Since it turns out that it isn't feasible to include check as many as apply questions in the big survey, I'm asking about supplements here. I've got a bunch of questions, and I don't mind at all if you just answer some of them.
What supplements do you take? At what dosages? Are there other considerations, like with/without food or time of day?
Are there supplements you've stopped using?
How did you decide to take the supplements you're using? How do you decide whether to continue taking them?
Do you have preferred suppliers? How did you choose them?
I used Cronometer for about 6 months until I found a few variations of my weekly food intake that got me to average out to 100% of the daily micronutrients I wanted each day. It comes with the RDAs as preset thresholds, but you can modify your targets as needed. (I used Yvain's Word doc and some of the other submissions to the minerals contest to set better targets).
I eventually got it to the point where the only supplements I was taking were:
I also occasionally take magnesium citrate supplements, even though I get enough through food, because I find they help with jaw-clenching which I do while I sleep, and 1/4 of a 3mg melatonin tab to counteract the "white/blue light before sleep suppressing natural melatonin production" problem. I bought some lithium orotate a while back, but haven't tried it yet
Getting away from vitamins/minerals, I use some no-added-vitamins Whey powder to help meet daily protein goals on days where I eat less or no meat and to rapidly refuel after weight training. I also occasionally cycle creatine to help with my weight training. (The benefits are very real and zero side effects.)
I take supplements during my feeding window (I do a 16/8 Intermittent Fasting protocol which I love) but am conscious of micronutrient absorption competition. Most supplements will upset my stomach if it's empty.
I decided to use them after tracking my micronutrient intake on Cronometer and seeing that my intake of some is below the levels I want them (with my goal being general health and longevity). I decide to continue using them if it is not a large cost, doesn't take too long, and doesn't cause any negative side-effects. (I stopped multivitamins because they were causing digestive issues, their effectiveness has not been substantiated by science, and the epidemiological correlation between multivitamins and all-cause mortality.)
I have some preferred vendors, but more important is the specific form of the supplement for bioavailability (for example, chelated zinc/magnesium > zinc/magnesium oxide). For certain types of supplements, the manufacturing process (and therefore the vendor) is more important because of the initial source (like EPA/DHA fish oil caplets. Have to be careful to get ones that have negligible amounts of toxins and heavy metals) and Melatonin (animal product and hormone).
A heuristic/methodology I use is to look up possible problems with things from the people who demonize them most (usually vegans and ancestral health / paleo people, but also sometimes Conventional Wisdom and government like in the case of Raw Milk) and then research their claims and learn more about the topic in general along the way. If I gauge the downsides to be negligible and the potential benefits good enough, then I give myself the OK to go for it.
I tracked everything meticulously for a long time, so now I have a pretty good idea of what most common foods contain, but now I'm a bit more relaxed about it all, knowing that my diet gives me well over 80% of the amount of everything that I want.
Hope this was helpful. Let me know if you have any followup questions.