Can vitamin D supplements reduce the risk of colds and the flu? Some positive studies on this have made headlines over the years, and researchers have proposed plausible biological mechanisms by which the vitamin may help protect against respiratory pathogens. But overall the results from clinical research have been inconsistent. The newest piece of evidence is a systematic review and meta-analysis in the journal BMJ, which concludes that D supplements may indeed help prevent such acute respiratory tract infections (RTIs), at least in some people.
As with many proposed effects of vitamin D, observational studies have suggested benefits, linking adequate blood levels of the vitamin with reduced risk of RTIs. Such studies can only find associations, however.
As for clinical trials comparing D supplements to a placebo, some have had positive results, but others have not found statistically significant effects. That’s where meta-analyses come in. These combine data from clinical trials to gain statistical power. Five prior meta-analyses looked at this subject; two reported benefits from D supplements against RTIs, while three found no significant effect.
The new meta-analysis is the largest yet on vitamin D for RTIs, combining data from 25 placebo-controlled clinical trials, which included nearly 11,000 people in 14 countries. It is also the most thorough, in that it took into consideration initial blood levels of vitamin D as well as how the supplements were taken—daily, weekly, or in a single massive dose every one to three months (called bolus dosing). The trials lasted anywhere from seven weeks to 18 months, and most provided 800 to 2,000 IU (international units) of the vitamin a day. Overall, the analysis found that the supplements reduced the risk of RTIs by 12 percent—a fairly small effect. The researchers estimated that 33 people would have to take supplements to prevent one person from developing any RTI. They found no adverse effects.
Subgroup analyses found that when the studies were limited to only those taking daily or weekly (not bolus) doses, the risk reduction increased to 19 percent, with about one in 20 people benefiting. And among people with very low initial blood levels of D (less than 10 ng/dL), who made up about 15 percent of participants, daily or weekly supplements cut the risk of RTIs by 70 percent, with about one in four benefiting. While bolus dosing has been a standard way of administering vitamin D in clinical research, there has been growing concern that the resulting wide fluctuations in blood levels may have undesirable effects.
Results from such subgroup analyses are less reliable than the overall finding, so additional well-designed clinical research is needed. More studies are underway. In the meantime, unless you know that your blood level of vitamin D is adequate (at least 20 ng/dL, preferably at least 30) or you regularly get sun exposure (which causes the skin to make the vitamin), consider taking a daily vitamin D supplement of 800 to 1,000 IU—or more if your doctor advises that. It’s very hard to get enough vitamin D from food alone.
For more about the “sunshine vitamin,” see Vitamin D: What's the Latest?