> previous reviews of the scientific literature have concluded that a well-planned and varied vegetarian diet can meet the needs of athletes, as it was for Roman gladiators or legionnaires.

What does this even mean? "Meet the needs" is relative to a certain quality of life. Even if the gladiators or legionnaires were vegetarian (questionable even with the paper's assumptions), who knows what their quality of life was. Probably not acceptable to our standards.