WASHINGTON (Reuters) - A nutrition advocacy group warned on Thursday that the popular Atkins diet may cause heart disease and could have killed a teen-age dieter. It urged the U.S. government to monitor the high-fat weight loss approach to see if it indeed causes heart disease. The Physicians Committee for Responsible Medicine stressed it could not prove the diet had hurt or killed anyone. But one dieter said he believed the approach clogged his arteries and the parents of a teen-ager who died while on the diet also blamed her meat-heavy regimen. The PCRM called on the U.S. Centers for...