Mark Kleiman, a blogger on public affairs, says:

So the FDA has what might be called the "quacks like a duck" rule: if you're going to claim some specific medical efficacy for your nutritional supplement or food product — that is, if you're going to market it as if it were a pharmaceutical — then you have to make a showing of safety and efficacy. If it's marketed as a drug, then its marketing is regulated as if it were drug marketing. Legally, a food sold as a treatment for a disease falls into the category of "drug."

General Mills, which makes Cheerios, has been marketing it as an aid to lowering cholesterol. The claim is quite specific: "You can lower your cholesterol by 4 per cent in six weeks." It's true that oat bran has some cholesterol-lowering effect. It's not true that eating Cheerios has been shown to be a safe and effective way of doing so, or that there's FDA-standard evidence for the quantitative claim.

I've bolded what appears to be the crucial bit.