For most adults, 2000 iu will raise your 25-OH-Vitamin D blood level by 20 ng/ml. If you are not eating lots of oily fish every day, or drinking lots of fortified milk, then your (as a Northerner) blood level sinks down to 20 ng/ml during the winter.

"Sufficient" level is 30 ng/ml. The Endocrine Society says 40-50 ng/ml is "ideal". Without other Vitamin D sources, you would probably need to take 4000-5000 iu per day to reach that level. Dr. Fauci is opting for 60 ng/ml.

USDA has said 400 or 600 iu per day is swell, although we know that would only raise the average person's blood level by 4 or 6 ng/ml. Most doctors heard Vitamin D is dangerous in medical school, and have not kept up with the research. Many research papers say 50-60 ng/ml has a definite protective effect for some cancers. 70 ng/ml has a strong positive effect on MS. (I'm proof!)

I think the USDA's criminally low recommendation is based on a lot of US workers doing field labor and getting a lot of sun, and just never got updated. National Health in the UK is sending Vitamin D3 to everybody in danger, by mail. Unfortunately, they are sending 400 iu which will do absolutely nothing!