Does dieting impact your productivity?

Absolutely! I did keto for a while: I was literally able to have productive 12+ hour days that way. Having perfectly stable blood sugar (or rather the lack of it) did wonders for my concentration.

The downside: I lost too much weight, got terrible keto rash, and had constant bad breath. Also no social food events at all.

I’ve since settled on a mixed diet with lots of animal protein and whole grain carbs. This is a sustainable compromise that still positively influences my ability to focus and my physical performance.

I'm not Muslim, but I participate in the fasting aspect of Ramadan every year. It absolutely impacts my energy level. Rather than try to feel normal, I just try to accept the situation and keep a good attitude about the whole thing. Works fairly well.

In my personal experience you can eat less and maintain energy better if you get sufficient protein (caffeine helps too, though).

Edit: and proper hydration is important for energy too!

but I have the impression it impacts my productivity and my mood: I get tired, I have low energy, even lethargic.

You are making something come true in your head when it might not be true at all. I used to think I needed breakfast soon as I woke up. Then I went a few days without breakfast till a bit later. It was fine.

What you start to believe adjusts the ceiling and floor of what you can do. Right now you've just moved that ceiling way down.

If you are feeling that way, it might be worth it to investigate your caloric intake and macros, and adjust expectations once your diet is set to a healthy level. Can you share a bit of your diet plan, if it's not too personal?

What I mean is, are you eating 1200 calories a day and near zero protein? That might explain your lack of energy. I've been known to do that before and learnt from the experience.

Yeah, I'm going to be one of those folks. Intermittent fasting always worked for me.

I think because it just keeps everything even or at least the hour-to-hour delta to a minimum. When you break it down into smaller meals, even if you are following the low calories through out the day, it can create a higher high/low that can swing through out the day.

Your mileage may vary.

Try taking cold showers, the adrenaline rush helps me get awake and be active. Plus regular exercise also helps towards a general good mood throughout the day.

This is something I’d talk to a professional rather than online.

Yes, I skip breakfast about 4 times per week. That's it.