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Old Tue, Apr-02-24, 00:22
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Default Is a healthy gut all in the mind?

Is a healthy gut all in the mind?

Our brains are in constant conversation with microbes in the intestines — and it has huge implications for our health, Dr Monty Lyman tells Anna Maxted

Until recently the prevailing scientific belief was that “the immune system stopped at the neck”. Put like that — by Dr Monty Lyman in his third book, The Immune Mind: The New Science of Health — it sounds ridiculous. But yes, he confirms, speaking on Zoom from his home in Oxford, “it’s been assumed for almost the whole history of modern western medicine that the brain and the immune system are completely separate”.

No immunology textbook mentioned the brain, and there wasn’t a squeak about the immune system in any psychiatry tome, says Lyman, 31, a medical doctor, researcher and an academic clinical fellow at the University of Oxford. The brain dealt with big threats (lions), the immune system with microscopic ones (viruses), and each stayed out of the other’s business.

Although many wondered along the way, like the American physician in 1885 who found that a “perfect counterfeit” rose in his clinic brought on his patient’s pollen allergy, or any one of us whose acne, migraines or digestive issues flare in times of stress. “There was a general feeling that the mind does affect the body, and that your general health affects your mind,” Lyman says, but it was unproven.

Yet over the past few years scientists have discovered new anatomy that shows the brain and immune system are intimately linked. “We now know that microglia are immune cells in the brain,” he says, and that our brains are surrounded by immune cells “that constantly taste the brain’s sewage” — the cerebrospinal fluid. “They monitor what’s going on.”

What this means, he says excitedly — he has the affable, unassuming air of a young, benign Hugh Grant — is that they can be viewed as part of the same system. “The brain and the immune system can both speak each other’s languages. And it’s an absolutely massive game-changer.” Lyman calls this the “defence system” — and the microbiome is its third component.

It’s crucial to our understanding of medicine, he says. “There’s no mental health condition that isn’t also physical, and there’s no physical health condition that isn’t also mental. That’s hugely important, whether you’re recovering from depression or a broken leg.”

Damage to one part of our defence system affects the others. One way we’ve all experienced our immune system affecting our brain is by “sickness behaviour”. Whether we have a cold or Covid, the immune system “recruits” the brain to aid its defence by making us feel tired, withdrawn, lethargic and pain-sensitive.

“Most of the symptoms that we get when we’re infected with a virus or a bacteria are mental, as well as the pain relating to whatever tissue is inflamed,” he says. “The amygdala — the brain’s defensive alarm system — can also be activated by immune molecules.” This not only makes us feel bad, it causes us to move more slowly and repels us from others — protecting the herd. Although if we insist on struggling into work, it alerts them. “We are exquisitely attuned to detecting illness in other people and being repelled from them as well,” Lyman says. “There’s evidence we can easily tell apart people who are sick just by their movements.” (The “sick sense”, naturally.)

As he says, it’s a chatty two-way street: our mind can rouse our immune system into action. Say you meet a lion on the street corner — “your immune system is activated as well as your nervous system when you go into fight or flight”. In such a situation antibacterial immune cells are assembled “from their barracks in the bone marrow”, in preparation for a potential bite.

Most of our modern stresses have a similar effect — “the antiviral immunity goes down”, Lyman explains. (Our ancient biology assumes that if we’re in flight or fight chances are a lion bite is imminent, so it assembles our antibacterial troops, not our antiviral ones.) This is why you do always catch a cold when work is hellish.

Alas, unlike the lion scenario — short, as you’ll be eaten, or escape — psychological stress often drags on. Bullying, social isolation or financial strain don’t just hurt our mental health, they harm us physically. “Psychological stress causes inflammation,” Lyman says, “and that’s an activation of the defence system.” He adds, “I’m having a bit of a flare-up of eczema because I’m going through quite a stressful period at work.”

Aware this is an anecdote, he quickly offers some scientific evidence — research in which mice were fed poisoned water that inflamed their colons. After complete recovery, activating the relevant neurons to stimulate this distressing memory caused their colons to reinflame, no poison water necessary.

In acute bursts stress and inflammation are beneficial. “The trouble is when stress becomes chronic,” Lyman says. It can cause chronic inflammation, which can lead to mental health conditions but is also a key driver of conditions like heart disease, diabetes and dementia, he adds. “Psychology is biology.”

