Really good points. Cars and diet. It's an amusing image to have cops patrolling outside McDonald's and pointing guns at fat people and telling them to move along. Cars are a great example of harm to oneself as well as to others both through accidents and pollution, etc.
I would be glad if a Constitutional scholar could illuminate on the point you mention. Yes, in the simplest analysis, I believe your comment about the 10th Amendment is true, i.e., what's not granted to the Feds in the Constitution is reserved to the States or the People. I think it's described as an "additive" document as opposed as a "subtractive" document, i.e., the Feds only have power explicitly granted as opposed to all powers except for what's taken away.
There is the "elastic clause" (the phrasing of which I can't cite off the top of my head) which specifies things like how Congress may pass laws that seem to impinge upon the rights of the States, so long as they have direct bearing on Congress's power over interstate commerce. This is highly debatable in many cases, e.g., commerce goes over roads, so do the Feds have direct control over roads and streets within the States, and does this extend into the driveways of individuals, etc.?
Another tidbit about drug prohibition is that Harry J. Anslinger, first "drug czar" and first head of the Federal Bureau of Narcotics, devised a "Uniform Narcotics Act" which he got all of the States to pass individually. This was mainly for budget reasons, since drugs weren't a problem at the time, and Congress didn't have money to waste during The Great Depression, so Anslinger used public money to launch a propaganda campaign (against marijuana, e.g., "Reefer Madness") to scare the public and the States into passing his law. Clearly this would get around the 10th Amendment, even if there were no other issues, and it would be difficult to repeal, in principle. Congress later passed the 1937 Marihuana [sic] Tax Act, which may well have depended upon the elastic clause for its justification.
I don't know if this helps much. But I would say that the Elastic Clause of the Constitution and Anslinger's trick of getting individual State legislatures to pass more-or-less uniform laws (similar to what is the case with alcohol in The South despite the repeal of The Volstead Act, incidentally, i.e., "Dry Counties" -- my dad is from a dry county, by the way) are a couple of ways to get around the 10th Amendment, for example, in the case of drug prohibition laws.
Oh, in a similar vein (there's a pun here somewhere, but I can't find it...), another Interstate-Commerce-derived law aimed at giving the Feds jurisdiction over vice crimes (or consensual crimes), in this case, prostitution, is The Mann Act. It stipulates that it is illegal to transport a woman across State lines for the purpose of prostitution. Obviously if prostitution is illegal in a given county were it takes place, it's actionable in county court, but even if not, the Feds can swoop in, in principle. This came up in, I believe, the second season of Showtime's Boardwalk Empire, by the way.
Anyway, I'd like to see somebody put these pieces together and answer your question. It's one I've contemplated. Thanks!!