Took vitamin D all winter and my level barely moved — what did I get wrong mechanically? Vitamins & Minerals

The one that catches nearly everybody: D is fat-soluble, and morning coffee on an empty stomach is close to the worst absorption scenario you could design. Moving it to the largest meal of the day is the single mechanical change I'd make first. That alone moved my own retest more than switching brands ever did.

Second mechanical thing: check how old the bottle was and whether it lived somewhere warm. Third: check whether both tests were the same assay at the same lab, because between-lab variation on this particular test is not trivial. A 'barely moved' result can be partly measurement rather than entirely absorption.

Consistency beats dose more than people expect. I was taking mine 'daily' and when I actually counted the blister packs at the end of winter I'd taken it about 60% of days. A pill box left on the dinner table fixed my numbers without changing anything else about the product.

Body weight affects how far a given dose moves the number. That's a conversation for your doctor rather than for us, but it's a legitimate reason two people on identical doses land in different places, and worth raising with them instead of assuming the supplement failed.

Before anyone suggests the brand was fake — that's the least likely explanation on this list. Timing, adherence and absorption account for nearly every case of this I've seen.

Anything involving repeat blood tests and dosing is genuinely a doctor conversation, and yours is already on it. Most of this thread is really about not wasting their time next winter.