Depression is one mental health condition that big studies suggest may in a quarter of cases — that’s 70 million people worldwide at any one time — be linked with inflammation. This subset with “inflamed depression” usually have raised inflammatory markers in the blood and don’t respond well to antidepressants.

“These patients tend to have greater fatigue, an increased need for sleep and increased appetite compared to others with depression — along with depression’s core symptoms of persistent low mood and a reduced capacity for pleasure,” Lyman says. Interestingly, a high number have insulin resistance — caused and driven by chronic inflammation — and some scientists prefer the term “immunometabolic depression” as “these people tend to have worsened glucose and cholesterol markers as well”.

Studies suggest that anti-inflammatory medications could help, he says. “But also it links to the fact that generally lifestyle choices that are good for our microbiome, positive for our immune system, will be useful for our mental health.”

Ah yes, our microbiome — the third defensive component. “All the evidence suggests that a diverse gut microbiome is strongly linked to reduced inflammation, so a more balanced immune system,” Lyman says, “and that reduction in inflammation also helps the brain.”

Our understanding that a healthy gut microbiome plays an essential role in training our nascent immune system (and that the state of our mother’s microbiome wields huge influence) is relatively new. Lyman says: “The immune system needs this constant training by good microbes to understand the difference between positive and negative microorganisms in the gut.”

Our gut microbes also talk directly with the brain. “Microbes can communicate with the brain via the vagus nerve, which snakes down from the brain and touches most of the organs in the body including the gut,” he says. This prompts a fabulous quote in The Immune Mind from John Cryan, professor and chairman at the department of anatomy and neuroscience at University College Cork: “What happens in vagus doesn’t stay in vagus.”

If we treat them right, our good gut bugs also produce beneficial molecules such as short-chain fatty acids (SCFAs), which reach the brain via our bloodstream and “play lots of important roles in terms of physical and mental health”.

In studies, mice bred to have no microbiomes were easily stressed: “They’re less sociable and more anxious, basically.” Lyman cites 2021 research by Cryan in which transplanting the microbiomes of young mice into elderly ones reversed age-related immune changes in their brains and improved cognition and age-related anxiety. If human studies provided results as conclusive, might Gen Z students sell their faecal matter to boomers to pay their way through college, Lyman muses.

In the meantime there are more salubrious ways to tend to our microbiome, immune system and mental health. “I talk about ‘gut gardening’, which is treating your gut microbiome as a medicinal garden,” Lyman says. “Eating fibre is the fertiliser for this medicinal garden, and fermented foods are sowing the seeds, introducing new microbes.” He makes his own sauerkraut.

He notes that we eat only 15g fibre daily, half the recommended amount, and that the standard western diet, high in refined grains, processed meat, sugary drinks and deep-fried food, is pro-inflammatory and “antibiotic” — ie bad for our gut bugs. Are you resistant to consuming 30g fibre daily or eating 30 different plants a week? As well as being in fruit and vegetables, fibre is also found in nuts, seeds, herbs, spices, grains and legumes. Lyman cites yet more of Cryan’s research, on middle-aged mice with some signs of inflammation in their brains. He slowed it by giving them inulin, a prebiotic found in vegetables like leeks.

But build your fibre intake slowly, gently. “Most western guts, we don’t actually have the microbial capability of breaking down the amount of fibre we should be having,” Lyman says. And do include kefir, kimchi, sourdough and kombucha. In one US study, eating more plant fibre led to gut bugs making more SCFA but not more types of bacteria. “It was fermented foods that increased microbial diversity and also reduced inflammatory markers — an enmeshment between the microbiome and the immune system.”

Looking after your defence system by living what Lyman calls an “anti-inflammatory life” isn’t just about food of course. It’s also about being active, sleeping well, nurturing your relationships. Both your brain and immune system thrive on connection and getting out and about. “New studies show there are cells in our bones that are activated by movement, that release healthy immune cells,” he says.

“Being sedentary is pro-inflammatory. We’re meant to be moving, gathering, exploring,” he adds.

Good habits can create a virtuous circle. Lyman cites a study showing that certain microbes can increase our willingness to exercise. “They activate receptors in the gut that increase dopamine related to exercising. But exercise in itself is anti-inflammatory and is associated with healthy microbiomes. It’s a loop.” He smiles. “We’re loops of mind and body.”

The Immune Mind: The New Science of Health by Monty Lyman
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