Lungs, asthma, respiratory condition

(© Marina Zlochin -

Typically, it’s late summer and into autumn when you start thinking about boosting your immune system. With colder weather, we’re expecting a surge of upper respiratory infections (URIs), such as the common cold and the annual flu season. We expect to spend more time indoors, in closer contact with more people, but every cough and sneeze can spread infectious respiratory droplets over alarming distances. 

Since COVID, however, “typical” virus prep is no more. Viruses, disease testing methods, immunizations, and disease-mitigation practices have all changed. It makes sense to optimize immune health year-round. Along with social distancing, masks, and vaccinations, is there a role for dietary supplements in infectious disease prevention or management?

Zinc, vitamin C, and vitamin D are popular supplements for people seeking to bolster their immune systems, but are they really helping? Let’s take a look at 10 of the common beliefs about respiratory infections and how to treat them — to decide which are fact and which are fiction.

1. Zinc supplements taken within 24 hours of the onset of symptoms of the common cold can decrease the duration of illness.

Fact. A recent analysis of multiple studies on the use of zinc lozenges or oral sprays for cold symptoms found that they shortened the duration of illness by about two days.

Zinc is important for the generation of T cells, a type of immune cell active against cells infected with bacteria and viruses. It also affects the functions of cells that line the respiratory tract — the first line of defense against germs causing respiratory infections.

2. Zinc supplements taken within 24 hours of the onset of COVID-19 symptoms will decrease the severity of an individual’s illness.

Fiction. A panel at the National Institutes of Health (NIH) stated:

  • There is insufficient evidence for the panel to recommend either for or against the use of zinc for the treatment of COVID-19.
  • The panel recommends against using zinc supplementation above the recommended dietary allowance (i.e., zinc 11 mg daily for men, zinc 8 mg daily for nonpregnant women) for the prevention of COVID-19.
Zinc supplement tablets on a plate
(Photo by Fida Olga on Shutterstock)

3. Zinc supplements should be ingested with a meal for maximum absorption.

Fiction. Zinc supplementation as part of a meal can significantly reduce zinc absorption when compared to water-based solutions of zinc. Dietary phytate, a natural substance in corn, rice, and cereals binds to zinc, severely restricting its absorption in the gut. Diets high in phytate can result in zinc deficiency, even with adequate zinc intake.

People with a good diet get plenty of zinc from meat, beans, lentils, seafood, and whole grains. They are unlikely to need zinc supplements.

4. Aging increases your susceptibility to zinc deficiency.

Fact. Elderly individuals are more susceptible to zinc deficiency than younger individuals, increasing their likelihood of acquiring life-threatening viral infections.

Zinc can interact or interfere with some medications and other supplements. Check with a physician before taking supplements.

5. Vitamin C reduces the incidence of the common cold in community populations.

Fiction. Vitamin C does not reduce the incidence of the common cold in community populations. Like zinc, however, vitamin C shortened the duration of symptoms.

Vitamin C contributes to the health of the immune system by triggering neutrophils and macrophages (types of white blood cells with immune functions) to fight infection and reduce inflammation. Macrophages kill and ingest germs, then clear away cellular debris.

6. Vitamin C supplements can help COVID-19 patients avoid hospitalization.

Fiction.  A panel at the NIH stated:

  • There is insufficient evidence for the Panel to recommend either for or against the use of vitamin C for the treatment of COVID-19 in non-hospitalized patients. Because patients who are not critically ill with COVID-19 are less likely to experience severe inflammation, the role of vitamin C in this setting is unknown.

The position of the panel is the same for hospitalized patients with COVID-19.

7. Consuming excessive vitamin C can cause kidney stones.

Fact. An article in JAMA Internal Medicine reported a connection between kidney stone formation and use of vitamin C supplements. Men who used supplements were twice as likely to experience kidney stones as men who did not use supplements. Use of a multivitamin did not increase the risk of kidney stones.

The vitamin C requirement for men is 90 mg per day, and 75 mg per day for women.

Foods High in vitamin C on a wooden board.
(© bit24 –

8. Low vitamin D levels have been linked to increased risk for respiratory infections.

Fact. One study with almost 19,000 individuals found an inverse relationship between blood levels of vitamin D and upper respiratory infections (URI’s) such as the common cold. The lower the level of vitamin D, the greater the risk of developing a URI. 

Another study found that, like zinc and vitamin C, vitamin D supplements may shorten the duration of the symptoms associated with a URI.

9. Low blood levels of vitamin D increase susceptibility to COVID-19 infection.

Fiction. Currently, data are insufficient to support a recommendation for or against the use of vitamin D supplementation to prevent or manage COVID-19. Some evidence, however, suggests that vitamin D supplementation helps prevent respiratory tract infections, particularly in people with 25(OH)D (vitamin D) levels less than 25 nmol/L (10 ng/m). Scientists are studying whether vitamin D might also be helpful for preventing or treating COVID-19.

Vitamin D foods
(© Leigh Prather –

10. Vitamin D supplements can adversely interact with some medications.

Fact. Vitamin D supplements may interact with some medicines. These are several examples:

  • Orlistat (Xenical®) is a weight-loss drug. It can decrease the amount of vitamin D your body absorbs from food and supplements.
  • Cholesterol-lowering statins might not work as well if you take high-dose vitamin D supplements. This includes atorvastatin (Lipitor®), lovastatin (Altoprev®), and simvastatin (FloLipid™).
  • Steroids, such as prednisone (Deltasone®) can lower your blood levels of vitamin D.
  • Thiazide diuretics (Hygroton®) could raise your blood calcium level too high if you take vitamin D supplements.

Tell your doctor, pharmacist, and any other healthcare providers about any dietary supplements and prescription or over-the-counter medicines you take. They can tell you if the dietary supplements might interact with your medicines. They can also explain whether the medicines you take might interfere with how your body absorbs or uses other nutrients.

While many of the studies are inconclusive, what you can take from this article with conviction is that, for your good health, eat a large variety of foods: proteins, complex carbohydrates, and healthy fats. Try to get your nutrients from fresh and unprocessed foods as much as you can. Exercise. Sleep. Build relationships. Play. Give. Keep reading.

About Dr. Faith Coleman

Dr. Coleman is a graduate of the University of New Mexico School of Medicine and holds a BA in journalism from UNM. She completed her family practice residency at Wm. Beaumont Hospital, Troy and Royal Oak, MI, consistently ranked among the United States Top 100 Hospitals by US News and World Report. Dr. Coleman writes on health, medicine, family, and parenting for online information services and educational materials for health care providers.

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