See BBC News report. "Researchers say the benefit only comes with exclusive breastfeeding - mixing breast and bottle will not achieve the same." ""Mothers should be advised by health professionals that, in addition to all the other benefits, exclusive breastfeeding helps prevent infections in babies and lessens the frequency and severity of infectious episodes," the researchers say."
Human breast milk is also low in sodium so is an excellent foundation for babies to escape becoming obese later on. See Children and Salt.