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My daughter had a D level of 5. her provider put her on D2 50,000 units once a week for years with no benefit. After doing some research I found out how important Vitamin K was to absorbing D. I purchased Vitamin D3 with K2 daily dosing and within 3 months her D levels were within range and has stayed that way.
I was diagnosed with MS in 2016, went on an immunosuppressive, and then found out how important Vitamin D was for the immune system. In particular, it is critical for decreasing inflammation which is a key component of MS. I went through increasing intake and measuring my blood level for months until I figured out I need 8000 iu per day to stay at 100 ng/ml, which is optimal for MS. (For most people, 10,000 iu per day gets you to 100 ng/ml, 5000 iu per day gets you to 50 ng/ml, etc. So you can see that 2000 iu/day only raises your level by 20 ng/ml, which may still leave you insufficient.)
Just before the Covid-19 outbreak, my doctor told me to stop the immunosuppressant because my B-cell level made me as low as an AIDS patient, and I was in danger of a viral brain infection. I did get Covid-19 but not bad enough to be hospitalized. Ever since then, my only MS medication has been Vitamin D! And I have had no more MS symptoms during the last 5 years.
https://www.youtube.com/watch?v=oz7u92mifPI