Evidence from clinical trials shows, with a wide margin of confidence, that a prolonged intake of 10,000IU/d of vitamin D3 poses no risk of adverse effects for adults, even if this is added to a rather high physiologic background level of vitamin D.
Vitamin D and cancer mini-symposium: the risk of additional vitamin D.
Vieth R.\nAnn Epidemiol. 2009 Jul;19(7):441-5. Epub 2009 Apr 11.
PMID: 19364661
doi:10.1016/j.annepidem.2009.01.009
Upper levels of vitamin D intake were set at 50 microg/d (2000 IU/d) for all ages. Some individuals would require higher levels than these to achieve serum 25-hydroxyvitamin D concentrations for optimal calcium absorption. So much new information on vitamin D and health has been collected since the requirements were set in 1997 that this nutrient is likely the most in need of revised requirements.
Vitamin D requirements: current and future.
Weaver CM, Fleet JC.
Am J Clin Nutr. 2004 Dec;80(6 Suppl):1735S-9S. Review. Erratum in: Am J Clin Nutr. 2005 Mar;81(3):729.
PMID: 15585797
Vitamin D and Cancer Mini-Symposium: The Risk of Additional Vitamin D.
Vieth R.
Ann Epidemiol. 2009 Apr 11. [Epub ahead of print]
PMID: 19364661
doi:10.1016/j.annepidem.2009.01.009
Vieth R. \nCritique of the considerations for establishing the tolerable upper intake level for vitamin D: critical need for revision upwards.\nJ Nutr. 2006 Apr;136(4):1117-22.\nPMID: 16549491 [PubMed - indexed for MEDLINE]
Conclusions: Healthy men seem to use 3000-5000 IU cholecalciferol/d, apparently meeting > 80% of their winter cholecalciferol need with cutaneously synthesized accumulations from solar sources during the preceding summer months. Current recommended vitamin D inputs are inadequate to maintain serum 25-hydroxycholecalciferol concentration in the absence of substantial cutaneous production of vitamin D.
Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol.
Heaney RP, Davies KM, Chen TC, Holick MF, Barger-Lux MJ.
Am J Clin Nutr. 2003 Jan;77(1):204-10. Erratum in: Am J Clin Nutr. 2003 Nov;78(5):1047.
PMID: 12499343