"He should stick to his expert area and not get involved in scientific issues."
Why not? Are you suggesting only scientists are allowed to have an interest in, or knowledge of, science?
"then he goes on to 'pretend' that he is an expert in nutrition by writing a book about food additives"
In what way is he 'pretending'? Does he state he is an expert at any point? Does he claim to be a nutritionist, or to have a scientific degree?