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Vitamin D and flu.

A 2010 blinded prospective cohort study found evidence that serum 25-hydroxyvitamin D levels of 38 ng/ ml or higher reduced the incidence and severity of acute viral respiratory illnesses, including flu, during the fall and winter of 2009-2010. This level is over twice the 15 ng/ mL (37.5 nmol/L) level recommended by the US National Institutes of Health as being "generally considered adequate for bone and overall health in healthy individuals." Vitamin D is known to increase production of antimicrobial peptides by macrophages and endothelial cells. It may also have other effects on immune function including the modulation of cytokine production and inflammation suppression. Historically, cod liver oil (a good food source of vitamin D) has been used to ward off colds and flu and to treat tuberculosis.

This 2010 study, conducted by James R. Sabetta and colleagues, involved 198 healthy adults living or working near Greenwich, Connecticut. Each of the participants donated 7.5 cc of blood monthly for 4 to 5 months beginning in September 2009, without being told what was being studied. The researchers recorded the skin pigmentation, age, sex, height, weight, and occupation of each participant. In addition, monthly reports kept track of each participant's use of medications, herbals, supplements, vitamins, and flu vaccine. Researchers asked participants to report any sign of respiratory infection immediately. After being evaluated (and, if necessary, treated) by one of two board-certified infectious disease investigators, these participants kept a symptoms diary during their illness. The research team looked at incidence of respiratory illness, the duration of each symptom, the total illness duration, and prescribed antimicrobials.

Participants with 25-hydroxyvitamin D blood concentrations of 38 ng/ml or more had about one-half the risk of respiratory infection, compared with those who had lower D levels, and "a marked reduction in the percentages of days ill." Less than 19% of the participants had serum D levels of 38 ng/ml or more throughout the study period. Interestingly, of the 30 people who took 1000 IU or more of vitamin D each, only a third had serum concentrations of 38 ng/mL or more. Light skin pigmentation, lean body mass, and/or taking vitamin D supplements correlated to higher serum D levels. Most participants had light skin (152 out of 195 who completed the study), a study limitation. Vitamin D was the only variable that made a difference in illness susceptibility: "There were no differences due to ingestion of herbals, supplements, or vitamins other than D, or, due to receipt of seasonal influenza vaccine or 2009 H1N1 vaccine before the observation periods." (Before observation, 137 of 198 received the seasonal flu vaccine, and 85 had the H1N1 vaccine.) This study adds to the clinical evidence of vitamin D's immune function benefit. The researchers call for intervention studies that look at cost-benefits of vitamin D supplementation for different populations living at different latitudes.

"Vitamin D levels depend 'on how big you are, your skin color, your diet and how much sun exposure you get,'Sabetta [told] Web MD. 'Individuals should get their viamin D levels checked. If you are gardening a lot, you probably are fine, but people in an office all day may need supplements.'" Exposing the skin directly to sunlight is the best source of vitamin D for most people. The US Office of Dietary Supplements recommends 5 to 30 minutes of exposure between 10 a.m. and 3 p.m. twice a week. Smog, shade, clouds, and darker skin (high skin melanin content) reduce exposure to the UV light that converts to D3. In addition, geographic latitudes above 42 [degrees] do not receive enough sunlight during late fall and winter to generate enough vitamin D. Even in the sunniest places, increased use of sunscreen as well as lifestyle changes that emphasize indoor activities have led to less sun exposure and lower vitamin D levels in recent years.

Few foods contain vitamin D. Fish (particularly salmon) and fish liver oil are the best food sources. Beef liver, egg yolks, and cheese also have small amounts. As a result, D-fortified processed foods have appeared on the market in the US. Many of these processed foods--such as breakfast foods, margarine, and milk--contain sugar, poor-quality fats, or have negative effects (i.e., lactose intolerance). A good-quality vitamin D3 supplement may be a more healthful alternative. Vitamin D is one of the fat-soluble vitamins that can build up to toxic levels in the body. Consistent serum levels of more than 200 ng/mL or 500 nmol/L may lead to hypercalcemia and hyperphosphatemia, according to the Office of Dietary Supplements.

Hendrick B. Vitamin D may cut risk of flu [Web article]. MedicineNet.com. June 16, 2010. www.medicinenet.com/script/main/art.asp?articlekey = 117281. Accessed July 20, 2010.

Office of Dietary Supplements. Dietary supplement fact sheet: vitamin D [Web document]. www.ods.od.nih.gov/factsheets/vitanind.asp. Accessed July 20, 2010.

Sabetta JR, DePelrillo P, Cipriani RJ, Smardin J, et al. Serum 25-hydroxy vitamin D and the incidence of acute viral respiratory tract infections in healthy adults. PLos ONE. June 2010;5(6). Available at: www.plosone.org/article/info:doi/10.1371/journal.pone.0011088. Accessed July 20, 2010.

briefed by Jule Klotter

jule@townsendletter.com
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Title Annotation:Shorts
Author:Klotter, Jule
Publication:Townsend Letter
Article Type:Clinical report
Geographic Code:1USA
Date:Oct 1, 2010
Words:873
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