The obesity epidemic may be slowing, but don’t take in those pants yet. Today, just over a third of US adults are obese. By 2030, 42 percent will be, says a forecast released Monday. That’s not nearly as many as experts had predicted before the once-rapid rises in obesity rates began leveling off. But the new forecast suggests even small continuing increases will add up. “We still have a very serious problem,’’ said obesity specialist Dr. William Dietz of the Centers for Disease Control and Prevention.