I was intrigued by this and did some very quick searching. Here is an article from 1948 which suggests that dietary differences are responsible for longer intestines in Japanese and Russians:
Here is a review which mentions several articles in which investigators looked at intestinal length among westerners and Japanese, but the authors concluded that it was difficult to perform direct comparisons due to differences in measurement techniques.
You should definitely send that to the author. After reading the (very enjoyable but inconclusive) article, I'm sure his 30 years quest could see some more data and, well, some more decades.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1513781/pdf/anns...
Here is a review which mentions several articles in which investigators looked at intestinal length among westerners and Japanese, but the authors concluded that it was difficult to perform direct comparisons due to differences in measurement techniques.
http://download.springer.com/static/pdf/803/art%253A10.1007%...