Capital D
By Jonny Bowden, PhD, CNS
Answers to your questions about this prominent vitamin

Sun and Vitamin DOne of the hot topics in nutrition today is vitamin D, which has been shown to have positive effects on cellular health, cancer prevention, immunity, bones, brain health, and even overall mortality. With all the press vitamin D has gotten, it’s no wonder that people want to know more about its benefits. Here are a few questions we’ve received:

Q: What’s the difference between regular vitamin D and D3?

The system of naming vitamin D is pretty confusing. “Vitamin D” is an umbrella term for a lot of different compounds, the most important of which are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). These two vitamins are actually very different. Studies have shown that D3 is the more potent, so that’s the one you want. Ironically, mainstream versions of vitamin D usually use the much less effective vitamin D2, so it pays to avoid drug-store brands and look for quality D3 supplements at your health food store.

And remember to take vitamin D3 with a fat-containing meal to ensure that it’s fully absorbed.

Q: What kind of blood test should I take for vitamin D?

The only test that means anything is the 25-hydroxy vitamin D—or 25(OH)D—test, which is recommended by the Vitamin D Council (vitamindcouncil.org). Most nutrition-oriented experts like to see levels in the 50 ng/mL—70 ng/mL, sometimes going as high as 80 ng/mL.

The chart below provides some general reference levels. (Note: The most common measurement standard in the US is ng/ml (nanograms per milliliter); the other measurement, nmol/L (nanomoles per liter), is often used in Europe.)

Vitamin D Levels

Q: A blood test showed that I’m low in vitamin D. My doctor suggested 2,000 IU per day. Is that enough?

It all depends on your blood levels. In general, you need to take 1,000 IU of vitamin D per day to increase blood levels by 10ng/ml, 2,000 IU per day to increase blood levels by 20 ng/ml, and so forth. So someone with a borderline level of 37 would need about 3,000 IU per day to move up to a much healthier level of 67. But it's not a guessing game. Further tests will show whether or not the dose you're taking is working, so you can adjust as needed.

Q: My doctor told me to scale back on vitamin D. I'm overweight, and she's concerned that it gets stored in fat cells. How should I proceed?

It's true that fat-soluble vitamins—D, A, E, and K—get stored in fat cells, and overweight people tend to have more fat stores. But that doesn't mean they have higher blood levels of vitamin D. In fact, because fat cells tend to hold tightly to vitamin D, the opposite is often true. In a study by expert Michael Holick, MD, obese and non-obese people were exposed to identical amounts of vitamin D.

The obese group was only able to raise their blood levels to about 45 percent of the amount that the non-obese group could. Holick says that obese people can need twice as much vitamin D to satisfy their needs, and he advises that they take 3,000—6,000 IU per day.

So you should definitely discuss this more with your doctor. Get her to test your blood levels before changing your supplement regimen. And ask to see any data she has that indicates that the amount of vitamin D you’re taking can be dangerous. To my knowledge, there has never been a substantiated report of toxicity occurring from as much as 10,000 IU of supplemental vitamin D per day (the amount I take). So unless you’re doing something crazy like taking 100 capsules of 2,000 IU each, you're probably safe.

D-Rich Foods:
From Highest to Lowest Amounts

  • Wild Salmon (Farmed salmon has been show to contain lower levels of vitamin D than wild salmon.)
  • Sardines
  • Goat's Milk
  • Cow's Milk
  • Shiitake Mushrooms
  • Eggs



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