As cold and flu season descends upon us once again — along with the ongoing COVID-19 pandemic — nutrition to boost your immune system is more important than ever. While interventions can be made using zinc, vitamin C, and vitamin D, there are also specific compounds in fruits, vegetables, and herbs that have antiviral properties.

Vitamin C

Vitamin C directly encourages the production and functionality of immune cells while protecting them from harmful free radicals. It is also an essential part of the skin’s defense system, acting as an antioxidant and helping to strengthen the skin barrier.

But vitamin C hasn’t been shown to reduce the incidence of coming down with a cold; that is, the research hasn’t been able to demonstrate that it prevents one from getting sick. However, there is evidence to support vitamin C’s role in decreasing the duration of cold symptoms.

The caveat to this is that, in order to be sick for less time, vitamin C supplementation needs to begin prior to the onset of symptoms. In other words, if you’re worried about a cold this winter, start taking a vitamin C supplement before you start to feel sick. You can also include fresh fruits and vegetables in your diet. Foods such as strawberries, grapefruit, kiwi, oranges, tomatoes, broccoli, and potatoes are all good sources of vitamin C.

The recommended daily allowance (RDA) of vitamin C is 90 milligrams for adult males and 75 milligrams for adult females. As an example, a cup of strawberries contains 85 mg of vitamin C, and about 2.5 cups of fresh fruits or vegetables would give you 150-200 mg in a day — at least twice the recommended amount!

In supplement form, vitamin C is most commonly available by the name L-ascorbic acid. It can be paired with a mineral, such as calcium or sodium, or other compounds such as flavonoids. There isn’t any evidence that one form of supplemental vitamin C is superior to the others, or that natural vitamin C is more easily absorbed than a synthetic version.

The tolerable upper intake level for vitamin C is set at 2000 mg per day for adults (19 years and older). Consuming more than this amount can lead to adverse side effects, such as gastrointestinal issues and diarrhea.1


There are over 50 different enzymes in the body that rely on zinc to catalyze their chemical reactions. Zinc also plays a role in the structure of some proteins. In regards to immunity, evidence shows that zinc reduces the duration of the common cold.

Zinc is essential for immune cell development, and a deficiency affects the immune system’s ability to function properly. Research suggests that a zinc lozenge, consumed within 24 hours of cold symptoms appearing, drastically reduced the duration of symptoms — much like the effects of vitamin C.

Zinc’s RDA for adults is 11 mg per day for adult males and 8 mg per day for adult females. Most individuals probably receive enough from diet alone, as red meat and shellfish — especially oysters — contain high amounts of zinc. Nuts and legumes — including soy beans, pine nuts, cashews, pecans, chickpeas, and brazil nuts — are all rich sources.

It should be noted that supplementing with high levels of zinc for several weeks can hinder the absorption of iron and copper in the body; therefore, it’s best to keep zinc supplementation short-term unless otherwise advised by your healthcare professional.2

Vitamin D

Also known as the “sunshine” vitamin, vitamin D is essential to keep your immune system healthy and protect against respiratory conditions in general. It directly enhances the function of immune cells including T Cells that fight off pathogens, such as viruses. It has also been shown in the research that a deficiency of vitamin D increases risk for upper respiratory tract infections, such as the common cold.

More recently, vitamin D deficiency or insufficiency was linked to an increased risk of contracting the COVID-19 virus. Further studies linked vitamin D deficiency to increased severity of COVID-19 symptoms, which was demonstrated by the patient’s need for hospitalization or intensive care interventions.

The RDA for vitamin D is 600 IU per day, for adults ages 19-70. After age 70, the RDA increases to 800 IU per day. While vitamin D is synthesized from exposure to sunlight (UVB rays, specifically), a number of foods are also high in this particular nutrient. Salmon, mackerel, sardines, and egg yolks are all good sources, and fortified foods such as milk or orange juice can be used as well. Sunlight hasn’t been shown to cause harmful levels of vitamin D in the body.

When supplementing, the tolerable upper intake level is 4000 IU per day for adults (age 19 and older).

The only way to assess individual vitamin D levels is through a blood test. Risk factors for vitamin D deficiency include cloudy, overcast weather and high altitudes and latitudes. These factors affect the amount of UVB rays that reach the ground, and are a common occurrence here in the Pacific Northwest. Because of this, supplementation with vitamin D is highly encouraged for most healthy adults.3

Onion and Garlic

These two plants are part of the Allium family, and both have been shown to have antiviral effects. Onion and garlic contain compounds called flavonoids, which are chemicals (the good kind!) produced by plants. These flavonoids have been shown to inhibit a virus’s ability to replicate itself, reducing the length of the infection.

