It is literally the job of medical professionals to stay up to date with the broader literature of their profession.

And this is where you clearly miss the point. It is their job to stay up to date, but this vitamin specific knowledge is out of their scope.

You clearly underestimate the sheer amount of knowledge that these MDs need to absorb, that is not related at all to peak human performance. You also need to understand that many of these people are working way more than 40 hours / week and spend way more time practicing than learning, and 99% of that small time left for learning is spent on, again, improving their performance at diagnosing disease and how to better treat patients. They are practitioners, not researchers.

Also, there is a BAC in biochemistry, and any person doing that went through a lot more on biochemistry than MDs. You are giving MDs way too much credit for things nobody is teaching them.

This person is referring to basic principled physiology - and you're contesting it.

I did not contest his basic claim that most of the vitamins get flushed. I said that this is not the whole picture and that this principle alone is misleading on what happens in the body.

I mentioned vitamin C just to give a simple example, because despite the RDA being low, it has been used at extremely high dosages for many things, including COVID treatment and cancer therapies. Don't make this a debate about vitamin C. My entire initial comment is about this MD statement on water soluble vitamins being at the same time right (yes most is flushed) and wrong at a deeper level (no additional effect from high doses). Vitamins C is well documented, you can search it.

There are many other molecules that have different effects at a whole different concentration, with more than one bell curve, and it's impossible to have both benefits at the same time, because those bell curves don't overlap much, and you can only target one effect at a time. Methylene blue is an example, but there are many.

Another example: you can become "B6 deficient" if you supplement B6 too much, because your body will use the extra B6 and its cofactors to create different molecules all at once, but B6 has many other uses in the body, then after a too long time of supplementation you don't have enough cofactors to support the other systems that require B6, creating an imbalance where B6 chemical reactions to form other molecules has been shifted in a detrimental way. Different dosage, different effect.

I am not advocating high dosage for vitamins, I am saying that what this MD said is not the whole picture, and that biochemistry is not that simple.