It is all good to say do not be deficient, but what does it take to get out of deficiency.  

This last month the UK said they would offer to supplement those who might have the highest potential to be vitamin D deficient.  Their plan is to offer 10 mcg (400 iu) of Vitamin D daily to this group of people, but the question begs to be asked.  Is that enough?  What do the vitamin D experts say?  

The data does not only suggest much higher supplementation, it points out very explicitly,
"As noted in the Results section, 12.5 μg/d is the approximate oral input required to maintain serum 25(OH)D at zero change, at least from starting values in the range we observed (≈70 nmol/L). I" (1)

So according to the study, the 10 mcgs being offered by the UK to those who may be deficient, is not even enough to keep these individuals where they are.  The Study also goes on and talks about safety of high dose daily supplementation.  So if you are replete(sufficient)(≈70 nmol/L*) in vitamin D you have to take in 12.5 mcg's per day just to keep current stores up.  What does this say about those who are deficient?   That 10mcg is not helpful, and might even be harmful by giving people a false sense of security.

*I think it is important to point out that 70-nmol/L is by no means a full replete state for the potential benefits needed for the immune system signaling at point of attack.(2)

"Sufficiency is defined as 25OHD3 levels above 30-100 ng/ml (75-250 nmol/l). Levels of 10-30 ng/ml (25-75 nmol/l) indicate insufficiency, those below 10 ng/ml (25 nmol/l) clear-cut deficiency. Therapy is equally easy. Cholecalciferol can be applied orally, and continuous daily substitution is recommended. The therapeutic dose ranges from 4,000 to 10,000 IU (100-250 μg)/day."(2)

But please do not take my word for it.  Read it here.

1. Robert P Heaney, K Michael Davies, Tai C Chen, Michael F Holick, M Janet Barger-Lux, Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol, The American Journal of Clinical Nutrition, Volume 77, Issue 1, January 2003, Pages 204–210, https://doi.org/10.1093/ajcn/77.1.204

2. Höck AD. Review: Vitamin D3 deficiency results in dysfunctions of immunity with severe fatigue and depression in a variety of diseases. In Vivo. 2014 Jan-Feb;28(1):133-45. PMID: 24425848.