Quercetin, one of the more prominent flavonoids, is found in onion skin. It prevents a virus from entering or attaching to a cell, and keeps the virus from replicating itself. If you enjoy making your own broth or stock during the winter months, you can throw some onion skin in to reap the benefits of quercetin.

Garlic’s antiviral effects have been studied for decades. In fact, a study published in 1985 found that garlic significantly minimized influenza A and B viruses — the two virus strains that contribute to flu season every year.

Garlic has also been shown to be an effective defense against the common cold, in addition to reducing the chances of catching one. It contains a compound called allicin, which is similar to the quercetin found in onions. Like quercetin, allicin also prevents a virus from replicating itself within a host cell. Additionally, it’s been shown to reduce inflammation in the body by modulating the immune system.4

There aren’t RDAs for quercetin or allicin; however, cooking up a big pot of soup or adding garlic and onion to roasted vegetables are great ways to include these compounds in the diet.


Elderberry has been traditionally used in herbal medicine to treat the influenza virus. Much like garlic and onion, elderberry contains high levels of flavonoids, which have antiviral properties similar to that of allicin and quercetin.

A study from 2004 had flu patients supplement with either elderberry syrup or a placebo syrup. Patients were instructed to take the supplement four times per day, for five days, which began 48 hours from the first sign of flu symptoms. The patients supplementing with elderberry syrup reported a reduction in flu symptoms within 3-4 days, compared to 7-8 days for the placebo group.

It is thought that the flavonoids in elderberry stimulate the immune system by increasing the production of cytokines, which are proteins that tell the immune system what to do. Flavonoids are also anti-inflammatory, so they can be effective in reducing the aches, pains, and fever associated with the flu.5

Elderberry doesn’t have an RDA either, but research has shown that dosages of 1200 mg daily can be used for up to 2 weeks, or 500 mg daily for up to 6 months.6 The supplements are widely available in grocery stores and pharmacies, in syrup, gummy, or capsule form, depending on your preference. For example, if you dislike swallowing capsules, a gummy or syrup may be better for you — but gummies and syrups may have more sugars or sweeteners compared to a capsule. Another consideration is that gummies may also include some of the immunity-boosting ingredients described above, like vitamin C and zinc.

Syrups are usually more concentrated than capsule or gummy versions, and the amount of elderberry in the supplement can be drastically varied; one manufacturer’s capsule may contain 150 mg of elderberry per serving, while a syrup version could have nearly 2000 mg or more.

It should also be noted that elderberry should never be eaten in its raw form. Raw, unripe elderberries can cause nausea, vomiting, and diarrhea; cooking, needed for supplementation, removes these effects.7

Your Personal Immunity Plan

  • Eating a balanced diet high in fruits and vegetables, to ensure optimal intake of vitamin C, vitamin D, and zinc.
  • Get your exercise! It’s one of the best ways to improve your immune system, in addition to nutrition.
  • Getting a blood test from your primary care provider to assess vitamin D status, and supplementing if needed.
  • Frequently cook with onions and garlic.

Having elderberry supplements on hand if you start to feel sick.

  • Prioritizing sleep, rest, and recovery.

Remember, these are all general guidelines and recommendations; as such, check with your healthcare provider before adding any supplements to your routine. They can help you ensure proper dosage and prevent any unwanted side effects.


1. Vitamin C. Linus Pauling Institute. Published 2021. Accessed November 22, 2021.
2. Zinc. Linus Pauling Institute. Published 2021. Accessed November 22, 2021.
3. Vitamin D. Linus Pauling Institute. Published 2021. Accessed November 22, 2021.
4. Sharma N. Efficacy of Garlic and Onion against virus. International Journal of Research in Pharmaceutical Sciences. 2019;10(4):3578-3586. doi:10.26452/ijrps.v10i4.1738
5. Zakay-Rones Z, Thom E, Wollan T, Wadstein J. Randomized Study of the Efficacy and Safety of Oral Elderberry Extract in the Treatment of Influenza A and B Virus Infections. Journal of International Medical Research. April 2004:132-140. doi:10.1177/147323000403200205
6. Elderberry. Natural Medicines.,-herbs-supplements/professional.aspx?productid=434. Published 2021. Accessed November 30, 2021.
7. Elderberrry. National Center for Complementary and Integrative Health. Published 2021. Accessed November 24, 2021.

Hyatt Training Portland personal trainer Maddie HaysAuthor Maddie Hays, MScN, is a certified Personal Trainer at Hyatt Training. She holds a master’s degree in nutrition from the National University of Natural Medicine and is a licensed nutritionist in Washington. As a nutritionist and trainer, Maddie is passionate about the relationship between food and exercise for optimal health. Learn more about Maddie, or get in touch with her by emailing us at

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