Throughout the hardships of the Great Recession, Salt Lake County officials were zealots about doing all they could to protect the county's AAA credit rating.

And they succeeded, significantly lowering the county's cost of borrowing by roughly $50 million a year.

But that AAA credit rating is being threatened, not because of a decline in the county's performance but because rating agency Standard & Poor's is contemplating changes to its ratings methodology.

The changes it's proposing — switching from per-household income to per-capita income to gauge the resiliency of the local economy — would hurt Salt Lake County and many of the state's other local governments simply because Utahns have large families.

"Your formula penalizes all bond issuers that exhibit a higher-than-average dependent population, notwithstanding our stellar reputation for sound fiscal management, prudent use of bonding capacity and long-term demographic advantages," said County Chief Financial Officer Darrin Casper in a letter sent last week to Standard & Poor's.

Or as Councilman Michael Jensen put it when the council unanimously endorsed Casper's letter to the agency: "Did you put the comment in about how stupid this is?"

Casper hadn't. The county has forged a good relationship with Standard & Poor's over the years, and he didn't want to disrupt that. But he wanted to convince the agency that its proposed formula change is statistically invalid as applied to Utah.

"There are positive benefits of having a younger-than-average population," he wrote. "Salt Lake County has a reliable, young, educated workforce that attracts business to our area. This is commonly seen as a positive, not a negative, from the perspective of our economic development teams at both the state and county level.

"A younger population also provides a steady stream of income earners over the long term to make future debt payments," Casper added.

Council chairman David Wilde was a bit bewildered by Standard & Poor's thinking, noting that "a lot of places are in trouble because of declining birth rates. To be in trouble because of a higher birth rate is counterintuitive to me."

Casper doesn't believe this change-in-methodology proposal will end up lowering Salt Lake County's credit rating, but he doesn't want to take any chances.

"It's a big deal. Our AAA bond rating means the world to us," he said. "If we're paying half to 1 percent more in interest every time we do a new bond issuance, it adds up to a lot of money."

mikeg@sltrib.comTwitter: @sltribmikeg