Personally, my doubts about the supplement industry aren't about the purity of the vitamin supplements, but the effectiveness of supplements in general.
You may enjoy reading Paul and Shou-Ching Jaminet's book _The Perfect Health Diet_. It includes an approachable survey of results for many supplements, as well as a considered take on why multivitamins seem less crucial than advertised.
One reason: metabolic interactions mean that proportionality matters, for example between A and D. Multivitamins are sometimes optimized for cost and pill size at the expense of this. Another reason is that people have varying baselines based on their diet, as well as how much micronutrient synthesis is happening in their gut. Because each vitamin (aka crucial micronutrient) has a dose-response curve, both too little or too much is bad; the book tries to chart out what is a good range for each and how you can achieve that through diet, lifestyle, and supplements. It is by no means a perfect book, but it is the best combination of approachable, credible, and well-researched information that I've come across.
The complexity of untangling these kinds of interactions is why nutrition research often seems to yield results that contradict earlier results. Biology is complex, but that does not mean progress in understanding it is impossible.
I, too, have similar concerns. For people who are taking supplements now have mostly qualitative data to show efficacy of supplements in general. Some people go beyond taking the steps of blood panels, for example.
What we are doing is finding ways to quantify how supplements are working for people. And we strive for is finding those metrics to better understand how people can improve in their health.
One reason: metabolic interactions mean that proportionality matters, for example between A and D. Multivitamins are sometimes optimized for cost and pill size at the expense of this. Another reason is that people have varying baselines based on their diet, as well as how much micronutrient synthesis is happening in their gut. Because each vitamin (aka crucial micronutrient) has a dose-response curve, both too little or too much is bad; the book tries to chart out what is a good range for each and how you can achieve that through diet, lifestyle, and supplements. It is by no means a perfect book, but it is the best combination of approachable, credible, and well-researched information that I've come across.
The complexity of untangling these kinds of interactions is why nutrition research often seems to yield results that contradict earlier results. Biology is complex, but that does not mean progress in understanding it is impossible.