Obviously no one should be taking health or nutritional advice from random sources on the internet, but where the hell did you pull that number out of? Current RDAs for most people is 600 IU: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessiona...
Note, the Endocrine Society currently suggests a minimum 25(OH)D level of 30 ng/ml (75 nmol/liter) and suggests much higher levels of supplementation if you need to raise it: "to raise the blood level of 25(OH)D above 30 ng/ml may require at
least 1500 –2000 IU/d of supplemental vitamin D."
Holick, Michael F., Neil C. Binkley, Heike A. Bischoff-Ferrari, Catherine M. Gordon, David A. Hanley, Robert P. Heaney, M. Hassan Murad, and Connie M. Weaver. “Evaluation, Treatment, and Prevention of Vitamin D Deficiency: An Endocrine Society Clinical Practice Guideline.” The Journal of Clinical Endocrinology & Metabolism 96, no. 7 (July 1, 2011): 1911–30. https://doi.org/10.1210/jc.2011-0385.
It's also worth noting that studies have shown that without sun exposure, people may require even higher levels of supplementation to maintain their 25(OH)D levels:
Heaney, Robert P., K. Michael Davies, Tai C. Chen, Michael F. Holick, and M. Janet Barger-Lux. “Human Serum 25-Hydroxycholecalciferol Response to Extended Oral Dosing with Cholecalciferol.” The American Journal of Clinical Nutrition 77, no. 1 (January 2003): 204–10. https://doi.org/10.1093/ajcn/77.1.204.
"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."
I think there is definitely something here. I never get sick in summer, spring and rarely fall. I run and bike with exposed skin quite a bit. In winter I get sick all the time :(
I think this winter I will run an experiment on myself.
