Yale law professor Jack Balkin proposes three. Here’s one of them:
The Moral Hazard/Adverse Selection Principle. Congress can regulate activities that substantially affect commerce. Under the necessary and proper clause, Congress can require people to engage in commerce when necessary to prevent problems of moral hazard or adverse selection created by its regulation of commerce. But if there is no problem of moral hazard or adverse selection, Congress cannot compel commerce. . .
Why not broccoli? There is no moral hazard or adverse selection problem created when people refuse to buy broccoli. It’s true that buying and eating broccoli might make you healthier, but people don’t wait until they are sick to buy broccoli. That’s because broccoli is not going to do them much good at that point. In this sense, broccoli doesn’t work like health insurance.