For instance, the gut microbiome helps “train” our immune system to distinguish friend from foe to prevent chronic inflammation, Lyman told Business Insider. Chronic inflammation is linked to symptoms including fatigue, pain, and persistent infections, as well as longer-term health concerns such as dementia, depression, and diabetes.

And in the early 2010s, scientists discovered that lymphatic vessels — the veins and arteries of the immune system — exist in the meninges, the protective membranes that surround the brain, proving the immune system and brain are linked, Lyman writes in the book.

“In the last 10 or so years, we’ve discovered new anatomy that links the immune system to the brain and discovered immune cells in the brain that we previously thought weren’t in there,” he told BI.

He hopes this knowledge will provide a new, more holistic framework for the way we look at our health.

“There’s no mental-health condition that isn’t also physical,” Lyman said. “And there’s no physical health condition that doesn’t have a mental aspect to it.”

With this in mind, it may be tempting to reach for products that promise to boost our gut health: The US digestive-health market is forecast to grow over the next five years and make up to $7.2 billion by 2028, Mintel, a market-intelligence firm, found. But most experts, Lyman included, emphasize that there’s no quick fix to health and that eating a healthy diet is the best way to achieve good gut health.

“Instead of thinking about it as trying to look for a silver-bullet pill, the idea of caring for the community within you, realizing that you are a community, and every time you have a meal, even if there are no other humans around you, you are partaking in a communal meal,” Lyman said.

Lyman does three things to cultivate a healthy gut microbiome for his mind and body, which he shared with BI.

Gut gardening

Lyman finds it helpful to think of his gut microbiome as a garden that needs tending to. He calls this “gut gardening” and makes sure he eats enough of the right foods to feed his microbial community.

“So providing them with the right fertilizer, which is the fiber, and also the right seeds, which is the fermented foods,” he said.

Dietary fiber, which comes from plants including fruits and vegetables, legumes, nuts, and seeds, is prebiotic, meaning it feeds the microbes that live in the gut lining. There’s strong evidence that a diverse microbial community is a healthier one, and eating a wide variety of plants helps create this biodiversity, Lyman said.

Meanwhile, fermented foods, such as kimchi, sauerkraut, and kombucha, are probiotics, meaning they contain live bacteria that can help populate the gut microbiome, he said.

Making his own sauerkraut

A 2021 study from Stanford University found that people who ate a diet high in fermented foods for 10 weeks had even more diverse microbiomes than those who ate a high-fiber diet. They also had reduced inflammation biomarkers.

“There was evidence that basically people didn’t have enough bacteria to break down all of the fiber,” Lyman said of the study. The daily recommended intake of fiber is 30 grams in the US, but most Americans eat about 15 grams a day, he said. So while eating more fiber is good, you should introduce it slowly and eat probiotic-rich foods at the same time.

After reading the findings, Lyman started experimenting with fermentation himself, and he eats fermented foods every day, he said. His favorite fermented food is sauerkraut made with cabbage, salt, and thyme.

Making more time for rest

Lyman has also started to be more intentional about what he commits to so he can ensure he has time to rest. “I’ve tried to be more proactive and ruthless in cutting out potential sources of stress, even if it’s at the expense of various ambitions and plans,” he said.

This is because chronic stress can cause chronic inflammation.