Most people get the common cold a few times per year.
Fortunately, there are several things you can do to reduce your risk of catching one. Most importantly, you can live a healthy lifestyle and follow a wholesome diet.
In fact, research indicates that certain nutrients can make a difference. These include flavonoids, which are found in virtually all plant foods.
Recently, researchers conducted a systematic review and meta-analysis examining the effects of flavonoids on the incidence of upper respiratory tract infections.
Here is a detailed summary of their findings.
Flavonoids are the largest category of polyphenol antioxidants in the human diet. Thousands of flavonoid compounds have been identified, all of them in plants.
They play a role in the plants’ defenses against viruses, bacteria, fungi and oxidative stress, and some of them may similarly benefit humans (1).
URTI is a collective term describing all infections affecting the throat, tonsils, nose, vocal cords, middle ear and sinuses. They’re usually associated with a cold.
They are common in the US, affecting about 23% of people every month, on average, with most people having 2–3 URTIs each year (4).
More than 200 types of viruses are known to cause URTIs. Initially, they are rarely caused by bacteria, but opportunistic bacteria usually infect tissues when their immune defenses have been weakened by a viral infection (5, 6, 7).
This was a systematic review and meta-analysis of randomized controlled trials examining the effects of flavonoid supplementation on URTIs.
This was a systematic review and meta-analysis of randomized controlled trials testing the effectiveness of flavonoid supplements against upper respiratory tract infections (URTIs) in healthy people.
The researchers searched for all relevant studies using several electronic databases. They then assessed the studies for eligibility, based on pre-specified criteria.
For example, the studies had to be randomized controlled trials with healthy human participants, aged 18–65 years old. Additionally, the participants had to supplement with flavonoids for at least 4 days.
Out of 387 studies, only 14 qualified. Six of them looked at the effects of flavonoid supplementation on URTI incidence, duration and severity, whereas 8 exclusively looked at the effects on markers of immune function.
Using the combined findings of these studies, the researchers evaluated the effects of flavonoids on URTI incidence, duration and severity, as well as their effects on immune function.
Bottom Line: This was a systematic review and meta-analysis of randomized controlled trials examining the effect of flavonoids on upper respiratory tract infections.
The Selected Studies
Here are summaries of the 6 studies that examined URTI incidence.
Nieman DC, et al. Quercetin reduces illness but not immune perturbations after intensive exercise. Medicine and Science in Sports and Exercise, 2007.
40 trained male cyclists from North Carolina supplemented with 1 gram of quercetin every day for 3 weeks. They started taking quercetin 4 days before a 3-day exercise period and stopped supplementing 2 weeks afterwards.
In the two weeks after the exercise period, URTI incidence was 5% among those who had supplemented with quercetin but 45% among those who got a placebo.
Rowe CA, et al. Specific formulation of Camellia sinensis prevents cold and flu symptoms and enhances gamma, delta T cell function: a randomized, double-blind, placebo-controlled study. Journal of the American College of Nutrition, 2007.
108 adults from Florida supplemented with an unspecified amount of L-theanine and epigallocatechin gallate (from green tea) twice a day for 3 months.
Supplementing with these flavonoids led to a 22.9% lower incidence of infections that lasted at least 2 days, while symptom days were 35.6% fewer, compared to a placebo.
Henson D, et al. Post-160-km race illness rates and decreases in granulocyte respiratory burst and salivary IgA output are not countered by quercetin ingestion. International Journal of Sports Medicine, 2008.
39 US adults competing in the 160-km Western States Endurance Run supplemented with 1 gram of quercetin every day for 3 weeks before and during the race and 2 weeks afterwards.
Supplementing with quercetin did not significantly affect URTI incidence.
Nantz MP, et al. Supplementation with aged garlic extract improves both NK and γδ-T cell function and reduces the severity of cold and flu symptoms: a randomized, double-blind, placebo-controlled nutrition intervention. Clinical Nutrition, 2012.
112 adults from Florida supplemented with 2.56 grams of a flavonoid-rich, aged-garlic extract for 90 days. At the end of the study, illness diaries showed that the incidence of colds and flu was not significantly different, compared to a placebo.
However, supplementing with the aged-garlic extract appeared to have reduced the severity of symptoms, since the number of symptoms reported was 21% lower and sick days were 58% fewer.
Nantz MP, et al. Consumption of cranberry polyphenols enhances human γδ-T cell proliferation and reduces the number of symptoms associated with colds and influenza: a randomized, placebo-controlled intervention study. Nutrition Journal, 2013.
For 10 weeks, 45 adults from Florida supplemented with 292.5–346.5 ml of proanthocyanidins per day, 30.6–50.9 ml of anthocyanins per day and 30.6–45 ml of flavonols per day from a cranberry beverage.
Supplementing with cranberry flavonoids did not reduce the incidence of colds and flu. However, those who supplemented with flavonoids reported significantly fewer symptoms, compared to those who got a placebo.
Riede L, et al. Larch arabinogalactan effects on reducing incidence of upper respiratory infections. Current Medicinal Research and Opinion, 2013.
187 German adults supplemented with 4.5 grams of larch tree extract for 3 months. The extract was water-based and contained unspecified amounts of taxifolin and quercetin.
Those who supplemented with larch tree extract were less likely to get a cold and be affected by the infection, compared to a placebo.
Finding: Flavonoids Reduced the Risk of Infections
This meta-analysis found that supplementing with flavonoids caused a 33% reduction in the incidence of upper respiratory tract infections (URTIs), on average, when compared with a placebo.
Supplementing with flavonoids also decreased sick-day count by 40%, although these findings were unclear.
Overall, flavonoid supplementation tended to reduce URTI duration and severity, but the evidence was inconclusive.
In the included trials, flavonoid supplementation ranged from 0.2–1.2 grams per day.
Bottom Line: The study showed that supplementing with flavonoids may reduce the risk of upper respiratory tract infections by 33%, on average.
Why Do Flavonoids Reduce the Incidence of Infections?
In the present study, the effects of flavonoids on infections were not explained by changes in markers of immune system function.
However, flavonoids have strong antioxidant and anti-inflammatory properties, improving health overall. In addition, many flavonoids have antibacterial, antiviral and antifungal effects (1).
The authors speculated that flavonoids may reduce the incidence of URTIs through their antiviral effects. In fact, flavonoids, such as quercetin, hesperetin and catechin, may act against certain types of viruses (8, 9).
Bottom Line: Why flavonoids reduce the risk of infections is not entirely clear. However, scientists have speculated that their antiviral properties may play a role.
This meta-analysis didn’t have any obvious methodological limitations.
However, the practical application of its findings is limited. Further studies are needed to examine the effectiveness of individual flavonoids and different doses.
Summary and Real-Life Application
This meta-analysis showed that supplementing with flavonoids may reduce the risk of catching a cold.
However, there is no conclusive evidence that flavonoids reduce the severity or duration of infections.
Keep in mind that flavonoids are a large group of antioxidants with different properties. Not all flavonoids are effective against the common cold and normal dietary intakes might not be sufficient.
Regardless, a diet high in flavonoids is associated with improved health overall. Increase your intake by eating a lot of berries, nuts, fruits and vegetables and drinking beverages like coffee, tea and cocoa.