Just to be clear, diet sodas are NOT SCD legal. I know that Elaine (who wrote Breaking the Vicious Cycle) allowed one diet soda every so often, but she mentioned saccharine sweetened beverages, which are pretty rare these days. I'm sure she didn't anticipate the wide variety of illegal ingredients which are present in diet sodas today, either.
Understanding why these drinks are illegal, though, is important, and if you ARE going to have the occasional diet soda it's still REALLY important that you READ the LABEL.
I was in for a very rude awakening when I started searching around for a diet soda that didn't have too much bad stuff in it. Here are some of the things you should be on the lookout for:
Sugar and corn syrup: In a diet drink? Say it ain't so! However, guess what? Diet orange Crush contains sugar! Yep. Those 25 calories per serving are from corn syrup. From CrushSoda.com:
"Diet Crush has 25 calories per 12oz. can due to the perfect marriage between corn syrup and aspartame."
That is an absolute no-no on SCD. Avoid!
Modified food starch or corn starch: In diet soda? Yah you betcha!
According to MugRootBeer.com, Mug brand sodas contain modified corn starch. That makes them gluten free, but not corn free. So this soda (and others containing food starch) would not be suitable for SCDers.
Gum arabic or acacia gum: These can be found in many diet sodas, including Diet Mountain Dew. Here's an excerpt of what wikipedia says about gum arabic:
"Gum arabic is a complex mixture of polysaccharides and glycoproteins that is used primarily in the food industry as a stabilizer."
Polysaccharides, as we know, are a BIG no-no on SCD.
OK, so what should we do? Make our own diet drinks? Well...
Packeted sugar substitutes usually contain maltodextrin (from corn) as a bulking agent. If you purchase a diet drink or soda, though, this bulking agent is not present, but it IS present in packets and granulated sugar substitutes sold in stores. Online, you can find low carb people grumbling about Splenda not being zero carb -- and that's why. The corn adds about 1 gram of carb per tablespoon.
The new Truvia sweetener has erythritol as a bulking agent, and so it may be a better choice -- if you use it occasionally.
So, just to be clear, diet sodas ARE illegal. But if you are an advanced SCDer who is going to occasionally have one, steer clear of any sodas with the above bold ingredients.
You may also want to consider Zevia. Elaine approved small amounts of stevia in some things, but as with everything, try a little bit and see how you react.