Science Reveals How Zinc Helps Colds and Flu

Like vitamin C, zinc is a beloved supplement during the cold and flu season. Zinc lozenges are commonly recommended for preventing cold or speeding recovery from cold.

Just like vitamin C, there is still an ongoing debate over the efficacy of zinc supplementation for preventing and/or treating cold and flu. Just as there are studies that have found zinc effective for shortening the duration of cold and flu symptoms, there are others that have found zinc useless for cold and flu.

But what is zinc and what are the health benefits of this trace mineral?

The Biological Roles of Zinc

Zinc is an essential nutrient needed in only very small amounts. In humans, it is found in over 300 enzymes and is the second most essential metal after iron.

Besides enzymes, zinc is also important to the formation of proteins. Specifically, it is found in proteins made from amino acids such as histidine, aspartic acid, glutamic acid, and cysteine. In addition, zinc is used in the regulation of nucleic acid metabolism and gene expression.

The body keeps a store of 2 – 4 g of zinc mostly in the brain, prostate gland, muscles, bones, liver, kidney and certain parts of the eye.

Because free zinc can be toxic in the blood, it is bound to transport proteins such as albumin and transferrin. Since these proteins also bind to iron and copper, excessive zinc intake can reduce the absorption of both metals and vice versa.

The recommended dietary allowance for zinc is 11 mg/day for men and 8 mg/day for women. Pregnant and breastfeeding women need more zinc (11 – 13 mg/day) while children need less (2 – 5 mg/day).

Zinc is most commonly obtained from the diet. Dietary sources of zinc include oyster, lobster, red meat and liver. Some plants do contain zinc too although their zinc content is determined by the amount of zinc found in the soils in which they grow.

Common plant sources of zinc include whole grains, seeds, nuts, beans, almonds, and blackcurrant.

Zinc Supplementation

Zinc can also be obtained from fortified foods and dietary supplements. Cereals fortified with zinc oxide are commonly recommended. However, studies show that zinc oxide and zinc carbonate are poor supplemental sources of zinc.

Better zinc salts include zinc acetate, zinc sulfate, zinc gluconate, and zinc picolinate.

Besides zinc tablets and capsules, other forms of zinc supplements include cold lozenges as well as nasal gel and sprays. However, in 2009, the FDA (Food and Drug Administration) warned that nasal zinc products may cause loss of the sense of smell.

Zinc supplements are usually used to address zinc deficiency.  In addition, zinc is also recommended for speeding up wound healing, for treating acute childhood diarrhea, for promoting recovery from the common cold and for delaying the progression of vision loss in old age.

Because zinc is only needed in trace amounts, zinc deficiency is mostly caused by poor absorption and chronic disorders such as liver disease, renal disease, diabetes, and cancer.

Even then zinc deficiency is still common and affects about 2 billion people worldwide.

Signs of zinc deficiency include skin lesions, diarrhea, cognitive decline, delayed sexual maturation, impotence, loss of appetite, impaired carbohydrate metabolism and depressed immune functions.

In fact, studies show that zinc deficiency can negatively affect all parts of the immune system. Therefore, zinc supplements are routinely recommended as adjunct medications in the treatment of diseases caused by a depressed immune system.

It should be noted that both low and excessive zinc intake can impair the immune system.

Zinc, Cold and the Flu

Is zinc really effective in the treatment of the common cold? While there is no consensus from past studies, the conclusions of two Cochrane reviews reflect the shifting view in the medical community over the efficacy of zinc supplements for the common cold.

A 1999 Cochrane review found no compelling evidence to support the use of zinc by cold sufferers.

However, an updated review published in 2003 revealed that recent, well-designed studies did find some benefits.

Currently, the recommendation of the Cochrane Collaboration shows that zinc supplements can indeed help people who come down with a cold as long as they start taking zinc within 24 hours of their first symptoms.

When taken over a period of 5 months, zinc supplements

• cut the duration of sickness
• reduce the severity of the sickness
• reduce the use of antibiotics by the sufferer
• reduced the risk of a cold by 40%

For all these benefits, studies show that the only major side effects of normal doses of zinc supplements are nausea and a metallic taste.

However, when taken in doses higher than 40 mg/day, the side effects reported included dizziness, drowsiness, hallucination, headache, sweating, anemia and loss of muscle coordination.

How Zinc Prevents Cold

Preventing the Entry of Cold Viruses

Cold viruses usually gain entry into the blood via the thin linings of the nostrils. Zinc by its particulate nature can coat these viruses and make them too big for the tiny holes in the linings of the smooth muscle.

This is really a physical inhibition of cold viruses. This mechanism of action is probably the reason why intranasal zinc products are more effective than oral zinc supplements.

However, the same mechanism by which zinc particles prevent the passage of cold viruses through the lining of the nostril may also be responsible for the dampening effect intranasal zinc products have on the sense of smell.

Blocking the Reproduction of Rhinoviruses

Scientists have demonstrated that zinc can inhibit the reproduction of rhinoviruses. Since rhinoviruses account for 4 out 5 cases of cold, this is a major mechanism by which zinc prevents and helps treat the common cold.

Reproduction is an important part of an infection. In this case, cold viruses need to spread in order for their viral load to lead to symptomatic cold infection.

