In the first six months of 2016, foreign central banks sold a net $192 billion of U.S. Treasury bills, notes, and bonds. This is more than double the pace from the same time last year. China, Japan, and Brazil were the leaders in selling U.S. debt.
With the U.S. debt running at approximately $19.4 trillion, this could be problematic. A large selloff of U.S. bonds would decrease their price, or in other worlds, increase domestic interest rates in general (bond prices and interest rates always move in the opposite direction).
With the debt roughly the size of the U.S. economy, interest spending on the debt is forecasted to be the fastest growing area of federal spending in the coming years, eclipsing Medicare and Social Security. In 2015, the U.S. spent $223 billion, or 6 percent of the federal budget, despite historically low interest rates.