It's that time of year. Sniffling, sneezing, coughing and runny noses abound. While there is no cure for the common cold because there are so many strains of viruses that cause it there are some remedies that can keep definitely help ease the symptoms, severity and duration. Zinc along with vitamin C are probably the two most studied nutrients for the common cold. Find out if it works and how it may kill cold viruses.
Does Zinc Work for Colds?
Zinc is one of the nutrients people take for colds that there seems to be some confusion around. The reason for the confusion is that there are so many forms of products and types of zinc on the market. The variety of formulations, doses, study type and perhaps quality of products in studies have also added to the confusion. As a result different findings from one study may have sparked a headline that looks to completely write it off as a farce. When we actually look at the higher quality evidence the overall trend is that it definitely works.
Two high-quality review studies combined over a dozen clinical trials and showed very positive results across the data for zinc for the common cold. A 2012 published in the Canadian Medical Association Journal combined data and looked at the overall effectiveness of zinc for colds in 13 randomized controlled trials found that zinc supplementation effectively shortened the duration of colds with lozenges and syrup products (1). A 2013 Cochrane review (14 trials, 1,656 participants) found that zinc lozenges given within 24 hours of when cold symptoms began also decreased the duration of cold symptoms (2). The type of zinc most commonly used in these studies was a gluconate or acetate lozenge or sulfate syrup. Interestingly in children and adults it appeared equally effective, with the best results occuring with about 75 mg/day of zinc. The best method was a continual intake throughout the day, where particpants would take a lozenge every 2-3 hours throughout the day. Of course this is a much higher daily dose than the recommended daily allowance (RDA) of zinc at anywhere from 2-12 mg/day depending on age and health status, but the duration was very short as well and has been found to be safe and effective in the forms discussed above.
A smaller more recent 2017 meta-analysis published in JRSM combined the data of 7 randomized trials and wanted to if higher doses above 75 mg/day were more effective. On average colds were 33% shorter in the zinc groups compared to placebos. Zinc acetate used in 4 of those studies found that colds were shortened by 40%, and in 4 studies using zinc gluconate colds were shortened by 28%. In the pool of studies 5 used zinc at a dose of 80-92 mg/day with a 33%, and two studies that used about twice as much zinc at a dose of 192-207 mg/day shortened cold duration by 35%. The authors of this study found very similar efficacy between the two types of zinc lozenges and didn't recommend a dose of more than 100 mg/day, as it didn't show significantly more benefit at twice the dose. The authors also encouraged people to try zinc lozenges as the results are very encouraging (3).
How Does Zinc Kill the Common Cold?
Zinc is an essential nutrient and brittle metal that is chemically very similar to magnesium with most of the zinc in supplements being in a 2+ oxidized form (lost 2 electrons, gained 2 hydrogens). The reason the chemistry is important is that this determines the most effective form of zinc (Zn2+) in shortening colds. The mechanism is largely dependent on the type of zinc being used.
In a study that compared different types of zinc ions the lozenges that had the Zn1+, or a zero ion state had less effect or no effect on colds. The Zn2+ form was the clear winner with the greatest effect of shortening colds. The authors also noted that the type of zinc also results in better outcomes in clinical studies, and gives our immune cells the type of zinc that our mast cells release in their granules when fighting off infectious invaders. Zinc's ion state determines how it interacts with cell receptors, proteins and amino acids that are involved in killing viruses or preventing viral replication (4). Other studies have also compared types of zinc and found that the flavoring agents used in syrups and lozenge products can affect zinc's ion state (2+, 1+, or none). Citric acid, mannitol, and sorbitol bind to (chelate) zinc and decrease it's potential to stimulate white blood cells' function, reduce it's antioxidant functions and anti-inflammatory functions (5).
Zinc's mechanism has everything to do with it's affect on activating white blood cells, signaling the immune system to kill the virus and blocking viral replication inside an infected cell. Viral replication involves six steps: attachment, penetration, uncoating, replication, assembly, and release. Interrupting any of these steps can result in the virus not being able to make more viruses in the cell that it infects and ultimately pass on the infection. Viruses essentially punch a hole in the infected cell's protective membrane, hijack it's DNA once inside and program it to make copies of itself so it can be released inside the body and go infect surrounding cells. It will continue to repeat this same process until the immune system stops it.
Although the mechanism of how exactly zinc kills the common cold virus isn't fully understood studies have been done that show it has potent anti-viral effects from a variety of mechanisms. Zinc cell studies have shown it's ability to block the rhinovirus' and RSV's ability to replicate by preventing capsid proteins (1,6), increase proteins (IFN-y) that activate neutrophil, natural killer cells and macrophages, block the viruses ability to bind to the mucosa in the nasal passage (via ICAM-1 inhibition), stabilize mast cells which prevents histamine release (6), and block inflammatory cytokines (prostaglandins) from being produced which can play a role in viral infection defense mechanisms as well (6,2).
Safety and Upper Limits
When it comes to the delivery method of zinc the lozenge formulations are definitely best as there have been cases where people lost their sense of smell (anosmia) with zinc nasal sprays and gels. Zinc deficiency will lead to more infections and a weaker immune system so it is good to get adequate zinc from beef, nuts and oysters and a regular multivitamin on a daily basis. The Institute of Medicine set the tolerable upper limit for zinc at 40 milligrams a day for adults, and less for teens and children. The use of zinc at daily doses of 50 to 180 mg for one to two weeks did not result in serious side effects with bad taste and nausea as the most common reported effects (1). Zinc lozenge use over 6-8 weeks could deplete copper levels.
References:
1. Science M, Johnstone J, Roth DE, Guyatt G, Loeb M. Zinc for the treatment of the common cold: a systematic review and meta-analysis of randomized controlled trials. CMAJ. 2012;184(10):E551-561.
2. Singh M, Das RR. Zinc for the common cold. Cochrane Database Syst Rev. 2013(6):Cd001364.
3. Hemilä, H. (2017). Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage. JRSM Open, 8(5), 205427041769429. /2054270417694291
4. Eby GA. Zinc ion availability--the determinant of efficacy in zinc lozenge treatment of common colds. J Antimicrob Chemother 1997;40:483-93.
5. Eby GA. Therapeutic Effectiveness of Ionic Zinc for Common Colds. Clinical Infectious Diseases. 01 February 2008. 46(3):483-384.
6. Caruso TJ, Prober CG, Gwaltney JM Jr. Treatment of naturally acquired common colds with zinc: a structured review. Clin Infect Dis. 2007 Sep 1. 45 (5):569-74.
