I switched over to LCHF/Banting for medium and long distance open water swimming and it’s worked very well. I did 5 miles of open water butterfly with no need for feeding along with a variety of 2 mile events both using front crawl and butterfly. Absolutely no problem with energy.
I was very worried about “bonking” so I repeatedly tested the energy by doing some limited fasting prior to swim practice. I couldn’t get myself to “bonk” even with 12 to 24 hours without food.
Further, both energy & speed for me has been very consistent for both shorter intervals (sprints) and longer intervals. My ability to hold faster intervals on 100 & 200 yard distances has improved drastically. I’m much less likely to peter out toward the end of practice…even when not eating all day.
Also dropped about 25 pounds without even trying. Strength remained constant in the weight room so I’m assuming it was mostly/all fat & water mass.
As always, individual results may vary!