The number of international students studying at U.S. colleges and universities last year saw its biggest increase in 35 years — an uptick reflected in Iowa, which experienced an 8 percent bump in foreign students and saw a $351 million economic impact, a study shows.Iowa State University and the University of Iowa host the most international students in the state, both ranking in the top 50 nationally out of 1,485 higher education institutions, according to a new 2015 Open Doors Report from the Institute of International Education.ISU ranked 44th in the nation with 4,425 foreign students in the 2014-2015 academic year — up 34 percent from 3,302 in 2010, according to the Open Doors report.