High-Fiber Diet

In the chapter “Many High-Fiber Foods” in How Not to Diet, I tell the detective story of the search for the keys that fit into two mysterious locks in the body, vital receptors that are abundant throughout our bodies—in the nerves, the gut, and immune, muscle, and fat cells. Spoiler alert: They were the short-chain fatty acids that gut bacteria produce when we eat fiber, and that are an important means of communication between gut bacteria and the rest of the body.

This could explain why fiber is so anti-inflammatory. For example, how is it possible that a single high-fiber meal can improve lung function in asthmatics within a few hours? Today we know that the good gut bacteria convert the fiber we eat into short-chain fatty acids; these enter the bloodstream and can then dock onto the receptors of inflammatory immune cells in our airways and switch them off.

Does that mean people who eat more fiber have a better immune system? How can we check that? Most people have received an MMR vaccination, which children have routinely gotten against measles, mumps, and rubella since the 1970s. Are there measurably more antibodies against pathogens with certain dietary patterns? Yes, for mumps. All participants had received the same MMR vaccine, but those who ate more fiber had significantly more antibodies against mumps, though not against either of the other pathogens.

To determine cause and effect, researchers gave volunteers an antibiotic cocktail when they received their annual flu shot, wiping out much of their gut flora. In those whose immunity had already been low beforehand, the antibody response was strikingly low. They responded much more weakly to the vaccine. Conversely, the antibody response to the flu shot was supported in people randomized to receive prebiotics (for example, fiber that the good bacteria feed on) or probiotics (the good bacteria themselves). Did that lead to a lower risk of infection?

In people with more fiber-eating bacteria in their gut, the probability of getting viral pneumonia or bronchitis was five times lower. A meta-analysis of randomized controlled trials on cause and effect found that prebiotics can reduce the frequency of respiratory tract infections overall. As for probiotics, children randomized to receive probiotic dairy yogurt, soy yogurt, or supplements had fewer and shorter upper respiratory tract infections, whereas in older adults only the duration of symptoms appears to have been shortened. In view of possible negative effects of probiotics, which I point out in the chapter “Prebiotics and Postbiotics,” I recommend feeding the good bacteria we already have instead, by eating foods that are naturally rich in fiber.

Do Vegetarians Have Better Killer Cells?

As we age, our natural killer cells tend to lose some of their capacity to proliferate and their punch. What can we do to maintain their functionality? I’ve discussed a few plants that presumably help, but what does a plant-based diet do? The natural killer cells of vegetarians were compared with those of nonvegetarians to find out which could destroy more leukemia cells, and the vegetarians’ killer cells won. They killed the cancer twice as efficiently. On average, each natural killer cell from a vegetarian’s blood killed two cancer cells, whereas the nonvegetarians’ cells took out one. The researchers concluded that “the lower cancer risk of vegetarians is partly related to the fact that they appear to have a better natural defense system.”

A more effective immune system doesn’t just protect against cancer by directly targeting tumors. Sometimes infections cause cancer, too. Take human papillomavirus (HPV), for example. Cervical cancer is now considered a sexually transmitted disease. Originally, this was suspected based on the different cancer rates among “nuns versus prostitutes,” but now we have proof through DNA fingerprinting that virtually all cervical cancer cases are caused by HPV, a sexually transmitted virus that also infects the penis, the vagina, the vulva, and the throat.

HPV is considered a necessary but not sufficient cause of cancer. It is so widespread that most young women become infected with it, but very few develop cervical cancer because their immune systems are able to eliminate the virus. 70 percent of all women overcome the infection within a year and more than 90 percent within two years—before the virus can cause cancer.

Could it be that people with a particularly strong immune system get rid of the virus even faster? That would explain the finding that HPV infection rates were significantly lower among vegetarian women. This is supported by one of many studies that observed a lower risk of HPV infections in people who eat primarily plant-based. It may even help simply to eat more vegetables.

Researchers tested women whose cervixes were infected with carcinogenic HPV strains, then repeated the analysis after three and after nine months and also assessed their diets. Higher vegetable consumption appeared to halve the risk of the HPV infection becoming chronic, thereby doubling the likelihood of overcoming this potentially carcinogenic infection. This could explain why vegan women are significantly less likely to develop cancers that affect only women, including cervical cancer. However, a comparison of natural killer cell activity in vegans and nonvegetarians was not able to reproduce the results of the previous study, so other anticancer factors or non-immunological mechanisms could be responsible for the lower cancer rates.

Plant-Based Protection from Pandemics

The Covid-19 pandemic offered a good opportunity to find out whether healthier eating can ward off infections. Details at see.nf/plantdemic, but in short, Harvard researchers collected data from nearly 600 000 participants and found that those who ate the most healthy plant foods and the least meat, eggs, dairy, and junk food not only had a significantly lower risk of a severe course of Covid-19, but also a significantly lower risk of getting infected at all—even after accounting for comorbidities and other lifestyle risk factors unrelated to diet, such as exercise, smoking, and socioeconomic status.

Dietary Supplements

Are there dietary supplements that can protect against infections?

Ginseng

Ginseng is remarkable because of its Latin name Panax, from panacea, “cure-all.” It can extend the lives of fruit flies and roundworms, but not those of mice. The two main types are American ginseng (Panax quinquefolius) and Asian ginseng (Panax ginseng), which can be further subdivided depending on processing method. White ginseng is the root of Asian ginseng that has simply been washed and dried. Red ginseng is the same root, but additionally treated with steam before drying. Various ginseng preparations have been shown to increase the population of B and T cells and the activity of natural killer cells, but how do they affect infections?