Factors Affecting Vitamin D Levels

Factors Affecting Vitamin D Levels.

Long known as an important factor in bone health, a quickly growing body of evidence now also shows that vitamin D may help lower the risk of cancer, heart disease, and even premature death.1, 2 Understanding optimal levels of vitamin D and the factors that effect them are an important part in getting the most from this nutrient.

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 individuals 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 dont 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 persons 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 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.


1.Heaney RP. Vitamin D in health and disease. Clin J Am Soc Nephrol. Sep 2008;3(5):1535-1541.

2.Melamed ML, Michos ED, Post W, Astor B. 25-hydroxyvitamin D levels and the risk of mortality in the general population. Arch Intern Med. Aug 11 2008;168(15):1629-1637.