--for various people in this thread:

AlwaysLearning, if it can be confirmed that you have low levels of D, B's, etc., for no particular reason, malabsorption should be investigated, and the most obvious tests for that are for parietal antibodies and for anti-gliadin and anti-transglutaminase antibodies associated with celiac/gluten sensitivity; both of these are more common than most doctors generally think and can lead to malabsorption of nutrients in the gastric tract.

Also, as regards skin biopsy, the ranges for what constitutes a "definite" small-fiber neuropathy were somewhat arbitrarily established during the original MacArthur research protocols at Johns Hopkins; specifically, intraepidermal nerve fiber density lower than the 5th percentile or higher than the 95th, compared to the original "normal" control groups, was considered definite evidence of small-fiber syndromes. There is a lot of variation in what constitutes normal in an individual, however. People's nerve fiber density does tend to decline with age, even in people without symptoms. And it's hard to know what a person's "starting point" was by the time they have symptoms and get a skin biopsy. (Someone may have declined from a "normal for them" 70th percentile reading to the 25th percentile, for example--but this would still be read as "normal density" on a skin biopsy enumeration, as it would be almost impossible to have an individual pre-neuropathy figure to compare to.) This is why savvy neurologists won't just go by the numbers--they'll ask for reports of condition of the nerve fibers observed--is there excessive branching, swelling, deteriorated fibers evident--and use a fuller clinical picture, including symptoms, to determine if there is a small-fiber neuropathy.