I want to briefly clarify our position about the 30-day trial. Although we hypothesise that there may be benefits to avoiding or minimising plants, it is definitely not necessary to avoid plants to gain the benefits of ketosis! None of the studies that form the basis of our beliefs about ketogenic diets had any such restriction. As we said in the explanation of the experiment, you can even formulate a vegan diet that is ketogenic.
The 30-day trial can be compared to the original Atkins (1972) induction phase  or the Eades' “meat weeks” in their book The 6-week cure for the Middle-Aged Middle. While both of those allow a small amount of certain vegetables, one basic idea is shared:
More carbohydrate restriction can be more effective, especially at the beginning, even if you ultimately find a comfortable long-term diet containing more carbohydrate.
In addition to minimising the carbohydrate amount, the main reason we recommend an all-meat diet for the 30-day trial is that it helps avoid these pitfalls:
- Accidentally misunderstanding carb sources (we've had several people tell us later they never realised that a food they were eating had that much carb in it).
- More generally, not having to count things, which can be a source of anxiety and error.
- Slippery-sloping: a little leads to a little more until the threshold is passed.
- If you happen to fall on the low side of carb tolerance, you might miss the benefits entirely by choosing the wrong arbitrary cut-off, and then conclude it doesn't work for you.
- Interference from food intolerances, fiber, or anti-nutrients (such as goitrogens or lectins).
Also, anecdotally, I and several people I have met had profoundly higher benefits from eating just meat. We don't know the mechanism for sure. Since you are going to all the trouble to try a keto diet anyway, you might as well find out if you are one of those at the same time.
If you are not one of those, you can go back to eating plants and no harm will have been done. I wish you much enjoyment. Vegetables can be exquisite, and I'd cry into my plate about my fate, if I didn't feel so fantastic and my choices weren't also excellent.
I categorise the Atkins diet into 3 major releases.
In the first edition, Dr. Atkins' Diet Revolution, there were no products, and there was no notion of “net carbs” (in which you subtract fiber from your carb count). You were allowed up to 2 cups of vegetables from a short list (mostly leafy greens). In the second major release, Dr. Atkins' New Diet Revolution, this becomes up to 20 grams, which he says is approximately 3 cups of salad, but which can be made up of any foods resulting in that count. The third major release, The New Atkins for a New You, came after his death, and was written by Drs. Westman, Phinney, and Volek. Its induction phase allows 20 grams of net carbs, which ultimately translates into a lot more vegetable matter.
There are groups to be found on the internet who live on what they call Atkins 72, because they have found that only that level of restriction works for them.