You can train your body to incredible lengths. I became much more sporty some 12-13 years ago. In the beginning, if I did some longer hike or skitour (3+hours w

You can train your body to incredible lengths. I became much more sporty some 12-13 years ago. In the beginning, if I did some longer hike or skitour (3+hours with 1000+ altitude difference), I got pounding headache no matter how much water I drank. I was used to eating 3x per days too. Rewind few years, doing many more (mostly mountain-related) sports/activities but cca 6x/week, I can wake up on rest day and go on without eating easily till evening without even noticing much difference, water/tea is enough. No headaches after sports unless I do something too intense or its too hot and I completely dehydrate myself. I still eat before some proper effort, didn't ever try doing them on empty stomach. A friend of mine did interesting effort on his own - he is into half marathons and marathons, and learned his body to not rely on carbs but purely fat. So unlike many others in the races, he isn't replenishing quick sugars but just fat. He enjoys no glycemic swings but rather flat levels of energy