U.S rate of "very low food security" rises in 2011 * Squeeze may worsen in 2012 as jobless aid dwindles * High jobless rate, slow economy keep hunger an issue WASHINGTON, Sept 5 (Reuters) - The number of poor Americans who repeatedly ran short of food shot up by 800,000 in 2011 to nearly 17 million compared with 2010, the U.S. government said on Wednesday. The Department of Agriculture said in a report that about 5.5 percent of Americans.