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Vitamin D Levels Fall Far Short in Winter An 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; the study also finds the RDA too low throughout the year. 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 RDA for vitamin D. UV sunrays stimulate synthesis of Vitamin D in the skin, but the melanin that colors human skin blocks those rays. Accordingly, 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. This landmark study finds the current Vitamin D RDA is woefully inadequate after taking participants’ diets, sun exposure, and skin shades into account. Findings suggest a need to raise the RDA five- to 10-fold, depending on skin shade, with dark-skinned people needing the most. Results fit with those of several recent investigations that found the current RDA grossly inadequate for the vast majority of Americans. Accordingly, dietary supplements, fatty fish, and fortified foods are the only way to boost vitamin D blood levels during the winter (unless you want to go the route of tanning lamps). Moreover, because so many people lead indoor lives, and stay covered with clothes, and wear sunscreen during sunnier months, they don’t make very much Vitamin D then, either. 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 RDA. 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 (which is the same as 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). 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: estimated vitamin D intake from foods and supplements, using diet surveys, actual sun exposure, based on reading from UV “dosimeter” badges worn by each of the participants, and the 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. Dr. Cinque's comments: They keep raising the recommmended optimal blood level of Vitamin D, and I am now seeing references as high as 100 ng/ml. For reference, the average American who is unsupplemented hovers around 18, and those with dark skin are lower yet. The idea of getting all the Vitamin D you need from the sun is increasingly being seen as naive. And note that there is a Catch 22 involved because the more sunlight you get, the more melanin your skin makes, which blocks the ultraviolet that fosters the Vitamin D. I'm all for having Vitamin D testing done. It's money well spent. Most people with low levels will need to take at least 5000 IUs daily for a while to get their blood level up to where it belongs. Once lifted, to say 50 or higher, it may only be necessary to take 5000 IUs every other day to maintain it. I intend to do a Vitamin D blood test on myself every year for the remainder of my life, and I suggest you do the same. It is likely to pay off in extra years of living. E-mail to a friend Next Article |
