Commodity prices have slumped to their lowest level since 2002, as investors await the Federal Reserve's first move to raise interest rates, which is expected as soon as September.

Gold has dropped to a five-year trough and oil to a three-month low. The Bloomberg Commodity Index hit a 13-year low Tuesday.

Higher interest rates hurt commodities as an investment class, because commodities don't offer income payments. That makes them less valuable compared to bonds and other investments whose income payments climb in tandem with interest rates.

Rising rates also can slow economic growth, decreasing user demand for commodities. The dollar's strength is weighing on commodities too, as many of them are priced in dollars.

“It is a question of choosing which asset class you want to be in and for many investors, commodities are not that asset class,” Edward Meir, a strategist at brokerage firm INTL FCStone, told The Wall Street Journal. Commodity funds suffered a $1.1 billion outflow during the second quarter, according to EPFR Global.

Elsewhere on the investment front, with the S&P 500 index having tripled since March 2009 and having escaped a 10 percent correction since October 2011, some experts expect sub-par market returns for years to come.

But not D. David Jilek, chief investment strategist for Gateway Investment Advisers. "History suggests an annualized return in the mid-teens over the next 10 years may be possible," he writes on MarketWatch.

Looking at the S&P 500 going back to 1928, 20-year annualized returns average 11.22 percent and 30-year returns average 11.24 percent, Jilek says. For the market to meet those averages for the periods ending in 2025, stocks would have to return an annualized 14.67 percent over the next 10 years to hit the 20-year average and 16.09 percent to meet the 30-year average.

To be sure, if the bull market lasts another 10 years, that would set a record by almost four years, Jilek notes.

So over the next 10 years, the current bull market may end, a bear market--defined as a 20 percent loss — may ensue, and stocks may still return an annualized 14.67 percent during the period, he says.

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