For me Examine.com is the best source on supplements/vitamins
take for example Vitamin D https://examine.com/supplements/vitamin-d/
as for your question regarding Supplement manufacturers test check labdoor https://labdoor.com/
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Looking at Vitamin D on Labdoor:
>6 of the 19 products in this report exceeded their label claims by greater than 40%.
>All 19 vitamin D supplements met or exceeded their claimed vitamin D3 content, ranging from +0 to +900.0 IU versus their stated label claims.
>The average label variance in this testing batch was 22%.
Wow.
Let's look at melatonin:
>Only half of the products tested (15 of 30) measured melatonin levels within 10% of their label claims. 7 products deviated from their claims for melatonin by at least 25%. 3 of those products recorded 40% or more melatonin than their label claims, and 1 product had less than 1% of its label claim for melatonin.
Vitamin B12:
>Overall, vitamin B12 measurements in this batch analysis ranged from 515 mcg to 6990 mcg per serving, deviating from label claims by an average of 27%. 3 products also recorded vitamin B6 and/or vitamin B9. 9 of the 13 products recorded more vitamin B12 than claimed, with one product recording as much as 74% of its label claim for vitamin B12. All products passed heavy metal screenings, and recorded vitamin B12 levels considered to be safe by the Institute of Medicine (IOM)1.