Can vitamin D supplements reduce the risk of colds and the flu? Some positive studies on this have made headlines over the years, but overall the results from clinical research have been inconsistent. The newest piece of the puzzle is a 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 benefits of vitamin D, observational studies have suggested benefits, linking adequate blood levels of the vitamin with reduced risk of RTIs. While such studies can only find associations, there are plausible biological mechanisms by which the vitamin may help protect against respiratory pathogens.
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 and involving nearly 11,000 people in 14 countries. It is also the most nuanced meta-analysis, 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 1½ years, and most provided 800 to 2,000 IU of the vitamin a day. Overall, it found that the supplements reduced the risk of developing RTIs by 12 percent—a fairly modest effect. The researchers estimated that overall about one in 33 people would benefit from the supplements. They found no adverse effects.
But subgroup analyses found that when the studies were limited to only those using daily or weekly (not bolus) dosing, 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) or you regularly get sun exposure (which causes the skin to make the vitamin), consider taking a daily D supplement of 800 to 1,000 IU (more if your doctor advises that). For more about the “sunshine vitamin,” see Vitamin D: What's the Latest?
Also see our quiz: Vitamin D: How Much Do You Know?