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That's why observational studies consistently show vitamin deficiency correlates with various types ill health, but controlled experiments consistently show that supplementation fails to improve it.

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Apparently it’s because of ethics committees. You can’t do a controlled experiment on people who are vitamin deficient, so iirc modern studies are such comparing people who are not deficient to people who take additional vitamins and find no effect. Which of course doesn’t mean that vitamin supplementation is worthless.

How about clinical reports/studies? Clearly doctors can give reports whether deficient patients' vitamin D or C levels improve with supplements.

My doctor literally gave me such an oral prescription as my vitamin D levels are low.

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