According to BMI, a person is underweight if their height relative to their weight is less than between 18.5%-19%. Thirdly, my search using the words “thin” or “skinny” yielded articles, blogs, and opinions that was mostly negative. There was a lot of thin hating or thin bashing as a backlash to the Hollywood thin craze that has seen once curvy starlets turning into “skinny scary”.

There is very little known about or addressing the 2% of the population that is underweight because of being naturally thin, outside of perhaps mentioning that we may have high metabolisms. According to Barb Troy, Marquette University’s Nutrition Specialist, edited from her website:
 

“Years ago when disordered eating wasn't a topic everyone had heard about, people would have just said you are a ‘naturally thin’ person. There are people who have that unique metabolism and body frame that keeps them very lean. Many times you can see a familial pattern too. The line here between ‘naturally thin’ and possible disordered eating patterns can be difficult to assess at times for an outsider or even for the person involved. The big distinction would be if the ‘naturally thin’ person exhibits signs of extreme fat phobia (either on food or on the body). Naturally thin people do not obsess about fat or calories eaten. Naturally thin people may love to exercise, but they do not obsess about how much exercise or how often, and can easily skip a day without feeling ‘guilty’. And finally, naturally thin people do not obsess about what the scale tells them that day.”