By Michael Anchors, MD, PhD
Many of the overweight patients who come to me are taking a vitamin D supplement on their doctor’s orders because the doctor measured the vitamin D and said it was low. Many of the same patients have had kidney stones recently. There is a link.
Vitamin D does many things, but one thing it does is to encourage the absorption of calcium from the gut.
Most of the vitamin D in the blood is bound to vitamin D binding protein. Only the unbound free fraction is hormonally active. Most doctors measure only the total vitamin D. What they should do instead is to measure the free fraction, since that’s the only fraction that matters.
In fat people the liver makes less vitamin D binding protein, compared to lean people. So in fat people, the TOTAL vitamin D is always low. This does not need to be treated because the free fraction can be, and probably is normal.
Taking the unnecessary vitamin D supplement is not harmless. It can cause calcium kidney stones. Obese people are more prone to kidney stones anyway, and the vitamin D pill makes this situation worse.
Fortunately most of the vitamin D in the pills is not even absorbed. Vitamin D is a fat-soluble vitamin. If the pill is not taken with oil or fatty food, the vitamin D is not absorbed. Maybe medical students should be required to take two years of biochemistry instead of just one.