By stopping their spread, zinc effectively prevents the few viruses that have gained entry into the lungs and bloodstream from initiating and perpetuating a cold infection.

Preventing Adhesion to Cell Membranes

Besides gaining entry into the systemic circulation, cold viruses need to adhere to cell membranes before they can infect human cells and cause infections. However, zinc blocks this docking mechanism.

Without the ability to gain entry into cells, cold viruses are more likely to be engulfed and destroyed by immune cells.

Therefore, zinc blocks an important mechanism by which cold viruses evade the cells of the immune system and cause widespread infection.

Boosting the Immune System

Lastly, the positive effect of zinc on the immune system can help combat cold viruses.

Studies show that zinc enhances every aspect of the immune system. Therefore, it can promote the release of killer immune cells, the production of specific antibodies against cold viruses and the reduction of inflammatory cytokines that cause some of the symptoms of the common cold.

This immune boost is most likely the reason why zinc supplementation reduces the risk of the common cold for some months after it has been stopped.

Studies on Zinc and Cold

Efficacy of Zinc in the Treatment of Cold

Given the large number of studies done to investigate the efficacy of zinc in the treatment of cold, the best way to determine if zinc is truly effective is to examine the results of meta-analyses and reviews.

Discussed below are 2 such reviews that were published in different decades.

The first one was published in the Journal of the American Pharmacists Association in 2003. The author examined studies published between 1980 and 2003 to reach the conclusion that current research supported the use of zinc (within 24 hours of the onset of the symptoms) for reducing the severity and duration of the common cold.

The second meta-analysis was published in the Canadian Medical Association Journal in 2012.

The authors reached similar conclusions as to the 2003 study demonstrating that zinc is still an effective cold remedy even after another decade of more research.

The same conclusions were also confirmed by a 2012 update of the Cochrane review on the efficacy of zinc in the treatment of a cold.

These meta-analyses called for more studies to determine the right tolerance profile for zinc supplementation for cold sufferers. Due to the common occurrence of mild side effects at certain doses, it is important to know the right dose for all stages of cold infection.

Zinc, Cold and the Immune System

A 2008 study published in the Journal of Infectious Diseases investigated the effect of zinc (as cold lozenges) on different aspects of the immune system.

The researchers recruited 50 volunteers within 24 hours of their coming down with a cold. The participants were divided into two groups with one group given a placebo while the other group received 1 lozenge (13.3 mg of zinc as zinc acetate) every 2 – 3 hours while they were awake.

The results of the study showed that the zinc lozenges reduced the duration and severity of the cold.

In addition, zinc reduced the number of days that the users experienced cough and nasal discharge compared to the placebo group.

Furthermore, plasma zinc levels rose in the lozenge group. Lastly, zinc supplementation boosted the immune system by favorably affecting the levels of intercellular adhesion molecule 1 (ICAM-1) and soluble interleukin-1 receptor antagonist (sIL-1ra).

The researchers, therefore, concluded that the antioxidant and anti-inflammatory properties of zinc were important to its efficacy for reducing cold symptoms.

A 2009 study published in the journal, Complementary Therapies in Clinical Practice, investigated the efficacy of zinc supplements for treating upper respiratory tract infections among US Air Force cadets.

The researchers recruited 40 cadets and over a period of 7 months gave them either placebo or 15 mg/day of zinc supplements (zinc gluconate capsules).

The results of the study showed that the group that received zinc supplements experienced fewer days of cold symptoms. The researchers attributed the efficacy of zinc to its immunomodulatory effect especially its ability to promote the activities of immune cells such as neutrophils, monocytes, macrophages, B lymphocytes, and T lymphocytes.

Cold Lozenges: Do They Work?

Because cold lozenges are the most commonly used zinc supplements during the cold season, it is important to determine whether they are effective at all or just as good as placebos. A 1998 paper published in Annals of Pharmacotherapy reviewed studies published between 1966 and 1997 that had investigated the efficacy of zinc lozenges.

The reviewers found 8 such studies and their results showed that 4 of those studies found zinc lozenges effective for treating cold while the other 4 found no difference between groups taking zinc and placebo.

However, closer examinations showed that the negative studies failed to properly use placebo as a control, used inappropriate zinc doses or used ineffective brands of zinc lozenges.

In their opinion, the reviewers concluded that lozenges made from zinc gluconate were effective for reducing the duration and severity of cold symptoms as long as adequate doses were used and the zinc lozenges started immediately after symptoms appeared.

Zinc + Vitamin C for Cold

Besides zinc, vitamin C is the other natural dietary supplement commonly used to combat cold. But is there a benefit to combining both supplements and is the combination safe?

A 2012 study published in The Journal of International Medical Research investigated the use of zinc-vitamin C combination for the common cold. The papers used data from 2 randomized, placebo-controlled, double-blind studies published on the subject.

In both studies, there were a total of 94 volunteers who were either given a placebo or a combination of 1000 mg/day of vitamin C plus 10 mg/day of zinc for 5 days.

The results of the combined research showed that the combination of zinc and vitamin C reduced the duration and severity of the cold. In addition, this combination also produced results superior to that obtained from using either dietary supplement alone.

Lastly, the authors of the study reported that the combination was safe and well-tolerated.