This is about a medical ketogenic diet for refractory epilepsy: Ketogenic diet - Wikipedia
If you look on the original research you can see that they would have zero carbs if that were practical, but very very low is really all that is really achievable in real life.
And before WW2 and insulin diabetics were kept on a similar diet but with hard core calorie restriction. I can remember seeing an article on some diabetic woman who was diagnosed pre WW2, I was amazed she managed to live so long through the whole experimental years. [Obviously she had amazing self control, as I seem to remember she was what is now considered insulin dependent through almost all of it.]Found her!