I heard a local veterinarian to say that Whiskas can cause kidney disease. I tried to research that claim, but so far I haven't been able to find academic articles dealing with this issue, even though there is a vast amount of anecdotal evidence.
I was wondering if anyone knows for certain if this is true or false. Apparently, older cats have propensity to suffer from kidney disease. However, some people say veterinarians have told them that Whiskas was responsible for the death of their cats. I suppose that veterinarians know about the incidence of kidney disease in cats, so it is surprising to read that kind of blanket statements being made by professionals.
Some people say that taurine is to blame. However, taurine is classified as an essential nutrient by the VCA Animal Hospitals. Others say that dry food or fiber is what makes cats sick, but again no evidence is offered to support this view.
UPDATE: In response to @DanS:
[...] Do some more research, are the salt levels [in Whiskas] high (magnesium etc.) are the protein levels high?
It is hard to answer precisely your question. As far as I know, there are not guidelines on daily recommended intake for cats. I even found some opinions saying that nobody knows (e.g. here). However, when it comes to protein there seems to be a magic number: 36%. It almost seems that food companies strive to reach that number. Of course, the question should be: 36% of what? In the package I bought, it says that the recommended amount is between 45g and 90g depending on age. This is equivalent to 16g and 32g of protein, respectively. Keep in mind that the product I bought is intended for kittens. Now, the only table of values I found was published (pdf) by FEDIAF (page 17.) It says the recommended amount of protein is 25.0g for adults and 28-30g when cats are still growing per 100g of dry food. As you can see, if my package contained 100g, it would have 35.5g of protein, an increase of 15% with respect to the highest recommended daily intake given by FEDIAF. Is this a significant increase? I don't know.
As for salts, I only know that Whiskas contains iodized salt but the product I bought doesn't say the amount. Other ingredients are specified in another paragraph but without percentages or amounts.