Recently there has been discussion in the news about taxing junk food (soft drinks, for example) in an effort to reduce the incidence of obesity in the U.S. Is the demand for junk food elastic or inelastic with respect to price?

Would think the deadweight loss of a soda/junk-food tax would be relatively large or relatively small? Would taxing junk food would be a good idea? Would it really help reduce the number of obese people in the United States?