When I first started fasting I had times when I found it difficult to concentrate and had a lack of energy.
What worked for me was to start with shorter fasts and consume less carbs when feeding. As time went on I was able to lengthen my fasts without feeling any negative effects.
As always, listen to your body and do what’s right for you. If I was fainting, I might also reach out to my doctor to get their opinion - only because it’s not something I commonly hear about in the fasting community and it’s definitely not common for me personally.
As a side note, everyone here is different. We have different histories, habits, and so much more. If fasting is new to you, take it slow. I was dazzled by people fasting for 30+ days and was so eager to just get it over with, but what I’ve found works for me has been slowly adjusting over time.