, we know that the substance itself isn't actually addictive, people don't have substance a withdrawals . But there are substances out there that are addictive in and of themselves. Those to me are where the gateway really lies.
The research / researchers you've cited are saying nicotine makes people move onto harder stuff, as post #148 points out in the last quote I provided (by Kandel). The exact statement was: E-cigarettes may be a gateway to both combustible cigarettes and illicit drugs.
There are two or three substances of the *other stuff* variety that I think of being addictive in and of themselves. Nicotine is challenging to see that way in today's world. 10 years ago, I don't think anyone here (or anywhere) would've thought there was a challenge to be made, but eCigs and the knowledge that comes with learning vaping and politics of vaping has greatly skewed that debate. To the point where leading researchers show up as either behind the times or having blatant political agenda.
Sugar strikes me as far more addictive than nicotine, and is seemingly a non-issue when it comes to 'protecting kids from the gateway effect.' In all honesty though, that paradigm (around sugar) may be changing right about now. I can foresee a day in the not too distant future where candy (or sugar) is researched and pronounced a gateway to illicit drugs. Based on similar agenda to what is currently occurring with nicotine / smoking / vaping.
I think gateway debate is greatly skewed by the illicit aspect and human judgments that go with that (or prejudice). It makes for difference of seeing user as criminal (deserving punishment) and/or patient deserving treatment. I find people in the know lean (heavily) towards treatment, but they aren't only people who are making decisions for how to handle things, nor does that perception generally hold the majority view as once an item is painted in negative light, it is challenging to change prejudices. I use smoking as prime example of what I speak here.
Besides the illicit prejudice at work is the lies. The lies we tell each other about these substances, and more importantly, the lies we tell ourselves. A substance such as sugar will greatly test this sort of deception, as nicotine already does, for neither is illegal, but both are in new paradigm where they could be (or already are) painted in such an awful (and highly deceptive) light, that the prejudice makes for what I call the true core of the gateway effect.
We ask users (noobs) to be their own, positive guides on the pathway that desires to use, and enjoy these substances. IOW, we as a society behave as incredibly immature gatekeepers.