WASHINGTON, July 23 — Four of every 10 patients who undergo weight-loss surgery develop complications within six months, the federal government said Sunday. The number of such surgical procedures has been rising rapidly, along with the incidence of obesity, which now afflicts 30 percent of adults in the United States, health officials said. Obesity surgery is helping thousands of Americans lose weight and reduce the risk of diabetes and other life-threatening diseases, said Dr. Carolyn M. Clancy, director of the Agency for Healthcare Research and Quality, a unit of the Public Health Service. But she added, “This study shows how...