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Question #90036 posted on 07/10/2017 1:22 p.m.
Q:

Dear 100 Hour Board,

What studies/research have been done to prove where the line is between healthy weight and unhealthy weight?
(basically, where is the scientific proof for the divisions between "unhealthy", "healthy", "overweight", and "obese", and how were those conclusions made?)

-Google keeps bringing me the wrong info.

A:

Dear you,

It doesn't look like any of us have been able to find this either! That's a really good question though - these are the types of questions that science relies on! If any readers have any information, please feel free to submit a correction.

-Zedability

posted on 07/11/2017 1:20 a.m.
According to my nutrition textbook (Williams' Basic Nutrition and Diet Therapy, 15th Ed.), the lines are drawn in terms of Body Mass Indexes. It gave no context as to why BMI divisions are cut where they are, however looking at it I think it's obvious they wanted nice multiples of roughly 5. And so:

BMI < 19 = underweight
18 > BMI > 25 = healthy weight
24 > BMI > 30 = overweight
29 > BMI > 40 = obese
BMI > 39 = extremely obese

I presume there is science behind the ranges and I realize that's what you're mainly asking for, but yeah I think this is enough to conclude that to some degree the lines drawn are arbitrary, as it is a continues line and in reality there is no one "tipping point."
It should also be noted that BMI is calculated by Weight(kg)/Height(m^2) and therefore is only a weight to height ratio, and healthy weights are dependent upon so many more factors than just height and weight. For instance, a 6ft 300 lb totally ripped body builder would be classified as "extremely obese" when obviously that's just not the case.
Since everyone's body type and optimal health is different (and that weight is not a sole nor even primary descriptor for health) I'm going to guess there isn't a perfect answer to your question, but that a simple measure was designed for quick reference with the intention that each individual must be observed individually from there.

If that doesn't quench your thirst for The Answer, for further research I'd recommend looking into how growth charts are created.
Best of luck!

-Corsica S.
posted on 07/11/2017 1:20 a.m.