I haven't noticed a real drop in energy, however I have noticed significant brain fog, lightheadedness and irritability in the afternoon. I also have significant feelings of hunger all day, to the point where it's distracting.
I realized I needed to be adding some kind of salt to my intake during the day to prevent the lightheadedness. The only zero calorie solution is basically adding basic electrolyte to my water, which means I end up drinking a weak salt brine most of the day which is not very pleasant.
I'm not sure if this is sustainable because it takes up so much of my mental energy. There doesn't seem to be much in the way of good resource for doing this kind of diet, so I'm excited to see what comes out in the next few years. Perhaps it's just a transition period and it will work out.