I've often heard that aspartame stays in your body forever and causes cancer. Is any of it true?
According to the National Health Institute, there is no evidence suggesting aspartame causes cancer:
The study they refer to is Opinion of the Scientific Panel AFC related to a new long-term carcinogenicity study on aspartame by The European Food Safety Authority. It can be found here.
If you're curious about this subject, the whole page on the National Health Institute's site is worth reading. It offers a comprehensive explanation of the finding, avoiding technical terms or linking to their dictionary when the term simply cannot be avoided.
If you give more weight to independent scientific research then yes, aspartame may cause cancer in humans. If you prefer industry-funded studies, then the answer is no, aspartame has no adverse health effects for humans.
Independent scientific research has overwhelmingly found health issues with aspartame, in particular that it does cause cancer in rats. A 2005 article from The Guardian references a 1996 review of aspartame research which found that:
The primary independent scientific studies which reported increased tumors in rats originate from the European Ramazzini Foundation of Oncology and Environmental Sciences (ERF), led by Morando Soffritti. They have conducted a number of studies on the carcinogenic potential of aspartame, concluding that aspartame is a potential carcinogen at normal dietary doses:
The primary industry-funded review cited when dismissing safety concerns around aspartame is the 2007 Burdock safety evaluation. As this review was funded by Ajinomoto (the largest manufacturer of aspartame), and conducted by the Burdock Group (an organisation whose commercial focus is on gaining regulatory approval of substances for clients) serious questions have been raised about its validity.
The scientists who led the Burdock safety evaluation had numerous conflict-of-interest issues, including lead scientist Bernadene Magnuson who is a consultant to the aspartame and soft drink industry, and has travelled the world funded by companies like Coca-Cola.
The ERF studies led by Morando Soffritti have also been criticised as flawed, but it is interesting to note that this criticism was led by the Burdock Group. In a letter to Environmental Health Perspectives Soffritti is highly critical of Magnuson and the Burdock Group, drawing attention to their industry funding and accusing them of misleading readers:
The other part of your question is whether aspartame "stays in your body forever". That is not a concern either cited by aspartame critics or mentioned in independent scientific studies, so the answer to that is no, it does not stay in your body forever.
However that particular question is representative of the sorts of claims which can easily obscure legitimate scientific concern over aspartame. By focusing on extreme and baseless claims, and emphasizing a link to the more fringe activist groups (who tend to hold extreme views on fluoridation, vaccination and the like), it becomes that much easier to denigrate opposition to aspartame, and dissuade others from digging into the real science on which concern over aspartame is based.
A good example which illustrate the problems faced by skeptics intent on knowing whether aspartame causes cancer is the Wikipedia article on the aspartame controversy. A number of editors very well-versed in Wikipedia rules have ensured that over the years this particular article maintains an industry point of view. For instance, instead of emphasizing the controversy's basis in independent scientific research, the article gives undue weight to individual activists and a hoax letter:
Furthermore, the ERF studies by Soffriti are downplayed and dismissed, without mentioning that the criticisms originate from the Burdock Group:
Various attempts at disclosing the industry links of that particular study are eventually removed by editors. A number of concerted efforts at bringing more balance to the article have failed, with a recent attempt resulting in creation of an aspartame page at sourcewatch.
A skeptic wanting to know whether aspartame causes cancer would do well to compare both the Wikipedia version and the Sourcewatch version to get a fuller pictures of the issues around this additive.
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In general with questions like this, you should try to look at all the available evidence and see what conclusions can be drawn from that.
Often you'll be able to find some studies that support one side, and some that support the other, but what does the overall body of research say? Of course it's possible you'll find a similar amount of research supporting either side, in which case the conclusion would be that we don't know yet.
Fortunately, the work has sometimes already been done for you, which is the case with aspartame. In 2007 a review of "the scientific literature on the absorption and metabolism, the current consumption levels worldwide, the toxicology, and recent epidemiological studies on aspartame" was done.
Specifically related to cancer the conclusion was: "The studies provide no evidence to support an association between aspartame and cancer in any tissue." And more in general: "The weight of existing evidence is that aspartame is safe at current levels of consumption as a nonnutritive sweetener."
There's still some open questions from a study newer than those cited in some other answers: Life-Span Exposure to Low Doses of Aspartame Beginning during Prenatal Life Increases Cancer Effects in Rats
This study found statistically significant increases in lymphomas and leukemias in rats fed several times a normal human intake over the course of the rats' lifetimes.
The study also raises concern with the methodology used in earlier studies. Specifically:
The Center for Science in the Public Interest, a group which lobbies for truth in labeling, summarizes the issue here, mentioning industry reaction to this study. They note that "It is likely that the new studies found problems that earlier company-sponsored studies did not because the Italian researchers monitored the rats for three years instead of two. The Italian tests remain controversial, with the industry contending that they were flawed in several ways and with the FDA stating its scientists couldn't evaluate the studies because the researchers refused to provide their original data."
So, eh. I don't think there's cause for a panic, but the question doesn't seem completely resolved. It's worth nothing that CSPI doesn't seem to give a blanket "fail" to all artificial sweeteners, currently listing Sucralose (Splenda) as appearing to be safe. Oh, and Neotame as well. In fact, since neotame is 40x sweeter than aspartame, requiring correspondingly lower dosage, that may be the safest bet so far.
Oh, and the idea that aspartame stays in your body forever is nonsense. It breaks down into aspartic acid, phenylalanine and methanol.
The wikipedia page about the "aspartame controversy", especially the safete/health part, has more references than I can easily type here. Check over there for the links included, it seems like a solid piece of work.
A short summary (for references see the original obviously, emph. mine)