The scientists fed rats drinking water that contained either no, i.e. 0mg, 500mg or 1.000mg aspartame per kg body weight day for a period of 160days. The rats from the 1.000mg/kg group showed...
[...] a significant increase in activities of alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP) and γ- glutamyl transferase (GGT). The concentration of reduced glutathione (GSH) and the activity of glutathione peroxidase (GPx), and glutathione reductase (GR) were significantly reduced in the liver of rats that had received aspartame (1000mg/kg.b.wt).The presence of "leukocyte infiltration in aspartame-treated rats (1000mg/kg.b.wt)" further underlines that high dose aspartame consumption over a period of 160 days induced "hepatocellular injury and alterations in liver antioxidant status" by increasing the detoxification burden on the liver to supra-physiological levels.
Yet, what does that mean for the average consumer? Let us put the figures into perspective: 1 liter of diet coke contains about 390mg of aspartame. The rats in the high dose aspartame group in the study (which is the only one where anti-oxidant status fell enough to induce histological side effects) received 1.000mg/kg of aspartame per day. In human equivalent dosages (HED), this is 162mg/kg, which would be 11.35g of aspartame or 29 l diet coke for a 70kg adult. Not that I want to trivialize the possible liver toxicity due to aspartame consumption, but let's be honest: you would probably long have died from hyperhydration before you would notice any of the toxic effects induced by the consumption of an artificial sweetener, which - far from being healthy - is yet not as toxic as some health-fetishists would have it.
In spite of the fact that a diet coke here and there won't kill you, you should still ask yourselves, whether or not you really need this "spawn" of a society where we want all the convenience and (unnaturally sweet) taste of unhealthy foods without the negative consequences for our health? I mean, you obviously chose the "diet" version because you want to avoid sugar; but do you still want the sweet taste, which (at least for some artificial sweeteners, eg. Nakagawa. 2009) has been shown to increase insulin secretion even in the absence of direct beta cell stimulation!? If you are willing to accept this compromise and do not care about the other chemicals your average can of diet coke "nourishes" you with, fine! But do not tell me later on, I had told you that diet coke was good for you ;-)