I got this abstract from the Archives of Internal Medicine, courtesy of several readers last week (along with related articles), so thanks to one and all.
Background Recent studies suggest a role for vitamin D in innate immunity, including the prevention of respiratory tract infections (RTIs). We hypothesize that serum 25-hydroxyvitamin D (25[OH]D) levels are inversely associated with self-reported recent upper RTI (URTI).
Methods We performed a secondary analysis of the Third National Health and Nutrition Examination Survey, a probability survey of the US population conducted between 1988 and 1994. We examined the association between 25(OH)D level and recent URTI in 18 883 participants 12 years and older. The analysis adjusted for demographics and clinical factors (season, body mass index, smoking history, asthma, and chronic obstructive pulmonary disease).
Results The median serum 25(OH)D level was 29 ng/mL (to convert to nanomoles per liter, multiply by 2.496) (interquartile range, 21-37 ng/mL), and 19% (95% confidence interval [CI], 18%-20%) of participants reported a recent URTI. Recent URTI was reported by 24% of participants with 25(OH)D levels less than 10 ng/mL, by 20% with levels of 10 to less than 30 ng/mL, and by 17% with levels of 30 ng/mL or more (P < .001). Even after adjusting for demographic and clinical characteristics, lower 25(OH)D levels were independently associated with recent URTI (compared with 25[OH]D levels of 30 ng/mL: odds ratio [OR], 1.36; 95% CI, 1.01-1.84 for <10 ng/mL and 1.24; 1.07-1.43 for 10 to <30 ng/mL). The association between 25(OH)D level and URTI seemed to be stronger in individuals with asthma and chronic obstructive pulmonary disease (OR, 5.67 and 2.26, respectively).
Conclusions Serum 25(OH)D levels are inversely associated with recent URTI. This association may be stronger in those with respiratory tract diseases. Randomized controlled trials are warranted to explore the effects of vitamin D supplementation on RTI.
In simpler terms, the higher the vitamin D level, the less cold and flu. Second, the association is looking more and more like an independent one; that is, regardless of your other risk factors, adequate vitamin D levels appear to be generally protective (this is what they mean when they say they "adjust" for age, gender, and other risk factors, etc.). In fact, the strongest association was found in those with asthma and chronic obstructive pulmonary disease (COPD).
It's important to understand what was done here, and the importance of data collection and storage over the long term. Basically, they were able to go back and query the database for those who had reported recent colds and flu (almost 19,000 records), and then compare those answers with the patients' vitamin D levels, also in the database.
This is decent epidemiological science; but it is, of course, only a precursor to gold-standard science: actual intervention by means of double-blind, randomized, placebo control. That means that the researchers intervene by supplementing half of a group with D and the other half with a placebo, with the spit between the group being random, and neither the researchers nor the subjects know who's getting the real thing and who's getting placebo. In this fashion, when done correctly and with statistically significant results, actual cause and effect can be established.
Here's an example of decent vitamin D science, a randomized controlled trial that demonstrated a 60% reduced risk for all cancer with supplementation of only 1,000 IU per day. As reported:
A four-year clinical trial involving 1,200 women found those taking the vitamin had about a 60-per-cent reduction in cancer incidence, compared with those who didn't take it, a drop so large — twice the impact on cancer attributed to smoking — it almost looks like a typographical error.
There's other "good science," too, only not in the way it was intended. These researchers determined that there was no noticeable benefit to supplementation at twice the USRDA of 200 IU per day (400 IU).
There's an old saying: "every experiment is a success." Get it?
And keep in mind: Getting plenty of sun may not be enough. Your ability to synthesize D diminishes with age, dark skin is less efficient, and latitude and time of year play a roll (father away from the equator, the less absorption, the less time per year the sun is high enough, and the less time per day when it is). The best way is to get tested and supplement until your level is 60 or better.