Hidden deep within all of us are vast coils called guts — intestines. The fitness magazine Tarzan this month invites us to contemplate them.

They’re in bad shape and getting worse, it seems. Even at best, the subject is an unpleasant one. Eating is fun, dining is culture, but digestion is something we’d rather not talk about, unless we have to — which is to say, when it betrays us.

“Things we’d rather not talk about” is a theme in itself. The women’s weekly Josei Jishin this month steps boldly into it, instancing the aches, pains and itches in unmentionable places, to say nothing of body odor, hair loss and the emotional stress they impose as we and the population at large age.

Unmentionable or not, the analysis of fecal matter, Tarzan observes, is key to the training of athletes at the highest level. Trainers rely on it in their quest for the perfect athletic diet. Bio-anthropologists also probe its secrets, and when American and Mexican scientists recently dug up buried mounds of human waste dating back 2,000 years, they saw in it fresh means of confirming a growing suspicion — to wit, that the bacteriological diversity in the human intestine is in sad decline.

It’s a whole new world in there, teeming with life, inhabited by 100 trillion-odd bacteria of roughly 1,000 types, some good, some not — but diversity, says Tarzan, is the main thing. Its loss puts the digestive process at risk, with collateral damage rippling through our anatomy and psychology, digestion being central to just about everything we do, consciously or unconsciously.

We moderns do everything wrong, Tarzan seems to feel. It’s interesting how far astray we’ve gone, given the means at hand — knowledge, productivity, technology — for perfection. Eating right, exercising adequately and resting appropriately are not difficult — or wouldn’t be if society as a whole didn’t militate against them, keeping us busy, hurried and harassed beyond the body’s natural capacity for stress. Japan’s rejection of its indigenous health foods — fish, soybeans, seaweed — in favor of fast food and deep-fried meat generates frequent warnings seldom heeded. The prime example of technology abuse is the ubiquitous smartphone, whose tendency to absorb us all day and into the night plays hell with the nervous system, with dreadful implications, Tarzan says, for the digestive system.

Pre-industrial humankind had a bacteriological diversity we today can only envy. That raises a question: Why enviable, since we’re so much healthier and longer-lived today? True, but progress is not irreversible, and we are in danger of reversing it. The gradual change down the centuries — rapid in Japan’s case — from plant-based to meat-based diets set us, biologically speaking, on a slow downward slope that steepened around 1995.

What happened then? Not one specific thing but a rising crescendo of small things whose impact showed in researchers’ surveys of wide swaths of population — one showing, for instance, a decline in children’s daily fiber intake from an average 20 grams in 1995 to 7 grams in 2020.

Broadly speaking, says Tarzan, the following factors favor bacteriological diversity in children: large families, pets, vegetables, mountains and rivers within view if not reach, and sleep rather than antibiotics as the default remedy for minor illness.

Urbanization, nuclear one-child families, fast food, a preference for antiseptic over soap and water — in short, hyper-cleanliness in an increasingly denatured environment — are almost certainly with us to stay, short-term, and it’s the rare individual who makes lifestyle decisions with intestinal bacteria in mind.

Still, two related facts of life may merit consideration. First, the intestinal environment is pretty well fixed during the first three years of life. Children deprived of the best then can’t, growing into awareness, backtrack. Too late.

Second, what we do to ourselves, bacteriologically and intestinally speaking — not to mention what COVID-19 is doing to us; its impact is still being assessed but is bound to be considerable — marks not only ourselves and our children but theirs, no one knows how far into the future.

Well, enough about the gut. The body imposes on us in so many ways; it makes one sick to think about it. At its best it’s a friend and ally, and Tarzan is right to urge us to stay in its good graces, but its favor is easily lost, and there are times when it seems bent on making life impossible. Like an obnoxious child it will do anything for attention. It aches, smells, itches — more and more as we age, Josei Jishin notes, and Japan is aging fast.

We all have bodies, we all suffer the body’s impertinence, and yet the symptoms in question, though shared, are hard to talk about, and therefore isolating. Whether social taboo causes personal embarrassment, or vice versa, is a chicken-and-egg conundrum, probably insoluble. It matters little. The consequences matter a great deal, though. From such trivia as mouth odor, athlete’s foot and the overpowering need to urinate when you’re not home alone — three among numerous issues Josei Jishin raises — come, at worst, depression, alcoholism and heightened irritability that strains social and family life to the breaking point, sometimes beyond. None of this is peculiar to our own time; it probably goes all the way back to the beginning of things; one wonders idly whether the Jomon folk of prehistoric Japan found ways to cope with it. They, of course, were less sensitive than we are.

Big in Japan is a weekly column that focuses on issues being discussed by domestic media organizations. Michael Hoffman’s latest book is “Cipangu, Golden Cipangu: Essays in Japanese History.”

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