I’m aware that what I’m about to ask is the intellectual equivalent of taking your date to a monster-truck rally — that is, sure evidence of low-brow benightednes and crude sensibilities — but on what Constitutional basis does the national government in the United States regulate the contents of school lunches?
That Uncle Sam does regulate school-lunch contents is beyond question. See this report  in today’s New York Times informing us that "A bipartisan group in Congress plans to introduce legislation today
that would prohibit the sale in school not only of French fries but
also of other fatty or sugary foods, including soft drinks."
Put aside all questions of the desirability of such legislation and ask "Is this legislation Constitutional?"
I’ve read the U.S. Constitution  several times, and nowhere — not remotely, not even as a penumbra emanating from its text — does it give to the national government the power to regulate the contents of school lunches. And yet, such a fact inspires no apparent hesitation in the typical member of Congress to regulate in this way.
Keep in mind that each member of Congress is sworn to uphold the Constitution. This oath, however, obviously matters less than does the fact that
Senator Lisa Murkowski, Republican of Alaska, has watched what goes on in the school her two teenage sons attend.
"We talk a lot about healthy nutrition ,
we teach the kids about the food pyramid, and then they go down the
hallway and get the high fat, high sodium and high junk available in
the vending machines," Ms. Murkowski said. "We need to be consistent.
People are beginning to connect the dots between rising health care
costs and obesity."