"He is definitely an expert." Medicine is a very big field. Indeed, so big that being a pediatrician, co-inventor of a rotavirus vaccine and publishing 100-odd

"He is definitely an expert." Medicine is a very big field. Indeed, so big that being a pediatrician, co-inventor of a rotavirus vaccine and publishing 100-odd papers doesn't make someone an expert on the very specific topic of human use of antioxidants. That is, unless the 100-odd papers are related to the human use of antioxidants.