I'm so happy to see people caring for their bodies so mindfully and intelligently!
Like many, my kid and I rely on food first (salmon for oils, kale for calcium, etc).
For reasons detailed by RetiredAt63, the pediatrician wanted my kid to take Vit D. As a preemie, he also ended up severely iron deficient, and his ped did blood tests before, during, and after supplementation. Ped was THRILLED with his fast turnaround on a food-centered one. I required the same until I took a year to revamp every aspect of my diet. Current ped would really like him to take the fish oils but, as is obvious by this thread, we've both been terrible at taking things daily.
Friends have had awesome responses to different Bs (3, 12 usually) for neurological symptoms such as OCD and depression.
I'm ridiculously healthy these days, as is my kid, and I'm so grateful to the information we have re: food, food-sourced supplements, and other supplements. Like, REALLY grateful that folks figured this stuff out for our benefit!