Companies in the Standard & Poor's 500 Index are paying less in interest on debt than any time in at least a decade, leaving investors more dependent on economic growth and corporate spending for equity gains in 2013.

Constituents of the benchmark gauge for American stocks such as Exxon Mobil Corp. and Walt Disney Co. cut interest expenses to 2.39 percent of sales in the 12 months ended Sept. 30 on average, the lowest level since at least 2002, according to data compiled by Bloomberg and Strategas Research Partners. With borrowing expenses at record lows, executives are finding it harder to squeeze costs, causing profit margins to contract for the first time since 2009.

Bears say projected revenue growth of 3.8 percent for S&P 500 companies next year won't be enough to drive higher profits without a decline in expenses. Bulls point to 8.9 percent profit growth estimated for next year, equity valuations at a 12 percent discount to the six-decade average and an economic recovery as reasons chief executive officers will start to spend the $1 trillion in cash they've built up since the recession.