percent body fat vs. simple bmi

[f00die]

Hello JeffN,
in other posts on the forum
you have indicated that there is no data supporting the use of percent body fat as marker for disease
viewtopic.php?f=1&t=57312&p=576688&hilit=percent+body+fat#p576688

Much of this discussion is based on a faulty assumption and a misunderstanding of BMI, body composition and the evidence on both in regard to health, aging, fitness, exercise, etc and our recommendations. As I said, and why I posted here in this thread, is to make an important point, that while some may not like BMI, and are quick to point out why, focusing on percent body fat, has no evidence at all to support it.

in this posting: viewtopic.php?f=22&t=6916&p=578973#p578973
the associated article states:

Banack and her coauthors used whole body dual x-ray absorptiometry scans, which they called a gold standard for adiposity measurement, to assess the body composition of 1329 postmenopausal Buffalo, New York, women ranging in age from 53 to 85 years. The researchers defined obesity as a BMI of 30 or higher or a body fat percentage greater than 35%, 38%, or 40%. (They tested 3 different cutoff points because no consensus exists on what percentage of body fat should be used to define obesity, according to the authors.)

is this now evidence that body fat percentage of those numbers indicates obesity-related disease risk
independent of BMI?
or, how do they support 35% vs. 25%?
is this now evidence that obesity can be defined by body fat percentage independent of BMI?
or am i misunderstanding the whole thing?
thanks

[JeffN]

The article has both good and bad points and I can see how it can be confusing.

To clarify, and as my discussion thread on BMI and this article points out...

- BMI is only a screening tool and when understood for that and used as that, it is very valuable. My Optimum BMI thread proves that out by showing how consistent it is as an effective screening tool in identifying many risks. That doesn't make it perfect, just an effective screening tool.

- BMI is the standard used because it is the easiest one to obtain as a screening tool as the methods used (weight, and height) having easy standardized methods to get a fairly accurate measurement. No such method exists for percent body fat. All current methods can have fairly high error rates and can be difficult to obtain. I discuss this in my BMI thread.

- There are no accepted standards for body fat. They admit that and that the ones they used they made up.

- Much of the criticism about BMI is that it doesn't always work and you can have lower BMI and high body fat, and higher BMI and lower body fat (ie., a body builder).

- In this study, they found a number of post-menopausal women who were not obese by the current BMI standard of 30, but could be classified as obese based on (their own standards of) percentage body fat.

- The study is looking at whether or not the cutoff off of a BMI of 30 is appropriate in post-menopausal women as defining obesity or should it be lower.

Here is the conclusion from the original study

"CONCLUSIONS
This study adds to the growing body of knowledge highlighting the limitations of using BMI to define obesity.13 These findings represent an important addition to the litera- ture because the body composition of postmenopausal women is distinct from premenopausal women or men at any stage of the life course. As life expectancies continue to rise, women are expected to spend more than a third of their lifetime beyond the menopausal transition.6 Understanding the effect of obesity in postmenopausal older women is critical to preventing morbidity and mortality and ensuring high quality of life as women age. Though BMI is frequently used as a measure of obesity, our findings indicate that using a BMI cut- point of 30 kg/m2 may lead to bias in measuring the effects of obesity on health outcomes in postmenopausal women.11 Our results add an important perspective focused on aging women to a growing body of literature, emphasizing the shortcomings of using BMI to define obesity."


Considering all the above, we could say, they identified a potential group (postmenopausal women) who may be at higher risk of having more body fat then BMI may indicate.

This doesn't in anyway show a relationship between percent body fat and any specific risk factors nor discount anything I have written.

If anything, you could say that it further supports my contention (along with several other groups) that a lower BMI in any range may be better, especially as one gets older. Or the opposite, a higher BMI in any BMI range may be a indicator of higher % body fat and potential risk then the BMI range may classify them.

I hope that helps

In Health
Jeff

[f00die]

...
This doesn't in anyway show a relationship between percent body fat and any specific risk factors nor discount anything I have written.
...
I hope that helps
In Health
Jeff

it does help
i encounter people who hold that being overweight (bmi 25-30) is ok as long as
waist/height < 0.5 (a measure of body fatness)
this article seemed to support their contention that it's the body fat and not total mass that matters
thanks for the clarification

[JeffN]

i encounter people who hold that being overweight (bmi 25-30) is ok as long as
waist/height < 0.5 (a measure of body fatness)
this article seemed to support their contention that it's the body fat and not total mass that matters
thanks for the clarification

Hmm...

I don’t see that at all. It sounds like you are making an assumption that the study is not making. They are just saying that there is a difference between BMI’s classifications of obesity and percent body fat (by their own made up classifications of it).

In other words, all they are saying is that some of those in the overweight category, as defined by BMI, had enough body fat to be classified as obese, based on categories they made up (as none truly exist).

That is it. No other inference was made. It is still just a screening tool.

They didnt link it to any increased risk or occurrence of anything.

They are saying

A = overweight based on BMI
B = obese based on their made up classifications
C = some risk or disease

A, sometimes may equal B

They didn’t say B = C

Or that any A that may = B also ='s C

Now, if they said, those who were overweight according to BMIs, but whose percent body fat would define them as obese (by these made up classifications), had as much of “X” ( CVD, DB, etc) equal to those with BMI’s over 30 with a percent body fat of obesity, which was a higher incidence than the women who were classified as overweight by BMI’s with a corresponding percent body fat of overweight, then you have something to look closer at.

In other words, they did not say...

A, sometimes may equal B

B = C

A's that = B also = C

B is a better marker then A

I hope that helps

In Health
Jeff

PS just by adding in waist circumference, they would find many, if not most, of those who are classified as overweight by BMI who are obese by body fat percent. Same with those who are healthy weight by BMI, who are really overweight by body fat. It is just that waist circumference is a harder measure to get in a screening situation.

[f00die]

Thanks again JeffN
very thorough
very clear
that was exactly what i was unclear about
i appreciate you taking the time