Japanese trading houses face billions of dollars in impairments due to the double whammy of an emerging market downturn and the ongoing commodities rout, after splashing billions of dollars on earlier acquisitions.

Known as "sogo shosha," the general trading houses supply everything from energy to metals to grains and textiles in resource-scarce Japan . The big five trading houses are Mitsubishi (Tokyo Stock Exchange: 8058.T-JP), Mitsui (Tokyo Stock Exchange: 8031.T-JP), Sumitomo (Tokyo Stock Exchange: 8053.T-JP), Itochu (Tokyo Stock Exchange: 8001.T-JP) and Marubeni (Tokyo Stock Exchange: 8002.T-JP).

Although they trade in a wide range of products and were traditionally importers of goods into Japan, conglomerates such as Mitsubishi, Sumitomo and Mitsui have expanded their footprints overseas, which means they have taken bigger hits from the commodities crash.

"They are trading houses but have become a lot more (like commodity) houses," said Seijiro Takeshita, a professor at the University of Shizuoka's management and information school.

In January Sumitomo took a $650 million writedown at a nickel project in Madagascar, while Itochu last year sold for just $1 a 25 percent stake in U.S. oil and gas producer Samson Resources that it had paid $1 billion for in 2011.

Jefferies Japan equity analyst Pham Thanh Ha estimates the big five firms' pre-tax impairments will come to 1 trillion yen ($8.5 billion) for 2015, after the companies increased assets by 8.1 trillion yen after the Global Financial Crisis of 2008.

So far, 12.5 percent of the assets accumulated in the past six-and-a-half years are impaired and "obviously there is more to come," he told CNBC's "Squawk Box" on Friday.

Goldman Sachs (NYSE: GS) estimated in a note this week that that four of the five largest Japanese trading houses could face 811 billion to 1.6 trillion yen in writedowns in the fiscal year ending March 31.

The investment bank also cut net profit estimates, slashing Mitsubishi's forecast by as much as 81 percent.

To counter the impact from the commodities rout, Japanese trading houses are shifting their focus to other areas.

Itochu, for instance, has increased its interest in the emerging consumer market, while less than 10 percent of Mitsubishi's earnings now come from the resources sector, said Jefferies' Pham.

Other than the trading houses, Japanese exporters in general will be hit by the global economic downturn.

"Certainly, it's comes to a (point) where things look really very nasty for the outlook of many of the Japanese companies," said Takeshita.