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Do bacteria make the man (or woman or child)?

'You are what you eat,' as the saying goes; but perhaps also where you eat


What happens when Japanese people start eating a Western diet? Could it mean that their famed long life span starts to decline?

It certainly could. It could even mean that the people of these islands become less Japanese.

I don’t mean to say that in a nationalistic sense — in the way that oddities such as Tokyo Gov. Shintaro Ishihara might argue there is something intrinsically patriotic about consuming miso soup and natto (fermented beans). Of course, food is closely linked with national culture, and in Japan it may be even more closely linked than in most countries.

No, I mean it in a more literal, biological sense.

After all, you are what you eat — quite literally.

The bacteria in your digestive system outnumber the human cells in your body by 10 to one. You are more microbial than you are human, in a way. There are about 1,000 times the microbial genes in your body than there are human genes. It’s hard to say where being human ends and being bacterial starts.

And many of those bacteria come from the food we eat. When we change the way we eat, we change our bacterial tenants.

When I am in Japan, eating Japanese food, I literally become more Japanese. I certainly become more like Japanese people. That’s because the huge bacterial component of our bodies — each of us carries about 1.5 kg of microbes in our gut — varies very closely according to country, and even region. Give a microbial scientist a bit of the contents of someone’s gut, and they will be able to tell you where that person comes from, and much more besides.

Actually, they don’t even need that — they only need some urine. Urine contains an array of different metabolites — the compounds left over after food is digested and absorbed — and it turns out that the kind of metabolites in urine can identify your ethnic origin, your gender, certain diseases you might be at risk of, and even your nationality.

A study published last week compared the “metabolic fingerprints” — made by analyzing urine — of 4,630 adults from Japan, China, Britain and the United States. Perhaps not surprisingly, the study found that people from Britain and the U.S. had similar metabolic fingerprints, or “metabolomes,” reflecting similar lifestyles, eating habits and incidences of high blood pressure and cardiovascular problems. They were found to be similar despite the geographical distance between the two countries.

In contrast, although adults in Japan and China have similar genetic profiles, and are geographically closer together than Britain and the U.S., they have very different metabolomes from one another and also from adults in the U.K. and the U.S. There are also major differences in the incidence of many diseases between Japan and China.

Japanese people living in the U.S. have metabolomes that resemble other people in the U.S., and which are different from their counterparts living in Japan. This shows that lifestyle is a dominant feature in determining metabolism.

“What our study really shows is how incredibly metabolically diverse people are around the world. British and American [metabolomes] are nearly identical. Japanese and Chinese people are totally different metabolically even though they are nearly identical genetically,” said the study leader, Jeremy Nicholson, of Imperial College London.

Nicholson’s team, which included researchers at Shiga University of Medical Science in Japan, found that the metabolomes of people who lived in Hawaii were equally similar to those among people on the U.S. mainland and also in Japan. They published their findings in the journal Nature.

And whom were the people most different among the 17 different groups from all over the world?

Sorry Ishihara, it’s not the Japanese.

A group of people from a region of south China were the most different, metabolically speaking, from anyone else in the study.

“They have a very different and much broader range of diet,” Nicholson said. “Very broadly speaking, the southern Chinese are the healthiest, and the people in southern Texas are the least healthy.”

I’d like to put a word in here for Okinawa, which still holds the record for the longest life-expectancy in Japan. I wonder if the metabolomes of Okinawans — at least, those on traditional diets and not burgers — is healthier even than those southern Chinese?

Nicholson says that metabolic profiling can tell us how specific aspects of a person’s diet — and how much alcohol they drink — are contributing to their risks of contracting certain diseases.

That’s the medical application we get out of this work, but what I love about it is the fact that, regardless of the color of your skin or your country of birth, you can alter your metabolome in a way that you can’t alter your DNA.

So what it all means is that if you live in Japan for a few months, your total genetic profile becomes more Japanese. That’s a kind of national identity I can subscribe to: one based on the food you eat.

Miso- and natto-eaters unite! Ramen-slurpers stand firm! Sushi and tofu lovers . . . revel in your Japanese identity. And that’s wherever you are from.

The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa ima mo shinka shiteru (The Evolving Human: How New Biology Explains Your Journey Through Life).”