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Vitamin D RDAs Fall Far Short in Winter
1/19/2010
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American study finds that in winter, people need five to 10 times more dietary vitamin D than the U.S. RDA, depending on skin shade; study also finds the RDAs too low throughout the year
by Craig Weatherby


According to a study from the University of California, Davis, people of all skin shades need much more vitamin D than the official recommendations would suggest.

The new study joins a fast-growing roster of findings that suggest an urgent need to raise the RDAs for vitamin D.

UV sunrays stimulate synthesis of vitamin in the skin, but the melanin that colors human skin blocks those rays. Thus, light-skinned people make more vitamin D compared with darker-skinned people, in response to equal amounts of sun exposure.

Human bodies manufacture vitamin D upon exposure to sunshine, but the UV sunrays in northern regions of the U.S. are so weak during the winter months that most people make no vitamin D at all.

Accordingly, dietary supplements, fatty fish, and fortified foods are the only way to boost vitamin D blood levels during the winter.

And because so many people lead indoor lives and wear clothes or sunscreen during sunnier months, they don’t make very much then, either.

UC Davis team finds need for much higher RDAs
A UC Davis research team led by assistant professor Laura M. Hall, Ph.D., made two findings.

Both are sobering in terms of average vitamin D intakes in America… and somewhat unsurprising to folks who follow vitamin D research:
  • Light-skinned people need at least 1300 International Units (IU) per day during the winter.
  • Dark-skinned people need 2100 to 3100 IU per day during the winter and throughout the rest of the year.
The recommendation for light-skinned people holds even if they get abundant sun exposure, so those who get little sun exposure need even more dietary vitamin D.

But the recommendation for dark-skinned people holds true even for those who get more sun exposure than average.

To put these recommendations in perspective, the current recommended daily allowance (RDA) from birth up to age 50 is only 200 IU. It rises to 400 IU for people 51 or older and to 600 IU for people 71 or older.

Thus, the UC Davis team’s findings suggest the need for a five- to 10-fold increase in recommended vitamin D intakes, compared with the current RDAs.

Vitamin D deficiency is linked to increased risk of osteopenia, osteoporosis, muscle weakness, fractures, common cancers, autoimmune diseases, infectious diseases, and cardiovascular diseases.

Most researchers recommend blood levels ranging from 90 to 120 nmol/L (i.e., 36 to 48 ng/mL).

And even these “optimal” levels are conservative, since healthy blood levels of vitamin D extend above 200 nmol/L (80 ng/mL).

Conversely, most vitamin D researchers define vitamin D deficiency as a blood level below 50 nmol/L (20 ng/mL).

New study takes sun exposure into account
The conclusions were based on data from 72 young adults from Davis, California, with a wide range of skin shades and sun exposure.

In a novel approach, the UC Davis team sought to assess three factors affecting vitamin D blood levels:
  1. Estimated vitamin D intake from foods and supplements, using diet surveys.
  2. Actual sun exposure, based on reading from UV “dosimeter” badges worn by each of the participants.
  3. Capacity to make vitamin D from UV sunrays, based on the measured light reflectivity of each individual participant’s skin.
The participants were divided into four groups based on the light reflectivity of their skin, and were studied for about two months in each of the four seasons.

Dark-skinned people with either low or high wintertime sun exposure were estimated to have vitamin D levels of 24 and 42 nmol/L, respectively, and these levels were estimated to increase to between 40 and 60 nmol/L in the summer.

People of European ancestry with low and high sun exposure in the winter were predicted to have vitamin D levels of 35 and 60 nmol/L, respectively, and these levels were estimated to increase to between 58 and 85 nmol/L in the summer.

Again, leading vitamin D researchers recommend blood levels ranging from 90 to 120 nmol/L (i.e., 36 to 48 ng/mL), so neither light-skinned nor dark-skinned people in the study would reach the experts’ recommended levels, even during the summer… despite the fact that they live in sunny California.

For more on this topic, search our newsletter archive for “vitamin D”.


Sources
  • Hall LM, Kimlin MG, Aronov PA, Hammock BD, Slusser JR, Woodhouse LR, Stephensen CB. Vitamin D Intake Needed to Maintain Target Serum 25-Hydroxyvitamin D Concentrations in Participants with Low Sun Exposure and Dark Skin Pigmentation Is Substantially Higher Than Current Recommendations. J Nutr. 2010 Jan 6. [Epub ahead of print]
  • Vieth R. Why the optimal requirement for Vitamin D3 is probably much higher than what is officially recommended for adults. J Steroid Biochem Mol Biol. 2004 May;89-90(1-5):575-9. Review.

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