Monday, February 07, 2011

Do Japanese have longer intestines?

The short answer is no.

But a surprising number of otherwise well-educated Japanese people believe this.  The argument is usually presented as something along the lines of "The Japanese were traditionally agricultural people.  Our ancestors didn't eat much meat, so we evolved a longer intestine to digest grains and vegetables."  The idea is usually presented as a contrast to Westerners, especially Americans, who are said to eat much more meat.  It is also sometimes used as part of an argument that Japanese should eat less meat.  In general, the Japanese intestine is said to be one metre longer than other intestines, or even 1.5 times longer.

Well, where to begin with such a claim?  There seems to be little definitive information available on racial differences in intestinal length on the net, though plenty of people have been asking the question.  There used to be a wikipedia article, but it was deleted, seemingly because of lack of evidence.  The belief is certainly a prime example of nihonjinron, the theory that Japanese are in some way unique, and by implication superior, in various cultural, physical and mental characteristics. 

It seems (to me anyway) to imply that people in other countries are more carnivorous, and by extension more aggressive or violent.  It was famously presented as a given fact by the Japanese minister for agriculture in Tsutomu Hata in 1987 when arguing against American beef imports.  Although in the resulting furore he lost his job, he was possibly only repeating what most Japanese perceive as common wisdom.  Around the same time it was claimed that Japanese snow was different, which was used to justify slapping a tariff on imported French skiis.

The theory that Japanse intestines are longer as an adaption to eating more grain and less meat seems superficially plausible.  Westerners do, in fact, eat more meat than Japanese, who even today tend to have a diet rich in seafood and vegetables.  However,  the theory is actually specious and pretty easily dismissed.

For one thing, the adaption aspect of this is decidedly dodgy.  Although human evolution has continued since the invention of agriculture (for example the spread of lactose tolerance through European populations), the lengthening of the human gut by a metre or more seems to be quite a stretch (as it were).  Gut tissue is metabolically expensive. An extra meter of it would require considerable resources, and some other body organ would suffer to provide the space.  Even if this were possible, it would imply massive selection pressure.  Actually it is theorized by some evolutionary scientists that reduction of the gut associated with increased meat consumption may have allowed early evolving humans to devote greater metabolic energy to brain growth and maintenance, thus preciptating 'a great mental leap forward'.  This is known as the "expensive tissue hypothesis".  Funnily enough, no Japanese person has ever suggested to me that their intestinal lengthening may have been accompanied by brain size reduction.  You probably won't find many references to brain shrinking in popular Nihonjinron books and articles. 

Secondly, it is not as if the Japanese diet was really that peculiar.  Sure, recently American or European populations have been eating more meat, but go back a few hundred years and common people in agricultural societies all over the world were subsisting on grains, vegetables, and much smaller amounts of protein.  Meat, in Japan and elsewhere, was reserved for the nobility.  This idea of Japanese 'uniqueness' just doesn't stand up to close scrutiny.

Going back further, none of the various human populations in the world are especially suited to to eating a large percentage of their diet in grain form.  Hunter gatherers didn't.  Indeed, if you follow the Japanese reasoning to its conclusion, the Egyptians and Sumerians (now Iraqis) should have the longest intestines in the world, since they live where grain domestication started.  Indeed, farming didn't come to Japan until substantially later, in about 300 BC with the Yayoi people, several centuries after the Golden Age of Ancient Greece and some 5000 years or so after domestication of the first grains in the Fertile Crescent.

All this is not to say, strangely enough, that many individual Japanese do not actually have longer intestines than Westerners.  There seems to be some evidence that intestinal length may depend on an individual's diet in their lifetime, with longer intestines associated with less meat consumption.  However this would be a cultural phenomenon, not a racial.

The issue is kind of frustrating because it seems like the kind of thing which should be easily testable.  However it could be quite difficult research to do.  Living or dead people?  How do you exclude cultural factors such as diet and body size?  Who do you include or exclude?  Do you exclude Chinese or Koreans from the long intestine sample? If so, why?  What is the standard variation in normal intestine length?  How many individual guts do you need to measure to arrive at a statistically valid conclusion?

In the absence of definitive medical evidence, I would hesitate to say that the Japanese are definitely wrong about this.  But it seems very likely to me that they are, and certainly a case where an extraordinary claim requires extraordinary evidence.


Anonymous said...

Great post. How much time did you spend on this?

Captaincassowary said...

Not longer than the amount of time a Japanese person would spend. Definitely not any shorter.

Anonymous said...

I see what you did there.

Anonymous said...

Interesting topic. While I agree with your conclusion, I must point out problems with the reasoning used by the proponents of this theory and your counter arguments. The conclusion, that the Japanese evolved longer colons because they ate less meat is based on an unproven premise, that the Japanese do in fact have longer intestines. We can end the argument right there. However, if we accept the premise for the sake of argument, then we must address 3implicit premises in the Japanese theory.

1. Meat is calorically denser than grains and vegetables. This is easily shown false by a review of the nutritional profile of macronutrients. The caloric density of protein is 4 kcal/g, same as for carbohydrates. Moveover, protein has a higher metabolic cost for digestion, so the same nominal energy value of protein is less than the same value of carbs, because the body uses more energy to digest protein. Gram for gram, carbohydrates provide more real energy than proteins.

2. Japanese people historically ate less "meat" than Westerners. By Western we can narrow down to European. The American diet is evolutionarily irrelevant given the short timescales and ethnographic diversity of the population. Japan has in fact the richest fisheries and coastlines of any major land mass. The productivity of the Japanese seas is unmatched and has been utilized extensively throughout its period of human habitation. The prehistorical Jomon people had a diet rich in fish, bivalves, seabirds, sea mammals, as well as deer and other land animals. It is safe to say that fish protein is nutritionally, if not taxonomically, equivalent to meat. Thus, human populations on the Japanese islands have had greater access to cheap animal protein than preindustrial Western populations as a whole.

3. Increasing meat consumption led to a shrinking gut. The theory that greater meat consumption caused shrinking of the prehuman gut has been superceded by a more modern understanding of the role that fire and cooking had in the changing diet of our ancestors. The increased consumption of meat did play a role, but the invention of cooking increased the food landscape by far greater degree. Cooking, by breaking down tough muscle fibers, killing pathogens in spoiled meat, breaking down cell walls in tough plants, and detoxifying bitter or mildly poisonous plants, greatly increased the available menu that created the energy surplus that enabled the evolution of the modern brain and gut.

Captaincassowary, the example of ancient agriculturalists that you use as a counter argument is misleading. As stated previously, carbohydrates in grains are not nutritionally inferior to meat, and the agriculturalist would not be under selective pressure to increase their colon to digest an "inferior" diet. If farmers had a poorer diet than hunter gatherers or pastoralists, then how would farming have taken over as the dominant lifestyle of the human species? How could civilization have risen but from the agricultural surplus made possible by farming? Remember that animal husbandry arose more or less simultaneously with the invention of agriculture. It's a lot easier to hunt for meat in the pen than in the forest or savanna.

In conclusion, the reasoning behind the theory that the Japanese people have a longer intestine than Western people because they eat less meat is proven faulty and untenable. The question of whether differences in gut length between races exist is solvable only by empirical study. To argue without this basis is futile at best. This leads to the question of why the Japanese people seem to habitually engage in this constant myth making. From their racial creation mythology to the deification of their monarchy to the distortions of history, the Japanese are guiltier than most of logical contortions to preserve their sense of their own uniqueness. Like you said, nihonjinron. What I would like to know if this is rooted in true arrogance, or defensiveness arising from an inferiority complex.

A bored biologist.

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