If you believe everything you read about the health-giving properties of the traditional Japanese diet — and if you were to eat traditionally every day — you might expect to live to at least 150, in rude health.

Maybe one day, and maybe soon — I’m sure the first person to live to 200 has already been born — but it hasn’t happened yet. The oldest living Japanese citizen whose age can be verified is 113-year-old Kama Chinen of Okinawa, though Guinness World Records lists Japan’s oldest-ever man as Shigechiyo Izumi, who died in February 1986 at the age of 120.

The fact that Japan consistently ranks top in the world longevity tables suggests that there is some truth in claims made for the traditional diet. It’s sorting out the truth from the hype that is tricky.

Those claims are regularly made; it’s a recurring theme in magazines and books, and not just in Japan. Green tea is supposed to prevent cancer; miso is said to protect against radiation sickness. It seems there’s an industry geared toward sensationalizing all things gastronomically Japanese. I wouldn’t be surprised to read that hanami (cherry-blossom viewing) helps boost mental ability or something.

This week, indeed, I read that natto (fermented soybeans) might help cure Alzheimer’s.

But what is the evidence for these things? And does it really matter if there is no strong evidence — after all, isn’t this Japanese-food promotion just a form of advertising? I think it does matter, so in this month’s column I’ll look at the scientific evidence for some of the claims made for Japanese foods.

First, what about the idea that natto can cure Alzheimer’s? Natto, it turns out, contains the enzyme nattokinase, which can apparently break down the plaques that form in the brains of Alzheimer’s sufferers. This led Alzheimer’s “experts” to tout natto as a potential wonderfood. The study I read this week has just been published in the Journal of Food and Agricultural Chemistry.

When you look at what the nattokinase actually did, however, there are a couple of serious problems with claims of it having any effect at all on Alzheimer’s. First, the experiment showed only that — in a test-tube — the enzymes can break down proteins that form the tangles and plaques in the brains of Alzheimer’s sufferers. There is no evidence that nattokinase can get into the brain and encounter the plaques, nor is there even any evidence that removing or breaking down the plaques could cure the disease — it might quite easily make it worse!

Second, and more serious, the neurodegenerative symptoms of Alzheimer’s are now thought to be caused not by the protein plaques, but by brain inflammation. So natto — delicious as it is — doesn’t to me look much good as a cure for Alzheimer’s. It would be wrong to give the impression to people diagnosed with this terrible disease, or to their relatives, that gorging on natto will cure them.

Next up, ocha (green tea), which is ubiquitous in Japan and integral to the nation’s culture, not least forming the centerpiece of the tea ceremony. It was widely drunk in China for medicinal reasons before it was introduced to Japan in the ninth century, but only relatively recently have these ancient health-benefit claims been put to the scientific test.

There is some evidence that drinking green tea reduces the risk of heart disease, but where I find the health claims most worrying is when it comes to a purported effect on curing cancer. While there is evidence that some of the compounds in green tea (flavanoids, in particular) can block cancer growth, we don’t know if they can pass through the digestive system unscathed to tackle cancerous cells.

Worse, there is some evidence that green tea actually stops a certain type of anticancer drug from working.

That drug is bortezomib — brand name Velcade — which is used to treat the second-most common blood cancer, multiple myeloma. Cancer biologists at the University of Southern California in Los Angeles found that Velcade was inactivated in mice with cancer that had been given green-tea supplements equivalent to those a person might take, or possibly the amount consumed by someone who drinks a lot of green tea.

Velcade is the only cancer drug that is affected chemically by green tea, so there is no need at all to stop drinking green tea, but the study (published last month in the journal Blood) made me realize quite how big the green-tea supplements business is.

The supplements are available in health shops, and contain green-tea chemicals called polyphenols at doses 50 times higher than in a cup of tea. Large amounts of polyphenols have been shown to cause liver and kidney damage, so the message is: drink green tea in moderation.

I’ve also seen claims that umeboshi (pickled plums) fight cancer as well as preventing hardening of the arteries.I don’t know what the evidence for this is, but I suspect it is not very powerful.I guess that most of the claims made for umeboshi are made by processors of the plums, and are based at best on nostalgia for stories of them being eaten by samurai.

None of this detracts from the pleasure and comfort of Japanese food, of course.As I write this, I have next to me a steaming bowl of miso soup, made with konbu (kelp) stock and containing regular wakame (seaweed).

With moderation and a balanced diet, someone out there is going to live an awfully long time.

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”).

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