America's largest retailers are facing a threat that is rarely discussed: student-loan debt.

Consumers make $80 billion in student loan interest payments a year, Moody's retail analyst Charlie O'Shea said on Bloomberg TV. This means that they have less money to spend at retailers like Walmart, Home Depot, and Costco.

Total loan payments add up to $160 billion annually.

"The average salary per college graduate has increased by only about 3% in the past seven years, while the average student debt burden has increased by roughly 53% over the same period," O'Shea told Bloomberg.

He says that America's more than $1.3 trillion in student loans could hurt the American economy more than a recession because it will last for decades to come.

Industry website Retail Dive notes that younger people are so burdened by student-loan debt that they're frugal.

"It's slated to get worse as retailers find that millennials in their thirties are hampered by debt in what should be their highest-spending years," writes Daphne Howland at RetailDive.com.

Student debt could also be part of the reason why sales are contracting at department stores like Macy's, Sears, Nordstrom, and Kohl's. Off-price retailers like TJ Maxx are thriving because consumers have less money to spend on clothing.

Macy's same-store sales fell 7.4% in the most recent quarter, Nordstrom's fell 1.7%, and Kohl's dropped 3.9%. Shares of Macy's and Nordstrom have lost about 50% of their value in the last year, and Kohl's stock price has fallen 40%, notes Business Insider's Hayley Peterson.

Shopping visits made to off-price retail stores increased by 4% in the 12 months ending April 2016, compared to the same period last year.