Life Extension Magazine®

Woman blowing nose with age-related declined immune response

Refuel Your Immune System

Supporting a healthy immune response requires fundamental nutrition.

Scientifically reviewed by: Dr Gary Gonzalez, MD, in August 2023. Written by: Michael Downey.

With age, immune function markedly declines along with the body’s natural defenses.1

Until a method to fully reverse this degenerative process is validated, the following nutrients can help support a healthy immune response:

  • Vitamin C
  • Quercetin
  • Vitamin D
  • Zinc
  • Probiotic Lactobacillus rhamnosus CRL1505
  • S. cerevisiae fermentate

Taking these daily may help defend against infectious agents.

Vitamin C

Virus in lung leading to respiratory tract infection

The activity of many immune cells is closely related to their vitamin C content.

This is especially true for phagocytes, the cells that engulf and destroy bacteria and other infecting organisms, and T-cells, which regulate and direct other immune cells.2

Studies show that some immune functions can be improved by taking vitamin C.3,4

For example, vitamin C deficiency is associated in some studies with increased frequency and duration of colds.4

In a human clinical study, oral intake of vitamin C has been shown to reduce the duration of colds by an average of 9.4%. It may also help prevent viral respiratory tract infections and reduce their severity.5

Evidence from basic research shows that vitamin C promotes a healthy immune system by:

  • Enhancing the function and promoting the growth, maturation, and survival of immune cells that fight infection,6,7
  • Increasing levels of interferons, the “warning signals” produced by the body that trigger protective immune mechanisms,8
  • Neutralizing excess free radicals caused by infections, limiting oxidative damage and reducing severity of illness,9
  • Aiding in the production of the structural protein collagen, which allows our skin and the linings of our respiratory and digestive tracts to maintain a protective barrier against infection,10
  • Lowering levels of histamine, a pro-inflammatory compound11 that plays a role in infections,12 and causes symptoms of allergy,13,14 and
  • As indicated by preclinical studies, vitamin C plays a role in reducing excess levels of other pro-inflammatory compounds, countering inflammation caused by infection and injury, and promoting tissue healing.

The human body cannot produce or effectively store vitamin C. What that means is, in order to maintain optimal immunity, it’s a great idea to replenish your supply through daily supplementation.


Research has shown that the plant flavonoid quercetin can support a prompt immune response to common colds and other upper respiratory tract infections. These studies found that adults taking quercetin were less likely to develop these illnesses.

One study found that only 5% of people taking quercetin got sick during a two-week period (after three days of intense workouts), while 45% of those taking a placebo developed colds.19

In another study of physically fit middle-aged and older adults, daily quercetin intake reduced the number of sick days taken for colds by 31% and reduced the severity of symptoms by 36%.20

Quercetin may also be effective against bacterial infections.

In animal studies, it’s been shown to decrease the infection rate and inflammatory response to Helicobacter pylori, the cause of many ulcers and some potential cancers of the stomach.21,22

Quercetin also reduced inflammatory responses and strengthened host defenses against Salmonella bacteria in a cell-based model of infection.23

Salmonella bacteria cause roughly 26,500 hospitalizations in the U.S. every year and are especially dangerous in older adults.24

Evidence from epidemiological studies shows that people with the highest quercetin intake have reduced risk for many different deadly cancers, including lung, colon, and gastric cancers.25-27

It can be difficult for the body to absorb quercetin.28

Researchers solved this problem by integrating it into a phytosome, a type of fatty substance that serves as a carrier. This makes it up to 50 times more bioavailable (absorbable) than standard quercetin.29

Vitamin D

Vitamin D fortifies the immune system, helping to protect the body from infections, and lessening their severity. It may do this by:30-34

  • Interfering with the ability of viruses to replicate,
  • Supporting and helping to repair cellular linings in the body, including lung airways,
  • Boosting production of proteins that are protective against infection, and
  • Helping to prevent the immune system from producing excess pro-inflammatory compounds in the lungs.

Meta-analyses of clinical trials have shown that vitamin D protects against respiratory tract infections.35,36

In addition, low vitamin D levels are associated with higher rates of many chronic diseases, including cardiovascular disease, cognitive decline, and cancer.37

Annual blood tests can let people know whether they are taking the correct dosage to ensure optimal blood levels of vitamin D.

If you do not already maintain an optimal blood level of 25-hydroxyvitamin D of 50 to 80ng/mL, then take between 5,000 to 8,000 IU of vitamin D daily, with meals.

