Artificial sweeteners are a topic of extreme misunderstandings and myths. Combine this with the typical effect of the internet, where myths that sounded good can easily get multiplied a thousandfold, you end up with a very tricky jungle of facts, fiction, myths and ideologies. A small attempt at fixing this.
Here, dearest internet, my own contribution to the wealth of badly-cited claims regarding food, as I didn't actually read all of those studies myself, but simply believed those who cited them. Thanks especially to Wikipedia for many pointers.
Do artificial sweeteners cause cancer?
The study this is based on seems to have used quantities completely out of proportion, and did use them on rats, where the cancerous reaction is likely based on a rat-specific reaction to sodium salts, hence the results of the study are basically irrelevant for humans. The current position of major health agencies in Germany is that there is no scientifically-backed connection between artificial sweeteners and cancer in the typically consumed quantities.
Cf. Martin R. Weihrauch et. al.: Künstliche Süßstoffe – Haben sie ein kanzerogenes Potential?. In: Medizinische Klinik 96, 2001, No. 11, p. 670–675 and Süßstoffe in der Ernährung, Deutsche Gesellschaft für Ernährung, 2007, accessed 2012-06-01
Do artificial sweeteners cause weight gain?
This is a very widespread claim on the internet, and apparently based on a specific study from 1986 by Bundle and Hill. A meta-study in 2007 compared 19 different studies, noting that 3 found artificial sweeteners to increase appetite, 3 found them to reduce appetite, and the remaining 13 were unable to find a clear link between artificial sweeteners and appetite.
Cf. F. Bellisle/A. Drewnowski: Intense sweeteners, energy intake and the control of body weight. In: European Journal of Clinical Nutrition 61, 2007, p. 691–700
The claimed link between artificial sweeteners and appetite caused the creation of a lot of myths, including a claim that they cause an insulin spike. The only clear link between artificial sweeteners and insulin levels was found with in-vitro tests of sweeteners and pancreas cells, though, and also only for a few artificial sweeteners, and not, say, the infamous Aspertame. Meaning this hypothesis is pretty much refuted.
Cf. ibid. and Willy J. Malaisse et al.: Effects of Artificial Sweeteners on Insulin Release and Cationic Fluxes in Rat Pancreatic Islets In: Cellular Signalling Vol. 10, No. 10,, 1998, p. 627–733 (ISSN 0898-6568/98)
Finally, the use of artificial sweeteners in pig fattening is often cited as a clear proof that they have to increase weight. The official reasoning given for their use is that they make normal pig food more interesting to piglets, allowing them to switch from their mother's milk to the fattening food earlier. A direct increase in appetite has been denied. A number of people, including some experts, claim that this is a lie.
Cf. various, e.g. Christop Drösser: Süßstoff für Ferkel, Zeit Online 2008, accessed 2012-06-01
This doesn't mean that artificial sweeteners are cool and you should use them a lot. It means that switching from e.g. Coca Cola to Coke Zero is not a health problem and can support a diet, but it's certainly not the only component of a diet. Be especially wary of using Coke Zero as your main source of water. Ideally, you drink water to satisfy your thirst, and drink coke zero (or milk, or orange juice, or …) for the taste primarily, in smaller quantities.