Fighting the Flu With Supplements: The Pharmacist’s Patient Counseling Guide

Questions about vitamins, minerals, and supplements crop up frequently during flu season. Patients’ confusion is unsurprising given that the FDA regulates these products not as pharmaceuticals but as food.¹ As a result, the agency’s authority is limited to removing supplements proven harmful only after market release. Furthermore, manufacturers may state unfounded claims, such as labeling products as “immune boosters.”¹ Pharmacists can protect public health and safety by understanding which supplements are effective against influenza and by knowing related counseling points for various patient populations.

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Flu-Fighting Nutrition and Supplementation

Vitamin C, vitamin D, and zinc support the immune system and are frequently the focus of information regarding influenza prevention and treatment.² The most potent source of vitamins and minerals is food, rather than supplements from a bottle.³ Table 1² lists examples of foods with nutrients that bolster immunity.

Supplementation is best reserved for patients with dietary deficiencies or restrictions.³ For example, reduced sun exposure causes vitamin D levels to trend downward in the winter, when influenza is more prevalent, so quite a bit of research into supplement use and influenza centers around vitamin D.⁴

A meta-analysis examining supplementation for influenza A and B revealed that optimal vitamin D dosing includes a loading dose below 60,000 IU and a daily dose of 2000 IU or higher. The meta-analysis also found that vitamins C and D reduce the risk of influenza and shorten its duration, whereas zinc shortens the duration of influenza but does not reduce the risk.⁵ The analysis covered adult patients. Table 2^6-8 shows intake values by age, including pediatric ranges. This table applies to male and female patients as well as pregnant and nonpregnant individuals.

Although the public commonly regards vitamins and minerals as safe, these micronutrients have potentially dangerous interactions with various pharmaceuticals. Aluminum, chemotherapy, estrogen, niacin, protease inhibitors, statins, and warfarin can interact with vitamin C. This supplement can also alter the results of fecal occult and glucose screenings.⁹ Vitamin D can interact with orlistat, statins, steroids, and thiazide diuretics.⁶ Zinc also interacts with thiazide diuretics, as well as antibiotics and penicillamine.¹⁰

Pediatrics and Pregnancy

Community health care professionals are best equipped to serve all patients by being familiar with research on different age groups and alterations in health.

To investigate supplementation in the youngest patients, researchers compared the effects of low- and high-dose vitamin D (400 IU and 1200 IU, respectively) on infants with influenza A. The 400 participants were aged 3 to 12 months.⁴ The 200 infants receiving 1200 IU of oral vitamin D drops exhibited faster improvements in fever, coughing, and wheezing, and had lower viral loads. As such, the research showed vitamin D to have an antiviral effect on influenza A with a low risk of adverse effects for infants.⁴

Vitamin D supplements are safe to use during pregnancy, with tolerable upper intake levels the same as those for nonpregnant patients.⁶ However, multiple studies have shown that maternal vitamin D supplementation during pregnancy and lactation does not inhibit respiratory infections in infants or children.^11,12

Another randomized trial investigated vitamin D supplementation in 248 children aged 2 to 5 years. Although the trial found that 2000 IU of daily vitamin D supplementation reduced influenza A incidence, the results had low statistical significance. The researchers noted that studies into vitamin D and influenza have varying results and suggest that vitamin D supplementation is more beneficial for patients with a deficiency of this vitamin.¹³

In an older age range, a randomized trial of 430 children aged 6 to 15 years supported this hypothesis. This trial’s results showed that 1200 IU of vitamin D significantly reduced the incidence of influenza A. Notably, the reduction in incidence was less prominent among participants consuming vitamin D supplements beyond those directed by the study. In other words, the population receiving the relatively lower dose showed greater improvement.¹⁴ The research team conducted its study from December to March of the research period, so the team concluded its findings with the suggestion of vitamin D supplementation in the winter for school-aged children.¹⁴

Older Adults

Data uncovering relationships between vitamins and influenza in older adults are varied. Results from one review are inconclusive as to whether vitamin D prevents influenza and whether it spurs effectiveness of the influenza vaccine in older patients. The review article suggests that in any case, older patients’ serum vitamin D should be assessed and maintained.¹⁵

A small study (N=85) related to methods for increasing the influenza vaccine’s effectiveness in older adults investigated the amino acid L-glutamine. The researchers tested secretory IgA (SIgA) levels following 30 days of L-glutamine supplementation in physically active older adults aged 60 to 85 years. SIgA is an immunoglobulin that prevents influenza infection. Results indicated that participants receiving L-glutamine supplementation had higher salivary levels of SIgA.¹⁶

In a 15-month trial involving healthy older adults, a daily multivitamin-mineral supplement did not reduce the incidence or severity of influenza A or B. Furthermore, participants receiving vitamin E supplementation had a longer illness duration and increased severity of symptoms. Vitamin E is one of the most common supplements for older adults, so a word of caution is beneficial for this age group until further research is available.^17,18

In another study, 107 residents in long-term care aged 60 and older were divided into 2 study arms receiving daily vitamin D. One arm received a high dose (3000-4000 IU), whereas the other received a standard dose (400-1000 IU). The high-dose arm’s incidence of acute respiratory infection was 40% less than that of the standard-dose arm.¹⁹ However, the high-dose subset had a fall rate 2.3 times higher than the low-dose subset (fracture rates did not increase).

