Your doctor’s prescription makes sense. But I can see why you’re confused.

When people are found to have low levels of vitamin D, it’s often recommended that they “tank up” on large amounts for several weeks. Then they can go to a daily dose of 1,000 international units (IU). Don’t worry: Your body stores of the vitamin are very low. So daily doses well over the usual 4,000 IU upper limit won’t overload you and be toxic.

Vitamin D comes in two forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). I recommend vitamin D3. It is believed to be more effective than D2 at boosting the biologically active form of the vitamin.

Do you need to take vitamin D? If your bones are already somewhat thin (osteopenic), the answer is definitely yes. In fact, research has convinced me that the majority of people who don’t get much sun exposure should be taking vitamin D supplements.

Most of the vitamin D in our bodies is made when sunlight hits our skin. So a lack of sun exposure can lead to a deficiency. And thin bones may not be the only consequence. Vitamin D deficiency may make us vulnerable to several major cancers, such as colon, breast, prostate and ovarian cancer. Also people with low vitamin D levels may be at greater risk of multiple sclerosis.

So I agree with your doctor. You should be taking vitamin D. And you’re on a reasonable dose.