I live in the sub-arcticd, have for 30 years. About a year ago, my doctor said many of her patients were testing low on Vit. D, told me to take a big dose every day, I started on 4,000 units. After three months of doing that, she tested me and I was right where I should be. In about 3-4 months I will be tested again. I am still taking 4,800 units daily.
One thing I have noticed is that my finger nails are more firm. I have had paper thin weak nails for years. I am also taking 1000 mg of calcium, due being female and aging.