Fact check: Do natural cold and flu remedies work?

The idea that you can “boost” your immune system is a popular one.

It’s no surprise, then, that a Google search for “immune booster” turned up more than one billion results. There was no shortage of ads, blogs and articles promoting certain foods and supplements with immune-enhancing ingredients.

With the flu, RSV and COVID circulating, it can be tempting to reach for a pill, drink or superfood that promises to supercharge immunity.

Can popular immune supplements and foods actually prevent or treat a respiratory infection? Here’s what the science says.

How the immune system works

Your immune system is constantly working to defend against infection-causing viruses, bacteria and other microbes. When it recognizes a threat, it mounts a response by releasing white blood cells and other immune compounds that destroy foreign invaders.

Claiming that a particular food or supplement can significantly prime your immune system to fend off infection, however, is simplistic and misleading.

That’s because your body’s immune system is a highly complex and finely-tuned network of many working parts, including organs, cells and proteins. Researchers still don’t fully understand all the intricacies of this system.

Does the science support “immune boosters”?

Among the multitude of supplements and foods often recommended to treat – or prevent – upper respiratory infections, here’s the evidence behind five.

Most studies have shown that supplementing with at least 2,000 mg of vitamin C daily, in divided doses, reduces the duration of cold symptoms by 24 to 36 hours, with a greater benefit seen in children than adults.

Taking vitamin C supplements has not been shown to prevent catching a cold.

Are you getting enough vitamin C?

Vitamin C supports immune function by protecting immune cells from damage caused by free radicals, unstable oxygen molecules generated during the immune response. It may also increase the production of immune cells that engulf and kill pathogens.

Many well-controlled trials studies have shown that zinc gluconate or zinc acetate lozenges reduce the severity and duration of cold symptoms, especially cough, nasal congestion, nasal drainage and sore throat. Zinc is thought to prevent cold viruses from replicating.

Zinc lozenges should be taken every two to three hours while awake, starting within 24 hours of cold symptom onset.

Don’t take a daily high dose zinc supplement thinking it will benefit immunity. The safe upper daily limit of zinc for adults – from supplements and food combined – is 40 mg. Long-term intakes above this increase the risk of health problems, including impaired immune function.

Consuming higher amounts of zinc over the short term (two weeks) from zinc lozenges has not been found to have adverse health effects.

Probiotics

Many clinical trials have shown that taking probiotics, as a supplement or a fermented milk product, reduces the risk of developing an upper respiratory infection (cold, flu, sinusitis, pharyngitis) in infants, children and adults.

There’s also evidence that taking probiotics can reduce the duration of upper respiratory infections by about 1.9 days in adults and children.

Most types of probiotics studied belong to the Lactobacillus genus. More research is needed to confirm which strains and doses are most beneficial.

Chicken soup

There’s no evidence to prove that eating chicken soup helps treat the common cold. But a theory gleaned from lab experiment conducted at the University of Nebraska suggests how it potentially might.

The study, published in 2000, demonstrated that bathing volunteers’ blood samples in homemade chicken and vegetable soup slowed the activity of inflammation-causing white blood cells. It was theorized that this could reduce mucus flow and help ease a stuffy nose.

Not much to go on. Still, there’s no reason not to each chicken soup when you’re sick. It’s nutritious and hydrating.

Honey

A 2014 review of three randomized controlled trials concluded that taking two teaspoons of honey at bedtime helped reduce nighttime coughing in children.

A 2021 review of 14 trials involving children and adults with upper respiratory tract infections found that honey was as effective – and possibly better than – other treatments used to reduce cough frequency and severity.

It’s not known how honey soothes coughing. It’s anti-inflammatory, antioxidant and/or antimicrobial effects properties may play a role.

Honey should never be given to children under age one owing to the risk of infant botulism.

While technically you can’t “boost” your immune system, you can certainly support it by eating a healthy diet that includes plenty of fruits and vegetables. Your immune system relies on a steady stream of nutrients to function optimally.

Exercising regularly, getting proper sleep and managing stress are other important ways to support a healthy immune system.

Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD