hi elly, maggie mae, and all
there is a body of research out there contending that 4000IU is about what an "average" person would use in an ordinary day, supposing the body's stores were high enough. this is why 4000IU has been called a "maintenance" dosage, and it explains why some people don't find their levels increasing.
i have heard (in the forums here) about someone building levels satisfactorily at supplementary doses of 2000IU per day, i think it was. i would suspect that individual could be getting a higher proportion of their daily d3 intake from sunshine and diet.
depending on your personal situation, you basically need to assess your daily sun exposure and fish intake before deciding on a satisfactory daily maintenance amount. then test at least once a year to make sure you are in the right ball park.
maintenance aside, some patients may require a high dose boost to get them into the right ball park before dropping back to maintenance. this is what i did.
i've seen a study which specified the strength of sunlight required to produce vitamin d3 [found it: http://www.ncbi.nlm.nih.gov/pubmed/2549141
]. in many northern latitudes, march/april through september/october seems to be the window of opportunity. skin bared to strong enough sun in significant enough amounts (bare arms and legs, maybe torso too) has been asserted to produce 10,000IU in half an hour. after that there appears to be a mechanism which prevents too much vitamin d3 from being created. this number has been proposed as a natural limit that should demarcate the tolerable safe upper limit for daily intake
absorbing *enough* sunlight through the skin for d3 production involves a number of challenges. as mentioned it has to be the right time of year, clear sky, uncovered skin, as well as pale skin, and youthful skin, in order for the highest levels to be generated. supplementing via diet or nutriceutical options can mitigate against these challenges.
i expect the endocrinologist would be looking at whether your liver and kidney are properly hydroxylating d3 intakes into the active metabolite required for optimal immune function.
, i asked my GP for this test. you don't need to wait to see your neuro, but there are potential issues when dealing with the family doc. interpreting the results is pretty easy, but getting the RIGHT results caused some issues for me in the "early days" - way back in '06
originally, my doc wrote "vitamin d3" on the requisition. at my lab, this defaulted to a test for 1,25-dihydroxyvitamind3 (1,25(OH)2D3). this is the active steroid hormone form, which is measured in pmol/L. the more valid measure of d3 status is the 25-hydroxyvitamind3 - 25(OH)D3 - which is measured in nmol/L, and which represents the body's available stores from which to make the more tightly-controlled 1,25(OH)2D3 (tightly controlled by your body, therefore it doesn't vary much, therefore it's a lame status measure). it took a little back and forth to explain to my docs with research in hand, but it panned out in the end and now when i get a test it's understood that we're testing 25(OH)D3.
that said, wrt interpretation, you want to be over 50nmol/L to prevent rickets, over 75 nmol/L to protect against osteoporosis, and it's hypothesized that at least 100 nmol/L 25(OH)d3 is beneficial for immune health. you don't want to go over 200nmol/L though (update: recently read a grant/holick document saying up to 250 is okay). at the higher levels you would have to start testing for hypercalcemia. basically in that scenario, you're taking in so much d3 that your diet can't supply the calcium needed to deal with it. it starts pulling calcium from tissue/bones instead, so you see more calcium showing up in the blood. even though the calcium level is elevated, it actually means you don't have enough. so, that's why you test, and that's why you keep taking good ratios of minerals when you're supplementing vitamin d3.
sounds like you're heading in the direction i'm going too. i am sick of the daily attention to d3. which is why my levels are back in the dumper these days. i'm going to do the 50,000IU x 10d booster, and then i think 25,000IU per week after that. all with associated tests for monitoring of course. FYI for anyone who doesn't know, there is a lag time of 3-4 months from max exposure to vit d3 to max serum concentrations of 25(OH)d3.
i'm hearing about lots of 50,000IU recommendations these days - it's good stuff! the word is getting out (after only three decades of research lol)
here are some good starter vitamin d3 cutaneous synthesis links:
Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin
Daily Duration of Vitamin D Synthesis in Human Skin with Relation to
Latitude, Total Ozone, Altitude, Ground Cover, Aerosols and Cloud
and their cool calculator:
Sunlight, season, skin pigmentation, vitamin D, and 25-hydroxyvitamin D:
integral components of the vitamin D endocrine system
i have another link somewhere in these threads with a very pretty overview of d3 endocrinology, hydroxylation processes in liver kidney etc. i will try to find it and post a link to it here also.