Feeding Immunity

It’s winter, at least in the northern hemisphere. That means cold and flu season is in full swing, and COVID-19 is still making a stand. Your immune system is your major line of defense, protecting you against pathogens novel and familiar.

Stunningly complex, the immune system has many moving parts, with many types of cells, communicating molecules, and fast-mobilizing players to protect you (and me) against a huge array of threatening agents. The immune system is typically seen as having two basic parts. An “innate,” or “natural,” compartment of nonspecific warriors swings into action to tackle all kinds of invaders by engulfing them and by setting up hostile conditions, such as inflammation in tissue under siege, and other general measures.

The so called “acquired,” or “adaptive” compartment of the immune system consists of special ops forces, various types of cells that target specific types of invaders, once they’ve been informed by frontline cells on the character of the interlopers. There are the B cells, which produce bespoke antibodies to neutralize the pathogen. And there are T cells that engage in mano a mano combat with specific antigens on invading cells.

All the immune cells in the body originate in the bone marrow. Once they encounter a specific pathogen, they develop a memory for it, enabling the body to swiftly recognize and mobilize against it, should that same bug reappear in the future. One of the first tasks of the immune system is to recognize microbes and identify whether they are harmful or not.

Maintaining a robust—but not hyperactive—immune response depends on many factors, some of which you have no control over: your genetic inheritance, your stage of life, even the time of day. But you have quite a say in some other factors—your stress level, physical fitness, and body fat, to name a few. Perhaps the fastest-acting factor to which you contribute is your nutritional status. What you choose to eat influences the function of the immune system—just as it does every other system of the body.

Nutrition is involved in immunity at many levels. Among the many ways diet influences immune response:

• Supplying fuel for the system to function.
• Furnishing building blocks for RNA, DNA, and such proteins as antibodies, cytokines, and cell receptors. Providing substrates for various other immune-active compounds.
• Regulating immune cell metabolism.
• Supplying nutrients—such as zinc and vitamin D—that have specific antibacterial or antiviral functions.
• Protecting against the oxidative and inflammatory stress of infection-fighting.

Several micronutrients have a particularly important role in immunity—vitamin D, vitamin C, zinc, and selenium are among them. Deficiencies of the nutrients are known to impair immunity.

Best known for its role in bone health, vitamin D plays a still-unfolding role in cell physiology and immune operation, most notably in the form known as D3. It stimulates production of substances that are part of the frontline innate immune response against bacteria and viruses and may keep pathogens from gaining a foothold.

Many studies have shown that vitamin D deficiency is associated with an increased risk of COVID-19 and other respiratory infections. Some small-scale experimental trials demonstrate that correcting D deficiency reduces the risk of hospitalization, ICU admission, and death from COVID-19 infection. While the vitamin bolsters innate immunity, it also keeps the inflammatory response under control, preventing the so-called “cytokine storm” linked to organ damage and death.

Vitamin D deficiency and its sub-acute version, vitamin D insufficiency, are widespread, especially in winter, when exposure to D-inducing sunlight diminishes just as the flu virus makes its seasonal rounds. In fact, some theorize that it’s the decrease in vitamin D levels that is the seasonal stimulus for the flu virus to spread.

It’s still largely an open question whether supplemental amounts of the vitamin—or how much or when— can protect against influenza infection.

Second only to iron as the most abundant trace mineral in the human body, zinc is essential for many enzymes in the always-on chemical factory known as the human body. It also plays a role in the creation of DNA, and it supports the activity of many types of cells in both the innate and adaptive immune response, particularly natural killer cells (innate) and antibodies (adaptive). It also has specific antiviral actions, including inhibiting the replication of coronaviruses. And while it’s doing all this, it’s preventing body cells from free-radical damage.

Evidence indicates that zinc will not protect against getting an upper respiratory infection. But zinc supplementation has been shown to shorten its duration.

Selenium is another trace mineral necessary for the immune response. Like zinc, it supports natural killer-cell activity of innate immunity and bolsters that quintessential adaptive immune function, antibody production. It’s also a major antioxidant, protecting cells against oxidative stress while marshaling defensive forces. Necessary as selenium is as an antioxidant and immune protectant, it’s needed only in small amounts, best obtained via diet— Brazil nuts, shellfish, and organ meats.

Not to forget Vitamin C. It plays a role in every aspect of immunity, from bolstering the barrier function of skin to rustling up antibodies. It’s also a powerful antioxidant that is quickly depleted during infections, when the body is under siege and rapidly producing infection-fighting cells.

Vitamin C deficiency is rare in the developed world, but there’s evidence that current recommendations for intake of the vitamin are too low. Further, vitamin C has a profound effect on the health of the gut microbiome, particularly in abetting microbial diversity—and a diverse microbiome is a key regulator of the immune system.

Even in healthy adults, vitamin C supplements significantly shift the balance of bacteria in the gut in healthy ways. No wonder many studies show that vitamin C supplementation reduces the severity and duration of colds.

Immune Players

• Cytokines. As intercellular messengers, some are key in stimulating inflammation, some in tamping it; they signal the brain to initiate sickness behavior, such as increased sleep.
• Mast cells. In mucous mem- branes, they kick off the immune response when activated by antigens, releasing many cytokines.
• Macrophages. Innate agents circulating in blood, they surround and consume bacterial invaders.
• B cells. Warriors of adaptive immunity, they produce antibodies specific to the invading bug, neutralize it, and mark it for destruction.
• T cells. Also adaptive agents, they directly kill and remove pathogens from the body and also trigger B cells.
• Natural killer cells. Part of the innate response, they destroy invading viruses and tumor cells without any prior priming or exposure.