I think the article's autor needed a person who had some authority. That is the reason  by which he quote Dr. Paul Clayton. A schientific opinion makes mor credible his argument.

The people in diets are mentioned as an empiric prove of his argument. The autor of the article use the study in which people on diets are the object as one of the supports of his position about carbohydrates.

With schollchildren happens the same as with people on diet, exept for, this time the argument he is supporting is about proteins and not carbohydrates.