Zinc and the common cold
The Irish Times
Health Supplement, p. 13
3 February 2009
DOES IT WORK? Very few studies have proved the effectiveness of zinc against the cold virus
IT’S THAT time of the year when everyone seems to have a cold. Everyone also has a favourite remedy or a surefire way to prevent getting a cold. Colds are caused by several different viruses which are not impacted by antibiotics. In addition to the personal discomfort of colds, they make a large contribution to the number of days lost from work and school. Hence the search for alternative ways to at least minimise the symptoms.
Evidence from studies
Twenty-five years ago, a controlled trial was published reporting dramatic improvements in cold symptoms when people took a zinc gluconate lozenge every two hours. However, within a few years, three more trials were published which failed to find positive effects.
The most recent review found seven studies in which zinc was effective – and seven where it was ineffective. What are we to conclude from all this research?
When the studies are examined by their quality, the best-designed ones show no benefit from zinc lozenges. Those with positive results all have at least one serious defect in the way they were conducted.
The studies also had other important differences which shed some light on why the results differed. This requires looking at some further details about zinc.
Zinc is an essential mineral, required in the body in its ionic (or charged) form. Zinc ions are believed to interfere with how cold viruses lodge themselves in the nasal passages. If this is how zinc works against the cold, the quantity of zinc ions released from zinc products is very important.
The earliest studies with zinc lozenges used zinc gluconate. Zinc ions have a positive charge and are found in compounds, along with different negatively-charged ions, like gluconate. As zinc gluconate lozenges dissolve, they release almost all of their zinc ions relatively easily.
However, they also have a chalky, bland taste which develops into a bitter, metallic taste after taking them for a few days. Many people find this taste very disagreeable.
Because of the taste problems with zinc gluconate, some manufacturers add other agents to mask the taste – additives such as glycine – or use a different form of zinc – most commonly, zinc acetate.
However, studies have found that zinc acetate releases its zinc ions very slowly, resulting in most zinc being swallowed and few zinc ions getting to the nasal passageways.
Another approach to reduce the bad taste has been to decrease the quantity of zinc in the lozenges. Studies with positive results tend to use lozenges containing 20-25mg of zinc, while some products on the market contain just 5-10mg of the mineral.
When the research studies were examined based on the zinc formulation used, the studies with positive results used zinc gluconate in higher doses. Because zinc ions are believed to work in the nose, nasal sprays and gels are now being produced. However, very few studies have examined their effectiveness.
The bad taste of zinc lozenges can lead to headaches and nausea in some people. Others can develop a sensitivity to zinc, which causes mouth sores.
Nasal sprays and gels have also caused nasal irritation, which can lead to nose bleeds or headaches.
A few cases have been reported where people lost their sense of smell after using zinc nasal gel.
While the overall picture is confusing, there is some guidance in the details. There is no evidence that zinc reduces the chance of getting a cold.
However, once the symptoms begin, a lozenge every two hours may reduce how long the symptoms last.
Lozenges containing 20-25mg of zinc gluconate, with few additives, seem to work best. However, the evidence is not clear-cut. Nasal sprays and gels deliver zinc to where it is believed to combat viruses, but little research is available on those products.
So what are we to do when we feel a cold coming on?
Treatments for the common are notoriously difficult to study. The placebo effect is particularly strong with conditions like the cold, where symptoms and their severity vary dramatically.
What we do know is that when you have a cold, get some rest and drink lots of fluids. Chicken soup has as much supporting evidence as some over-the-counter remedies. Unfortunately, the old saying remains true: there’s no cure for the common cold.