However I have no problems on keto, or even just a consistently low-GI low-carb diet. If anything, fasting seems to improve my discipline and "depth-first" thinking.
Absolutely.
> Even small fasts seem to sap my concentration and critical thinking. Once I eat again I feel like I think better.
That's kind of normal when you start with (intermittent) fasting, may take at least some days until it gets better. Especially if are on a high carb diet and already developed some degree of insulin resistance your body is unable to fully switch into fat burning mode while the glucose stores are already depleted. This results in low energy and food carvings. Once that is regulated well your mental abilities likely won't suffer.
It's key to not refeed carbs at all during fasting (no juices, honey, etc).
I also occasionally do keto between the fasts which might have helped my metabolism switching to ketosis more quickly.
Ketosis is - in brief - a metabolic state where the body primarily used fats to burn for energy. Our bodies are perfectly suited to operate on ketosis. But this is only reached if glycogens are completely burned which means to u need to avoid refeeding carbs entirely.
Basically it is simpler to eat nothing at all, than to eat at a caloric deficit.
Think of it this way: our bodies have evolutionarily adapted to be able to live off our fat reserves (when overweight).
I recommend reading material or watching videos of Dr. Jason Fung, a Canadian nephrologist who makes compelling well-sourced arguments and explanations.
I'm only speaking of IF, but yes. Multi-day fasts may be different, but I actually think best right after my workout during my IF window.
The effects are so pronounced I'm considering just eating a single meal each day right before bed.
On a carbs based diet, it feels like there's an animal inside me craving for food all the time. Then I feel drowsy and sleepy if I don't eat anything for even a few hours. As a developer, that's terrible. I feel really bad.
On a fat-based diet (e.g less than 40g of carbs/day), I feel absolutely great, and I don't feel hunger. When lunch comes, I feel just empty, but no craving at all, no hunger. Then the fasting becomes easy.
The caveat: switching from carbs to fat is not pleasant, takes a few days and isn't any fun. If you break it with a piece of cake, then you'll have to make the switch all over again and it won't be pleasant again.
That said, I have gone through fasts and said f-it at hour-mark 34 because I was on a deadline.
It's not a religion, so do what works for you - but I would at least try it.
Even small fasts seem to sap my concentration and critical thinking. Once I eat again I feel like I think better.