One of the things keeping scientists on their toes as they try to come up with updated recommendations for vitamin D intake is that many different factors affect an individual’s blood level of the vitamin. These include:

  • Where you live. Northern parts of the country get less vitamin D producing ultra violet B (UV-B) sunlight than do southern parts. Above about 37 degrees latitude (around San Francisco, California, or St. Louis, Missouri) UV-B is too low in late fall through early spring to produce any vitamin D.
  • How much sun you get. People who work or spend a lot of time outdoors typically have higher vitamin D levels than those who don’t often get outside.
  • Your skin color. Darker skin produces less vitamin D because melanin, the substance the gives skin its color, basically acts as a UV-B sunscreen.
  • Your age. Vitamin D production in skin gets much less effective as you age.
  • Your weight. Overweight, and especially obese, people have lower levels of vitamin D in the blood because the vitamin can be taken up and held by fat cells.

With so many factors playing a role in a person’s vitamin D levels, it can be hard to come up with blanket recommendations for an entire population. Because about half or more of the United States is low in the vitamin, shooting for 1000 IU of vitamin D per day is likely a good, safe place to start. But some scientists looking at the data are starting to feel that a bit of personalization may eventually be in order. A simple blood test could show if someone is at or near levels that provide larger health benefits; after that, intake recommendations can be adjusted accordingly.

Such testing makes a lot of sense, just given the huge differences in potential vitamin D production in, say, Fargo, North Dakota and San Diego, California. But given the time and cost such a practice would involve, it will take some time for the number-crunchers, policy makers, and researchers to sift through the data and settle on an evidence-based cost-effective approach.

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