A "national eating disorder" would be an eating disorder that is associated primarily with a specific nation. One that is possibly the result of conditions and practices in a specific nation.
Is that a credible, believable possibility?
On the surface it seems implausible since the world has shrunk so much in the last forty years, giving us the impression that we are all so much alike. However this is a false impression. Nations and cultures have retained their individuality in spite of this increasing "global" emphasis.
For example, it is possible that Anorexia Nervosa could be identified as a "National Eating Disorder" or at least a "Cultural Eating Disorder." While it is a complicated diagnosis that involves much more than weight and appearance, it is not found in societies that do not place such an emphasis on thinness as we do in Western Culture. There are still nations / cultures in which "beauty" is very different. In most undeveloped nations "Thin" is seen as unattractive and "Heavy" is seen as attractive because hunger is so widespread that if you can eat enough to be heavy you are doing great. "Beauty is in the eye of the beholder."
I hope this makes sense and helps.