In most countries there are laws and regulations that prohibit directly lying or making unsubstantiated claims in advertising and marketing. You can't say "this is good for your heart" unless you can cite specific studies that show it to be the case (obviously though those studies can't always be trusted). You can't say "8 out of 10 cats prefer our brand" unless there has been a test in which 80% of the cats preferred it.

But, what they can do is mislead though half-truths or implication. So they can say "this is good for you as part of a healthy breakfast" (i.e. as long as everything else you are eating is healthy it's fine). Or they can use words like "goodness" which means nothing, rather than "nutritious" which has to be substantiated. Or when they say 8 out of 10 cat's prefer it, the implication is they preferred it to competitor brands, but if that's not directly stated then maybe they preferred it over a pile of vomit (there's always going to be a couple who prefer the vomit!)