The Japanese spent more than they saved in the 12 months ended March 2014, the first time that's happened since the data set began in 1955, with the savings rate at a negative 1.3 percent in the last fiscal year.

"It's something to keep an eye out for in the medium-term because Japan's debt has been funded domestically, and very cheaply. But foreign investors would require a more appropriate risk premium," said Toru Yamamoto, Daiwa' Securities chief rates strategist.