What is difference between vitamin D2 and vitamin D3?

DEAR DOCTOR K:

My doctor advised me to take a vitamin D supplement. Why do I need vitamin D? Also, my pharmacy sells vitamin D in two forms: D2 and D3. What is the difference, and which one should I take?

DEAR READER:

This is an area full of controversy because not enough research has been done. I typically recommend getting vitamins from food, but vitamin D is found naturally in only a few foods. Fatty fish is the main dietary source, and milk, many juices and breakfast cereals are fortified with it.

We get most of our vitamin D from the sun: When sunlight strikes the skin, skin cells make vitamin D. But these days, people get a lot less sunlight than they used to.

Other factors also affect how much vitamin D your body makes. For example, how far you live from the equator, the season and the time of day all affect how much sunlight reaches your skin. Your age, skin color, how much skin you expose and your sunscreen use also influence your production of vitamin D.

The amount of vitamin D in your body is reflected by the levels of the vitamin in your blood. These levels can be divided into the following categories: clearly low, possibly low, on the low side of normal and on the high side of normal. Here’s what we know:

People with clearly low levels of vitamin D are at greatly increased risk for osteoporosis, a bone-thinning disease, and should take vitamin D supplements. That’s not controversial.

Here’s where the controversy starts. Many studies also show that people with possibly low blood levels of vitamin D, or levels on the low side of normal, are at greater risk for several major diseases, compared to people with levels on the high side of normal. Those diseases include high blood pressure, heart disease, multiple sclerosis, Type 1 diabetes, depression and colon cancer.

What’s controversial is this question: If your blood level of vitamin D is possibly low or on the low side of normal, does taking vitamin D supplements reduce your risk of developing these major diseases? My own view is that we just don’t know. Fortunately, research is underway to answer that question.

Until the answer is in, what should we do? A committee of experts convened by the Institute of Medicine recommends: (1) people below the age of 70 should take 600 international units (IU) per day of vitamin D, and (2) people age 70 and older should take 800 IU per day. If you can’t get enough from foods and modest sun exposure, it makes sense to consider a daily supplement.

Vitamin D comes in two forms: D3 and D2. D3 is the form made naturally by the body. It is also the form most often used to fortify milk and other foods. D2 is made from plant material. The Institute of Medicine has said that D2 is just as effective as D3. Other experts recommend D3 because it is the form most easily absorbed and used by the body. I agree with them.