Fatness and body mass index and being mean with cutoffs

Comments and Key points

The authors made an assumption that I disagree with. They set their BMI cut-off values to maximize sensitivity for obesity, by saying:

"sensitivity is given more importance than specificity since a false positive is not considered as serious as a false negative."

Paraphrased, this means they think it is more acceptable to call some "normal" people as "overweight", rather than risk under-calling "overweight" people as "normal". This is opposite to how most people think. Most overweight people don’t mind being called normal, but normal people are angered if wrongly labelled as overweight.

People using science to be mean, I hate that.

They concluded that a BMI cutoff of 25.0 for men and 23.0 for women is optimal, to define "obesity" as a body fat percentage over 25% in men or over 33% in women. (Note, these body fat percentages are commonly used arbitrary values.) Lets restate that. They didn’t use science to test any variations other than 25% body fat in men and 33% in women. They just adopted those numbers because they are “known”. Then they did goal seeking with BMI cutoffs to see how many people would be normal or overweight, as they tried various cutoffs.

I think their terminology is confusing because most people would say these body fat percentages are "overweight" (not "obese").

Back in 1996, these study authors used the word obese in a hurtful way.

This study’s data sample is weighted towards young adults, with average age of approximately 31 years.

I have gleaned from this article’s ROC curves, the following possible cut-off values. In my opinion, a Body Mass Index threshold should have about 90% specificity and at least 60% sensitivity, to gain popular acceptance.

I marked my favorite spots using BOLD in the table below.

Cutoff BMI	Men		Women
Cutoff BMI	Specificity	Sensitivity	Specificity	Sensitivity
23			84.4%	81.5%
24			89.5%	77%
25	68.8%	78.6%	92%	72%
26	81%	68%	97.8%	51.8%
27	88%	57%
28	93.5%	43.6%

This criteria is satisfied for women at a BMI of 25.0. For men, there is no perfect answer, but I would choose 27.0 for men.

I originally made this page in 2003 and updated it in 2014, and now these cutoffs that I suggested long ago, are starting to be used in the mainstream.

I’m happy for you. Now lets party.

Wait. This article is too short and had no references. Even if I could praise the CDC and NIH, nobody would care. Anyway, there’s something still valuable about this article. I like how I was able to figure out that BMI of 27 is probably a better overweight threshold for men, than 25.