Most populations weren’t engaging in fasting for extended amounts of time. You can pretty easily go 3-days without electrolytes, probably more like 5 as the body will have enough stores for those short amounts of time.
Otherwise there is some degree of electrolytes in most water sources… some more than others.
My guess is that our pre-agrarian ancestors had sub-optimal electrolytes when they were involuntarily fasting. I’d think they also had to sustain mental focus and physical energy in small bursts while hunting for food to end the fast, and that active hunting was pursued by the most fit.
Current life demands that most of us maintain focus while navigating commutes, being productive at the office, and conducting most of the aspects of modern life. To another poster’s point, hunter-gatherers also likely didn’t carry as much excess weight as much of current society. Not all of our advances are beneficial, but using science to safely and comfortably conduct a fast, and thereby making it possible for those of all different levels of fitness, seems like an improvement to me.
I’m grateful for electrolytes, because they make it easier for me to maintain a fast beyond 24 hours. Without electrolytes I get brain fog, heart palpitations, dizziness and muscle cramps.
My first attempts to fast resulted in GI distress because of two key mistakes: I used Epsom salts (much more comfortable to take magnesium via pill) and later I attempted to consume them quickly (rather than nurse them over time). I’ve since improved my approach by learning from this sub.
Lots of people remind others on r/fasting to take electrolytes because it has significantly helped us, and it’s common to hear issues that new fasters experience that can be avoided by employing electrolytes.