Disclaimer: I'm not an expert on this, just describing my own experience (which may be subjective/biased/completely misinterpreted by me).
If you're mostly sedentary in a first-world country, going off sugar/carbs-dominated diet brings your insulin sensitivity to its natural levels (which, very probably, it hasn't been since the moment you started eating adult food). For me this had the amazing effect of becoming more alert and productive mentally and more aware of my body physically. As an athlete, I suddenly discovered that I can be underperforming because I'm lacking fuel, and not because I'm tired, and that if I "fuel up" by eating carbs, I can expect a surge of energy that will allow me to perform well. Needless to say, the discovery was amazing, I felt like a finely tuned sports car or something.
Back when I was eating a regular western diet, my insulin sensitivity was low, and "injecting" fast carbs made me drowsy, not full of energy.
Needless to say once I experienced that new state, I got hooked, so sticking to good eating habits haven't required tremendous will power for me. Getting there, though, did.
I noticed this effect when I lost weight in the last 7 months, I dropped 60 odd lbs by switching to a diet of eggs, meat (mostly chicken), fish, low fat/carb dairy (cottage cheese, 0 fat greek yoghurt etc) and a large amount of vegetables (mostly steamed).
After a few months on that diet anything rich in sugar gave me a massive short lived energy rush, I actually didn't much like the feeling so now I only eat complex carbs and hardly anything with a lot of sugar (lots of cruciferous vegetables, not a lot of high sugar fruits).
I've found that throughout the day my mood and general feeling is pretty constant rather than up and down like a yo-yo.
I had a similar experience when I removed sugar and almost all carbs for 5 months. Switched to mostly protein and fat (meat/tuna/eggs/cheese/vegetables)
One major change was much higher "energy"/"alertness levels after lunch and after dinner. No more sleepy low energy "food coma" feeling after meals.
Where I would usually feel like laying on the sofa after a long day's work and dinner, now I had the urge to go out on my bicycle, even at night and in winter cold.
Although I have to admit that like most people who do drastic diet changes I had a relapse and went back to eating sugar again after the five months. This relapse happened gradually, where I naively convinced myself that I was no longer addicted to sugar, and started by having a treat once a week, which quickly became once a day ...
The mental change is still ingrained in me though, I now treat sugar like a dangerous addictive substance and I'm trying to get rid of it like any other "substance abuser".
I feel carbs in general can be my best friend if I use them wisely. But yes, pure sugar is just too potent. Sort of like taking pure heroin instead of an ibuprofen to treat a headache.
I recently did this as well and my experience has been the same. Did it mainly to manage my energy levels better. Now less reliant on caffeine and less prone to energy swings. Also in much better shape.
By western diet I meant the gratuitous consumption of highly potent energy sources in the form of white bread/soda/cake/sugar/etc., which is common in North America (can't speak about the rest of the Western world, I hear that Europeans are lean compared to North Americans).
Anyway, my approach is simple and unscientific, but it seems to work for me. I just treat any carb-rich food as fuel (there is a difference as to "slow" vs "fast" carbs, but it's not important for the purposes of this discussion :)).
So, since it's fuel, I use it as such. For example, if I'm in my sedentary/low stress mode, I eat just a token amount of them. If I'm preparing for a gruelling match or bulking up at the gym, I'll load up on carbs (ideally the day before the match). If I'm cutting, I reduce the amount considerably.
As for fat/protein/fiber, that's pretty much an invariant. I just eat a normal healthy amount. I do increase protein intake on the days I work out a lot, though.
If you're mostly sedentary in a first-world country, going off sugar/carbs-dominated diet brings your insulin sensitivity to its natural levels (which, very probably, it hasn't been since the moment you started eating adult food). For me this had the amazing effect of becoming more alert and productive mentally and more aware of my body physically. As an athlete, I suddenly discovered that I can be underperforming because I'm lacking fuel, and not because I'm tired, and that if I "fuel up" by eating carbs, I can expect a surge of energy that will allow me to perform well. Needless to say, the discovery was amazing, I felt like a finely tuned sports car or something.
Back when I was eating a regular western diet, my insulin sensitivity was low, and "injecting" fast carbs made me drowsy, not full of energy.
Needless to say once I experienced that new state, I got hooked, so sticking to good eating habits haven't required tremendous will power for me. Getting there, though, did.