That would be libel and could get you in trouble if you meet some really stringent "ifs". First, the soda company you accuse needs to prove that at the time of writing you either knew it was a lie, or did not care. Secondly, they have to prove you caused them actual damages to their product or image (since people don't want cancer, they would probably drink more soda if they honestly believed you). Finally, since the soda company is a public entity for the purposes of defamation law, they would have to prove that your actions were malicious... so if you claimed this in what is clearly a farcical literature or intended it to be, you're off the hook.
I put in "public entity" to point out that the rules change if the person was not a public entity at the time of the defamatory report. Lets say I'm a reporter and write an article describing Old Man Jenkins, a farmer who know one really knows, is stealing money from the bank and dressing up as a local supernatural monster known as The Creeper to scare away people from the crime scene. Since Mr. Jenkins is not known until I publish the story so his forced entry into the public stage is impugning his good character and calling down a gaggle of meddling kids and their dogs to his property, I can be sued for defamation by Mr. Jenkins and he would not have to prove malice intent on my part. This would become even more apparent when some Meddling Kids and their Dog trap the Creeper a bale of hay and pull off the mask, revealing it to be Mr. Caswell, the Bank manager.
It also gets tricky as you can only lie about facts, not beliefs. If your book says "I believe soda cures cancer" rather than "Soda kills cancer" you are not lying, as you do in fact believe that statement. In the U.S. you do have a right to express your own beliefs no matter how stupid they are.
Similarly, had I reported "Old Man Jenkins is suspected to be the Creeper" rather than "Mr. Jenkins is the Creeper" I also have not lied. Someone (me at the least) does suspect him of being the Creeper thus, the statement is true. On the flip side, suppose I report "Mr. Caswell is the Creeper and Bank Robber", prior to Caswell receiving a guilty verdict in a court of law, I am committing libel. Mr. Caswell is only accused of a crime, but is still innocent until proven guilty by jury. Yes... even if we have photographic evidence and a signed confession.
Finally, remember that J. Jonah Jamerson takes great umbrage at being accused of Slandering Spider-man in his paper. Because he never spoke those words. He libeled Spider-man because he printed those words.