I'd take my chances with the occasional regular cola, before I would put any of that diet crap into my body. I'll pass on something that breaks down into methanol and formaldehyde, thanks very much.
There are sodas that use sucralose rather than aspartame.
This from a physics forum
In the body Aspartame metabolises primarily to Phenylalanine ~50%, secondarily to Aspartic Acid ~40%, and Methanol ~10%.
I am unable to find out what happens to Aspartame in the acidic environment of our stomach. According to Wikipedia, "Under strongly acidic or alkaline conditions, aspartame may generate methanol by hydrolysis." which I have confirmed with several other research resources. I do not know how 'strongly acidic or alkaline' it must be. I would like to find this out. I am also unable to obtain pure aspartame to test its reactivity with different acids or chemicals.
~10% x 111mg = ~11.1mg of methanol after metabolization
Wikipedia - If ingested, as little as 10 mL of pure methanol
can cause permanent blindness by destruction of the optic nerve,
and 30 ml is potentially fatal.
http://en.wikipedia.org/wiki/Aspartame
So for a quantity of Aspartame that would cause permanent damage and severe symptoms (if fully metabolized and absorbed) I shall calculate approximately how much you need to consume.
Methanol's density is 0.791 g/mL
Mass = density * volume
Mass = 0.791*10.00 (for 10ml)
Mass = 7.91
Mass conversion: 7.91 g = 7,910mg
7,910mg / 11.1mg = 712.612612-(just keeps going)
713~ cans of Pepsi would produce a toxic methanol dose after metabolization (assuming nothing else would affect it)
(also assuming you hadn't already suffered from some other effect from some other ingredient [like blood dilution from all that water? lol])
My takeaway from this is the old expression is that
The dose makes the poison, a principle of toxicology, as expressed by Paracelsus, i.e. that a substance can produce the harmful effect associated with its toxic properties only if it reaches a susceptible biological system within the body in a high enough concentration (dose).[1]
Having said this I still prefer diet soda sweetened with splenda (sucralose) based on my perception that there are more health complaints about aspartame than sucralose. My sense of regular cola is that it is not a natural product. The production of sugar (basically the major component of sweetened soda) is quite an industrial process and was certainly not a significant part of our diet until the last few hundred years. I recommend "Good Calories, Bad Calories" by Gary Taubes to get in to the health effect of excess sugar in the diet.