The research team hypothesized that falls increased due to improved mobility with higher vitamin D levels, although they said more research is needed.¹⁹ Additional studies have reported increased fall rates with vitamin D supplementation but have likewise failed to determine a root cause. Older adults taking vitamin D should be informed that improved musculoskeletal function could increase their risk for falls, especially if they have a fall history.^19-22

Conclusion

Confusing product labeling and conflicting research make challenging work of understanding which complementary health approaches are beneficial. Supplements can even be dangerous; for example, intranasal zinc gels and swabs, marketed to shorten cold duration, can cause a permanent loss of smell.²³

Patients will benefit from understanding that the FDA regulates supplements in the same way as food. As such, botanicals including echinacea, elderberry, ginseng, and green tea may have misleading claims despite not having been proven effective.²³ Pharmacists can also teach patients how to follow a diet rich in immune-supporting vitamins and minerals to stay flu-free even before seeking supplements.

References

1. Berg S. What doctors wish patients knew about vitamins and supplements. American Medical Association. October 1, 2021. Accessed March 29, 2024. https://www.ama-assn.org/delivering-care/public-health/what-doctors-wish-patients-knew-about-vitamins-andsupplements

2. Wempen K. Fight off the flu with immune-boosting nutrients. Mayo Clinic Health System. November 16, 2022. Accessed March 29, 2024. https://www.mayoclinichealthsystem.org/hometown-health/speaking-ofhealth/ fight-off-the-flu-with-nutrients

3. Get nutrients from food, not supplements, from the June 2015 Harvard Health Letter. Harvard Health Publishing. June 1, 2015. Accessed March 29, 2024. https://www.health.harvard.edu/press_releases/get-nutrients-fromfood-not-supplements

4. Zhou J, Du J, Huang L, Wang Y, Shi Y, Lin H. Preventive effects of vitamin D on seasonal influenza A in infants: a multicenter, randomized, open, controlled clinical trial. Pediatr Infect Dis J. 2018;37(8):749-754. doi:10.1097/INF.0000000000001890

5. Abioye AI, Bromage S, Fawzi W. Effect of micronutrient supplements on influenza and other respiratory tract infections among adults: a systematic review and meta-analysis. BMJ Glob Health. 2021;6(1):e003176. doi:10.1136/bmjgh-2020-003176

6. National Institutes of Health Office of Dietary Supplements. Updated September 18, 2023. Accessed March 29, 2024. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

7. National Institutes of Health Office of Dietary Supplements. Updated September 28, 2022. Accessed March 29, 2024. https://ods.od.nih.gov/factsheets/Zinc-HealthProfessional/

8. National Institutes of Health Office of Dietary Supplements. Updated March 26, 2021. Accessed March 29, 2024. https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/

9. Vitamin C. Mayo Clinic. August 10, 2023. Accessed March 29, 2024. https://www.mayoclinic.org/drugs-supplements-vitamin-c/art-20363932?p=1

10. Zinc. Mayo Clinic. August 10, 2023. Accessed March 29, 2024. https://www.mayoclinic.org/drugs-supplements-zinc/art-20366112

11. Morris SK, Pell LG, Rahman MZ, et al. Effects of maternal vitamin D supplementation during pregnancy and lactation on infant acute respiratory infections: follow-up of a randomized trial in Bangladesh. J Pediatric Infect Dis Soc. 2021;10(9):901-909. doi:10.1093/jpids/piab032

12. Sulaiman Z, Noor NM, Ismail SB, Mohd Lukman ASF, Irfan M, Hussain NHN. Maternal vitamin D supplementation for the prevention of respiratory tract infections in offspring: a meta-analysis. Korean J Fam Med. 2022;43(3):174-182. doi:10.4082/kjfm.21.0029

13. Huang YN, Chi H, Chiu NC, et al. A randomized trial of vitamin D supplementation to prevent seasonal influenza and enterovirus infection in children. J Microbiol Immunol Infect. 2022;55(5):803-811. doi:10.1016/j.jmii.2022.01.003

14. Urashima M, Segawa T, Okazaki M, Kurihara M, Wada Y, Ida H. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr. 2010;91(5):1255-1260. doi:10.3945/ajcn.2009.29094

15. Lang PO, Samaras D. Aging adults and seasonal influenza: does the vitamin D status (h)arm the body? J Aging Res. 2012:2012:806198. doi:10.1155/2012/806198

16. Paixão V, Almeida EB, Amaral JB, et al. Elderly subjects supplemented with L-glutamine shows an improvement of mucosal immunity in the upper airways in response to influenza virus vaccination. Vaccines (Basel). 2021;9(2):107. doi:10.3390/vaccines9020107

17. Graat JM, Schouten EG, Kok FJ. Effect of daily vitamin E and multivitamin-mineral supplementation on acute respiratory tract infections in elderly persons: a randomized controlled trial. JAMA. 2002;288(6):715-721. doi:10.1001/jama.288.6.715

18. McKenzie J, Keller HH. Vitamin-mineral supplementation and use of herbal preparations among community-living older adults. Can J Public Health. 2001;92(4):286-290. doi:10.1007/BF03404962

19. Ginde AA, Blatchford P, Breese K, et al. High-dose monthly vitamin D for prevention of acute respiratory infection in older long-term care residents: a randomized clinical trial. J Am Geriatr Soc. 2017;65(3):496-503. doi:10.1111/jgs.14679

20. Sanders KM, Stuart AL, Williamson EJ, et al. Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial. JAMA. 2010;303(18):1815-1822. doi:10.1001/jama.2010.594

21. Bischoff-Ferrari HA, Dawson-Hughes B, Orav EJ, et al. Monthly high-dose vitamin D treatment for the prevention of functional decline: a randomized clinical trial. JAMA Intern Med. 2016;176(2):175-183. doi:10.1001/jamainternmed.2015.7148

22. Wimalawansa SJ. Infections and autoimmunity—the immune system and vitamin D: a systematic review. Nutrients. 2023;15(17):3842. doi:10.3390/nu15173842

23. National Center for Complementary and Integrative Health. Updated November 2016. Accessed March 29, 2024. https://www.nccih.nih.gov/health/flu-and-coldsin-depth