What you need to know

Zinc supplement and apple that can boost the immune system

Support a Healthy Immune System

  • In order to live long, healthy lives, we need our immune system to function at peak form on a daily basis. Several nutrients can help do that.
  • Vitamin C helps increase levels of antibody-producing cells (lymphocytes), boosts function of infection-engulfing neutrophils, and helps NK (natural killer) cell activity.56,57
  • Quercetin reduces inflammatory immune cells, cuts histamine levels, relaxes airway smooth muscle, inhibits replication and infectivity of cold-causing viruses, and reduces senescent cells and their pro-inflammatory signaling.58-60
  • Vitamin D interferes with virus replication and modulates the immune response via receptors on various immune cell types, thus supporting antimicrobial defenses while limiting inflammatory signaling.61
  • Zinc is key to maintaining the integrity of the immune system. It helps with the normal development and function of natural killer cells, lymphocytes, neutrophils, and macrophages.62
  • A probiotic strain, Lactobacillus rhamnosus CRL1505, significantly boosts levels of secretory IgA—critical antibodies that target both viral and bacterial invaders in the upper respiratory tract—thus providing a security system against cold and flu viruses within mucosal membranes.44,45,47,48
  • S. cerevisiae fermentate supports NK (natural killer) cell activity, production of secretory IgA, a balanced response to environmental allergens, and defense against colds.50-55


Shield with zinc periodic symbol boosting immune system

Zinc deficiency is quite common in the elderly.38 It is thought to result from reduced zinc consumption and absorption in older individuals.39

This may compromise the function of the immune system and contribute to atherosclerosis, cancer, neurological disorders, autoimmune diseases, and other age-related conditions.40,41

The decline in immune function that happens with aging has been associated with both disease and death.42

By restoring zinc levels, aging adults may be able to partially slow immune function decline and protect against chronic inflammation.

Oral intake of zinc in the elderly has been shown to boost the stress response of white blood cells, providing an immune system anti-aging mechanism.43

Probiotic L. rhamnosus CRL1505

The immune system makes proteins called antibodies that fight bacteria, viruses, and toxins.

One of the most common antibodies, called secretory IgA (immunoglobulin A), is found in mucosal membranes.

IgA acts as the body’s built-in security system within mucosal membranes that line the nose and upper respiratory tract.44

Having adequate IgA levels is critical because these antibodies target both viral and bacterial invaders in the upper respiratory tract. This IgA activity can prevent cold and flu viruses from gaining a foothold and wreaking havoc on the respiratory tract.44

Scientists studying the beneficial live micro-organisms known as probiotics identified a specific bacterial strain that, in a preclinical model, significantly increased levels of secretory IgA.45

Originally isolated from goat’s milk in northwestern Argentina,46 the bacterium Lactobacillus rhamnosus CRL1505 has been shown in preclinical studies to help inhibit viruses and bacteria that can cause:45,47

In a clinical trial, one group of healthy male and female children consumed a yogurt drink five days a week that contained 100 million CFU (colony-forming units) of L. rhamnosus CRL1505. A second group consumed a drink that did not contain the probiotic.

The children ranged in age from two to five years, a population that is particularly susceptible to respiratory infections.48

Over six months, compared to the placebo group, the children in the probiotic group had:48

  • 61% fewer cases of tonsillitis and pharyngitis (a throat infection),
  • 55% fewer cases of cold or flu,
  • 49% fewer infections,
  • 46% fewer cases of fever, and
  • 33% less need for antibiotic use.

Daily intake of L. rhamnosus CRL1505 can provide protection against viral and bacterial infections.

S. cerevisiae fermentate Fights Allergies and Provides Immune Benefits

Smiling woman supplementing with S. cervisiae fermentate

The immune effects of S. cerevisiae fermentate were discovered by accident.

A company in Cedar Rapids, Iowa, had been producing a specialized yeast culture when it became apparent that its factory workers—who were exposed to the yeast daily, through inhalation—were taking far fewer sick days than its office workers.49

S. cerevisiae fermentate helps promote the body’s immune response when it encounters environmental allergens, like pollen.

At least six placebo-controlled clinical trials have validated its ability to protect against allergies and colds.50-55

In one study, subjects took either 500 mg of S. cerevisiae fermentate daily or a placebo for five weeks during the beginning of allergy season.50

Subjects in the placebo group did not see a change in their seasonal allergies.

The group supplementing with the S. cerevisiae fermentate saw improvements. Half of the treated male volunteers reported a complete absence of allergy symptoms, which returned within two weeks once they stopped taking the yeast fermentate.

In two other studies, subjects receiving S. cerevisiae fermentate reported shorter duration of symptoms and better response to immune challenge, compared to those taking a placebo.53,54

S. cerevisiae fermentate appears to work by promoting natural killer cell activity as well as the production of secretory IgA—two key players in our body’s immune defenses.50,51

In one study, subjects taking 500 mg of S. cerevisiae fermentate daily for eight weeks experienced an increase in secretory IgA.50

A placebo-controlled, double-blind study found that yeast fermentate increased markers of natural killer cell activity—after just a single 500 mg dose.51


Aging weakens the immune system, leaving us vulnerable to viral and bacterial infections, including colds, flu, and upper respiratory infections—and increases the risk of cancer.

Select nutrients help the immune system function optimally to kill pathogens, and can help prevent the chronic, low-level inflammation that is associated with numerous degenerative diseases linked to aging.

These critical nutrients include vitamin C, quercetin, vitamin D, zinc, the probiotic strain L. rhamnosus CRL1505 and S. cerevisiae fermentate.

They can help support a healthy immune system and may offer protection against viral and bacterial infection, cancers, and other illnesses.

If you have any questions on the scientific content of this article, please call a Life Extension® Wellness Specialist at 1-866-864-3027.


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