As winter approaches, the chance of catching a cold or flu increases. Ever wondered if there were proven immunity boosters that could prevent winter illnesses? To help you out, I’ve done an extensive literature search to find out what has worked and what has failed in clinical trials.
Deviating a little bit from probiotics, I decided to check out what supplements can help stave off a cold. Following a fairly rigid set of guidelines (see below), I sought out immune boosters with clinical evidence for either the prevention or treatment of respiratory illnesses. Included is also a list of popular supplements that do not have any clinical evidence to back them up on PubMed.
Guidelines for Supplement Inclusion
Only data from randomized, double-blind clinical trials performed in humans after 1980 were included in the initial search. Favorable trials considered were those with positive significant results for one of the following parameters associated with respiratory illnesses:
- Frequency of infection
- Illness duration
- Symptom severity
- Number of sick days (for a day care or workplace study)
For completeness, I mention the negative trials as well in the description. Prioritization of the supplements in each section is based on the number of successful studies, the robustness of the data and the safety of the supplement.
Best Immune Boosters for Prevention of Respiratory Illness
Hands down, probiotics are the most investigated and most promising of the supplements investigated for the prevention of respiratory illness. More than 80% of the studies that I found had positive results. I found 7 studies that showed that beneficial bacteria species worked to reduce the incidence of the common cold when provided daily for a number of weeks1-7. A number of studies also indicated that probiotics could reduce the severity of symptoms2, 5, 7-9 and the illness duration2, 4, 8, 9. One study even found that probiotics could reduce the incidence of pneumonia10. Two studies did not have positive results, and they used Yakult11 and Lactobacillus fermentum (PCC(R))12.
A variety of bacterial strains were used in these studies, therefore a good quality probiotic supplement would probably be enough to see an effect. One easy to obtain strain was used successfully in a Finnish study. Babies taking Lactobacillus GG had a 17% reduction in respiratory illness with complications and the average number of days sick was reduced by more than a day7. The most robust study combined a multi-strain probiotic with a prebiotic5.
North American ginseng, not to be confused with Siberian ginseng, was tested in 2004 as a preventative for respiratory illness in nursing home residents. It was found to lower the incidences of respiratory infection13. A second study in healthy adults confirmed these findings14. In 2006, a ginseng product, “COLD-fX,” was also found to be effective in the elderly15.
3. Vitamin C
The third in this line-up is good, old vitamin C. In a trial with 168 volunteers taking vitamin C daily, there were significantly fewer colds and a shorter cold duration16. In a twin study, the sibling taking vitamin C had the same amount of colds as the untreated partner, however, the cold duration was reduced 19%17.
Another safe immune booster is garlic. In a preventative study, those taking aged garlic daily had reduced and less severe respiratory infection symptoms than those taking the placebo18. These results were supported by another study that found similar results19.
Baker’s Yeast, besides being used as a leavening agent in bread, has been considered a great source of nutrients. However, like probiotic bacteria, some yeasts have the ability to interact with the immune system in a beneficial way. In a 2010 clinical trial, a yeast fermentation product (EpiCor) was given daily to adults at risk for respiratory illness. The supplement reduced the incidence of cold and flu20. More recently, another baker’s yeast derived supplement (Wellmune) was given daily to women with moderate stress. The supplement reduced respiratory symptoms (viral and non-viral) and improved mood21.
6. Green Tea
Green tea is made from the leaves of the plant, Camellia sinensis and is known to be full of healthful flavonoids. One study using green tea flavonoids found that daily supplementation could be protective against influenza22. Another study using capsules of green tea protected adults from cold and flu, lowering both the symptom severity and reducing the illness duration23.
7. Vitamin D
Although the health benefits of vitamin D are well known, vitamin D falls lower on my list. The reason is that vitamin D is really most effective in those that are already deficient in the vitamin. This was illustrated in 2010, when it was noted that vitamin D protected children from influenza, especially in those already taking less of the vitamin24, 25. This would suggest that a well-nourished and healthy individual would not see such a huge benefit.
