When you first arrive in Japan, you are exposed to a lot of facts which may be myths and myths that may be facts. The average foreigner is highly suspicious of most of the little tidbits of information tossed her way. The main reason for this is that, by and large, we're taught in our home countries that we're all the same under our skin and a human is a human is a human. The Japanese, on the other hand, are taught that they are different and unique from other humans and readily accept that there are physiological variations based on ethnic background.
There is some factual data in regards to medical conditions and differences which are clearly ethnically-based so the notion that we may be constructed differently isn't that absurd. There are a lot of examples but a few obvious ones are Sickle Cell disease which mainly affects people descended from or born in sub-Saharan areas and Tay Sachs disease which mainly affects Jewish people. If diseases and certain physical attributes can "discriminate" based on ethnicity, why can't internal biology?
One commonly-mentioned and much disbelieved myth (or fact) is that the Japanese have much longer intestines than those from western cultures. This notion has been used to explain why Japanese people shouldn't consume certain foreign foods. Most foreigners scoff or react incredulously when they are presented with this belief. The credibility of the notion isn't helped by the fact that Japanese people rarely are able to explain why this should be and that there is no readily accessible medical data in foreign languages to support the idea. Some of the Japanese people I've spoken to have stated that such data exists but only in Japanese and only in medical journals. Last night, I got a rudimentary explanation from a student for the first time and did some research into this claim based on what she told me.
One can attempt to view this (bowel length difference) idea objectively without hard evidence and speculate on its feasibility based on related information. Mainly, one can look at the animal world and how intestinal length relates to dietary habits. Herbivores are known to have much longer digestive tracts than carnivores. The reason for this is that they need the added length to break down the vegetable matter they consume in order to extract the nutrients. Carnivores have shorter intestines to allow them to digest meat more rapidly before it deteriorates in their bodies and causes illness.
All of the information regarding herbivores and carnivores in the animal kingdom is absolute fact. However, people are omnivorous (but actually closer to being herbivores than carnivores in terms of their digestive systems). The Japanese contend that they have longer intestines because they are descendants of people whose diets were skewed much more toward a vegetable-based diet than western people who are the descendants of people who included much more meat in their diets. As a theory, this isn't too hard to believe since the modern Japanese diet still contains a lot of soy-based proteins, seafood, and vegetables.
The possibility that longer bowel length is a fact among Japanese (or other Asian) people compared to western people opens up a lot of discussion and theory about how their health may differ compared to westerners. It's been speculated that their slim physiques may be related to longer intestines as digestive processes are metabolically intense. It's also possible that it can increase longevity if more nutrients are extracted from vegetables that are consumed.
The only real question is whether or not someone has ever done comparative autopsies on a statistically valid number of bodies and measured intestinal length and proven this theory. Somehow, I doubt anyone has because the truth is that proving it isn't all that important unless you're Japanese and have health concerns related to your dietary habits. Since most Japanese people already accept this theory as fact, they've already incorporated it into their mindset in regards to health maintenance.
I'm not suggesting that this "myth" is true but rather that we should be a bit more open to the possibility that some of the notions which initially strike us as preposterous may not be so silly after all. Even if they are actually not true, there may be at least some sound logic behind them which make them sound reasonable to the Japanese who believe such ideas.