Federal Reserve Board Chair Janet L. Yellen (Brendan Smialowski/AFP/Getty Images)

The Federal Reserve on Wednesday announced it's going on the financial equivalent of a diet.

After the devastating Great Recession, the Fed took two unprecedented steps to rescue the U.S. economy and jump-start growth: It cut interest rates to zero for the first time ever, and it beefed up its balance sheet from about $900 billion in assets to $4.5 trillion. The Fed has already lifted interest rates to above 1 percent, a sign it believes the U.S. economy has finally bounced back from the crisis and recession.

Now, the United States' central bank plans to start trimming down the $4.5 trillion collection of assets.

“It's past time to reduce the Fed's balance sheet, because its role in the economy is unnecessarily large,” said Tate Lacey, an economist at the Cato Institute, a libertarian think tank that has criticized the Fed for bulking up its balance sheet so much.

For the Fed, however, it's not as simple as shedding all its assets at once. If the Fed moves too quickly, it risks unsettling stock and bond markets and driving investors away from investments with more potential to stimulate economic growth. But if the bank moves too slowly, it could end up still sitting on a bloated balance sheet when the economy hits its next downturn — leaving it without some of the tools it needs to push the economy toward recovery.

“If I had to bet, I would bet things would go smoothly, but I wouldn't bet the farm on it,” said Joseph Gagnon, a senior fellow at the Peterson Institute for International Economics who used to work at the Federal Reserve.

As Federal Reserve Board Chair Janet L. Yellen and the bank's other leaders attempt this tricky process, they have no precedent to fall back on for a sell-off of this size. There was no guidebook for the Fed's balance sheet to grow so large, and there's no guidebook now for how to shrink it back down.

After nearly a decade of unprecedented efforts to pull the economy out of the recession, the Fed's work is yielding results. The economy has added more than 15 million jobs since the worst of the crisis, and the United States is in the midst of the third-longest economic expansion in the nation's history.

The Fed is trying to avoid disrupting this growth by telegraphing its moves clearly to avoid the type of uncertainty that tends to unnerve investors and rattle markets.

On Wednesday afternoon, the Fed confirmed that the slimming down process will commence in October. The Fed already outlined a plan in June to start small, unloading $10 billion worth of assets a month and then scaling up gradually over time until it is shedding $50 billion a month.

Analysts say that incremental nature will help prevent the Fed from knocking the economy off its growth path.

“The good news is the Fed has been planning this for a long time,” says Carl Tannenbaum, chief economist at Northern Trust and a former Federal Reserve staffer. “It's going to be a slow, careful and gradual process.”

While many worry the Fed might go too quickly, Tannenbaum says it's also problematic if the Fed moves too slowly. The current process the Fed outlined will take four or more years to shrink the balance sheet to $2.5 trillion or $3 trillion, the level many economist say they expect will be the new normal for the Fed.

But if another recession hits the United States before then, the Fed may not have the tools it needs to stimulate the economy. The Fed currently owns about $2.5 trillion in U.S. government bonds, known as Treasurys, and the rest in mortgage-related securities. The Fed started buying up these assets in late 2008 to stabilize markets and keep interest rates low. It stopped the big buying spree in 2014. All the Fed's purchases caused bond prices to rise and yields (the interest paid on the bonds) to go down.

Having such a big balance sheet already makes it difficult for the Fed to use this tactic again if a recession were to hit soon. The Fed is trying to get back higher interest rates and a lower balance sheet so it can deal with any future crises or downturns with a full set of tools.

The sheer size of the Fed's holdings has caused so much concern about how the central bank could rattle markets when it unloads the assets. The Fed owns 15 percent of America's federal debt, according to calculations by Northern Trust bank, and about a third of mortgage-backed securities.

So far, Wall Street seems fine with the Fed's plan. The U.S. stock market is trading at record highs, and the bond market is also showing signs of calm. Investors don't seem to be spooked by the Fed or even the devastating hurricanes, gridlock in Washington or North Korea's threats about nuclear war.

“If there’s a time when the market is telling [Yellen], 'Get on with it,' is it not now?” wrote Michael Block, chief strategist at Rhino Trading Partners, in a note to clients.

If all goes well, few Americans will notice what the Fed is doing because it will be quiet and orderly — with economic growth and job creation holding steady and the central bank readies itself for the next recession. The only indication anything has changed might be for home buyers looking to get a 30-year mortgage.

“For the average person, it means mortgage interest rates might go up,” Gagnon says.