After earning hefty profits on its commodities trading for nearly 18 years, heavyweight trader Goldman Sachs Group Inc. (NYSE: GS) now finds itself on the hot seat, defending this crucial source of revenue. And while that may not be good for Goldman, it’s also bad for investors.  Let me explain…

It all started back in 1991, when J. Aron & Co., Goldman’s commodities-trading division, recommended that a large institutional client invest about $100 million in commodities.  The vehicle “du-jour” was Goldman’s own investment vehicle, the Goldman Sachs Commodity Index (now the S&P GSCI Commodity Index).

The GSCI is a 24-commodity dollar-weighted index, comprised of 70% energy (oil and natural gas), 8% industrial metals (aluminum, copper, lead, nickel and zinc), 3% precious metals (gold and silver), 14% agriculture (wheat, corn, soybeans, cotton, sugar, coffee and cocoa) and 4% livestock (cattle and hogs).