Whey protein

It's really hard to get what I think is enough protein (2.2g per kg of lean body mass, given my age, sex and activity level), without supplementation, especially while limiting my eating window and/or calories as I currently do. Whey protein as opposed to other sources because it's highly bioavailable and has an advantageous distribution of amino acids, and your actual requirement is not really for protein, it's for the amino acids you'll break it down into. Also as you age, your protein requirements to stave off age-related sarcopenia go up, and your ability to eat tends to go down. Also I lift, which adds even more to my requirements. 30g, offset from meals.

Leucine

Probably the most important branch chain amino for muscle protein synthesis. I get a decent amount in my diet and some more in my protein supplement, but I suspect I could use more, so I take more. 5g, with my protein shake

Taurine

A non-essential amino acid that isn't involved in synthesizing muscle protein or any other protein, supplementation of which appears to have quite a lot of benefits. 2g/day

Creatine monohydrate

Creatine is a critical part of replenishing ATP for cellular energy, particularly in muscle cells (all cells but I take it to keep my muscles going). If I eat ad libitum, and all beef, I get plenty in my diet, but I'm not currently quite doing that, so I'm supplementing 5g/day in my protein shake.

Collagen peptides

Lots of potential age-related benefits including skin elasticity and joint health. Also seems to help increase muscle mass with resistance training, though I couldn't explain how. I get a lot in my diet so I take half the recommended dose, 10g/day in my protein shake.

Electrolytes

I sweat a lot, particularly when I take long mid-day walks in Texas in July. Sometimes I get really tired and can't understand why, realize I've been slacking on electrolytes, pound some, suddenly feel amazing. In my opinion it's easy to do too much of this though - your main electrolyte is sodium, which can spike your blood pressure, which is not necessarily a good thing especially if you're doing it all the time. 1 serving before and after any serious sweating.

Vitamin D3

Only during the winter or if I can't get outside for a long period for some reason. Not currently taking it. Getting lots and lots of sun instead. It's hard, if not impossible, to get enough of this in your diet to meet your needs.

I'm actually a bit anti-supplement. My thinking is if you aren't getting enough of something in your diet, you're probably eating incorrectly (or at least suboptimally). But now that I'm restricting myself for the first time ever, I'm pretty sure I'm not getting enough of some of this stuff, even if I would by simply eating more of what I'm already eating. A year from now maybe I won't be taking any of this stuff because I'll be getting plenty of all of it from food.