8. Vitamin A
Another huge anti-cold and flu standby has been vitamin A. Vitamin A is well studied, and the early studies indicate a benefit26-29. Unfortunately, there is evidence that indicates that vitamin A should be used with care. In general, the studies reveal that a person’s health status may affect the benefits of vitamin A. Recent evidence shows that vitamin A is not well utilized by the body during a respiratory infection30. In one study, children with a high infection rate responded differently to vitamin A and had less protective benefits31. Furthermore, in children with low weight, vitamin A could even increase lower respiratory tract infections32.
Zinc falls almost into the same category as vitamin A. There’s a lot of positive evidence (especially for cold treatment), but there is also conflicting data. Preventative zinc supplementation in children (10-15mg per day) does seem to reduce the incidence of respiratory infection and even limit symptom severity33, 34. Yet, children from the urban slums of Bangladesh had more acute lower respiratory infections when taking zinc for 14 days35. Furthermore, nasal preparations can harm the ability to smell.
10. Vitamin E
The last option is vitamin E, better known as the skin vitamin. In a single clinical study in the elderly, daily supplementation seemed to help against the common cold36.
Best Immune Boosters to Treat a Respiratory Illness
Zinc is well known for stopping colds in their tracts. Research backs up this idea. Of the 11 studies that I found, 8 had positive results37-44 and the rest didn’t see an effect45-47. In the majority of the studies, zinc was administered within the first 48 hours of cold symptoms. Success was seen with most forms of zinc, and the most common method of administration was via a lozenge, which can be bought on Amazon. In one study using zinc lozenges every 3 hours, there was a three day reduction in the cold duration38. I would not recommend using a nasal preparation. Although zinc nasal gels have had success in clinical trials, they could hurt your sense of smell.
Although there is a fair amount of conflicting data about Echinacea, it’s still worth a shot. Of the 8 studies that I found, one half were positive48-51, and the other half had no effect52-55. The reason that Echinacea gets the benefit of the doubt is that the quality of the material cannot be compared between these studies. Some use an extract, others use tea and some use unrefined material. What doesn’t seem to work is unrefined material54, juice53 and some extracts55. Success has been seen for high quality extracts48, tea50 and concentrated extracts51. Like zinc, it is given during the onset of a cold.
The most intriguing studies were for elderberry. An early study in 1995 showed that an elderberry extract helped control an influenza outbreak in 199356. A later study in 2004 supported these findings. Adults taking elderberry syrup during the early onset of flu had a shorter illness and less medicine was needed57.
4. Vitamin C
Even though vitamin C is wildly popular for cold treatment, conflicting studies put it at fourth place. Studies in 1994 using moderate doses (200mg per day), and a 1999 study with megadoses (3g per day) support that it can reduce the duration and severity of colds58, 59. However, a more recent study found that megadoses of vitamin C were ineffective60.
5. Pelargonium sidoides
A more exotic newcomer to the immune booster family is the South African Geranium or Pelargonium sidoides. In common cold treatment studies, extracts taken during cold onset could reduce cold severity and the duration of the sickness61, 62. There are several extracts on the market.
6. Andrographis paniculata
Andrographis paniculata is a plant native of India and Sri Lanka and has been used since ancient times to treat respiratory infection. In a trial performed in 1999, taking the herb during the beginning of illness reduced the number and severity of cold symptoms63.
Supplements That Didn’t Make the Cut
A number of supplements are recommended on many websites as immune boosters. However, many of these have only been examined in in vitro assays and/or in animals. While they could be useful, I did not find clinical evidence for their usefulness in human respiratory illness. The following well-known immune boosters fell into this category:
- Aloe vera
- Glycyrrhiza glabra
- Grapefruit seed
- Olive leaf
- Rhodiola Rosea
Don’t Forget Your Lifestyle
This article focuses on supplementation; trials looking at lifestyle changes weren’t investigated. Despite that, a good recommendation for improving the immune system would be to improve general health through diet, exercise, relaxation and sleep. Supplements are helpful, but having healthy habits are essential.
Please note that the product links included are my Amazon affiliate links.
Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment.
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