I've been hearing that type of argument since the 70s, after they banned saccharine.

Let's suppose a chemical causes slow damage to the body, maybe greatly increasing your risk of cancer or alzheimers if you drink a can of diet soda per day. How could we possibly know if it is safe?

We often test chemicals on rats, but the problem is that they only live a few years. No one wants to wait 20+ years to see the results of an expensive study on longer lived mammals anyway.

The solution is we feed the rats excessive amounts of the chemical and see if it causes problems. This isn't a great way to do it, but what other option would you suggest?