DEAR DR. ROACH: My blood work showed that I was slightly low in vitamin D, so my doctor recommended that I take a supplement. My pharmacist was concerned and told me that vitamin D does not get absorbed in people with dark complexions and tends to get deposited in the arteries.

The pharmacist recommended that I take vitamin D with K2, which allows vitamin D to be absorbed more efficiently. My cardiologist told me that vitamin K wasn’t well-studied, but some studies showed that it can actually thicken in the blood. In any event, the cardiologist didn’t believe it was a big deal. So, now I’m back to just taking vitamin D. Am I doing the right thing?

• S.P.

ANSWER: There’s a lot of confusion here. First off, a person with darker skin is, indeed, more likely to have low vitamin D, especially when in temperate zones above the line of Los Angeles to Atlanta in the United States. This is not due to an inability to absorb vitamin D; it’s an inability to activate vitamin D in the skin through sunlight. There is no problem with the absorption of vitamin D supplements based on skin color.

Some people don’t absorb it well, so I usually check the blood level when a person has very low vitamin D. I don’t know what the pharmacist meant about vitamin D being deposited in the arteries.

The form of vitamin K that is important for bones is vitamin K2. K2 helps calcium stay in the bones and out of the blood vessels. It also has nothing to do with vitamin D absorption. It also has nothing to do with blood clotting; that’s vitamin K1.

People with a vitamin K1 deficiency can develop excess bleeding or excess clotting, but giving vitamin K1 (in reasonable doses) to a person without the deficiency does no harm. In addition, it does not predispose people toward clotting. (Some people misuse the term “thick blood” when they mean that the blood is more likely to clot.)

Vitamin K2, by the way, is made in the intestines by your healthy bacteria, which uses vitamin K1 as a precursor. Most people do not need supplementation. Studies on vitamin K2 supplementation in the U.S. have not shown an improvement in bone density or fractures, unfortunately. Still, it is not harmful, and there may be a benefit that is too small to be seen in these studies.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu.