Immune function has been top of mind for many people during the pandemic, as a strong immune system can more readily fight pathogens such as the coronavirus. And while everyone knows that what we eat can affect our weight and energy level throughout the day, not everyone realizes the extent to which diet affects the immune system. “Seventy percent of the immune system is located in the gut,” says David Heber, MD, PhD, professor emeritus of medicine at UCLA Health. “Nutrition is a key modulator of immune function.”
Immune cells in the gut interact with the microbiome, the diverse array of bacteria and fungi that live in the gastrointestinal tract and are directly influenced by an individual’s diet and lifestyle. The foods we eat affect the diversity and composition of the gut microbiome, which in turn affects immune cells. Those gut bugs are healthiest and support strong immunity when we consume plant foods that are high in fiber.
“The microbiome and the immune system are critically intertwined,” says Jonathan Jacobs, MD, PhD, assistant professor in the Vatche and Tamar Manoukian Division of Digestive Diseases and co-founder of the UCLA Microbiome Center. “What’s present in the gut determines what education immune cells get.”
Dietary diversity and microbial diversity go together, Dr. Jacobs says. The typical Western diet, which is high in animal proteins, sugar, processed foods and saturated fat, results in lessdiverse gut bacteria and promotes inflammation and chronic disorders, he adds. A fiber-rich diet, on the other hand, supports the microbiome and reduces inflammatory response.
“Gutbacteria subsist on complex carbohydrates and fiber that our own cells are unable to digest,” Dr. Jacobs says. Those fibrous sources are plant foods, from apples and broccoli to yams and zucchini.
Carrying extra weight also affects immune function, says Dr. Heber, founding director of the UCLA Center for Human Nutrition. Fat stores, once thought to be inert tissue, actually secrete hormones and chemicals that stimulate inf lammation. Medically known as adipose tissue, fat is now understood to be a “metabolically active endocrine organ,” says Vijaya Surampudi, MD. “Obesity affects the immune system directly,” she says. The low-grade inflammation obesity stimulates is an immune-system response. Maintaining a healthy weight through a plant-based diet boosts the microbiome and the immune system.
Nutrition experts point out that it also is essential to consume sufficient protein to support our muscles. “We are doing our bodies a disservice when we do not have enough protein with each meal,” says nutrition specialist Michael C. Garcia, MD. Like fat, muscle is an endocrine organ that directly affects the immune system, he says, and muscle is made from protein. Dietary protein can come from animal products - wild-caught fish is an excellent source and is healthier than farmed varieties - but plant sources are better overall for the microbiome.
Other recommendations include eating seven servings a day of colorful fruits and vegetables; having protein at every meal; using healthy fats such as olive, avocado or canola oil and including such things as avocado slices on salads in place of dressing; and natural spices and herbs, which support gut-bug diversity, in food preparation.