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Need clarification on body fat and body mass index



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3NORNS
Posts: 351
3/10/12 5:58 P

BMI is crap, is all anyone really needs to know. one country - norway, i do believe - doesn't even use it any more.

waist-to-hip ratio is a much better predictor of health concerns (as it takes into account not only how much fat is there, but also how it is distributed).

otherwise, if you need to know if you're at a good weight, the mirror is the best guide.

ETA: sometimes i think doctors these days are losing their brains. my daughter's specialist always starts the visit with raves about how wonderful she looks but as soon as she weighs her, now i get the lecture about my daughter being "obese" (which she absolutely is NOT). she weighs like a ton of bricks - she's 54lbs and she'll be 5 in a week - and the doctor goes on and on about how she weighs as much as a 7yr old child but then never says anything when i ask okay, fine, but she's also nearly the height of a 7yr old child and she LOOKS fine - all she does is tap her stupid growth chart and continue the lecture.

so i ignore her and keep on doing what i'm doing and hope that one day her scale blows up in her face and she'll have to actually use her eyes.

Edited by: 3NORNS at: 3/10/2012 (18:02)

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ARCHIMEDESII
SparkPoints: (135,675)
Fitness Minutes: (204,345)
Posts: 20,175
3/10/12 3:01 P

BMI is merely one way to measure a person's over all health. However, it's not a very good one because BMI does not take lean muscle into account.

Let's take two women who are 5ft 5" tall and weigh 150 pounds. If their doctor does a BMI, they'll both come out with a 25. That's the high end of normal bordering on what the charts would consider overweight. Now, one woman is fairly sedentary. the only exercise she gets is from her daily activities at work and home. She has a body fat percentage of 35%.

Let's look at the other woman. She's very active. She engages in regular strength training. As a result of the strength training she does, she has a body fat percentage of 20%.

Put these two women side by side and they look very different. The woman with the 20% body fat who exercises looks much leaner than the woman who doesn't exercise. but, they both have a BMI of 25 !

This is not an unusual scenario. doctors use BMI as a rough guideline when working with their patients. This is because most of their patients are probably over weight and don't get enough exercise. BMI should not apply to those who are active, once again because BMI doesn't calculate lean muscle. And having muscle makes a big difference. A body builder who's 6% body fat would have an obese or even morbidly obese BMI. I'd hardly call a body builder fat.

So, don't let your doctor freak you out when they talk about BMI. there are better ways to measure a person's health such as waist to hip ratio.



GIMME!
Posts: 111
3/10/12 2:50 P

BMI and body fat percentage are 2 completely different things. BMI is a calculation that only uses height and weight, someone else had posted the equation. If you had a low percentage of body fat, and had a lot of muscle (like body builders) you would be probably be considered obese (or at least overweight) according to this equation.

It's impossible to tell what your body fat percentage is from just your BMI.



UNIDENT
Posts: 33,498
3/9/12 5:52 P

Geez, you try to get pre-emptive, and someone still chimes in with the old canard about feathers. I guess WOF didn't bother reading the responses, just jumped on the saying.

It's always important to read a whole thread . . . :)



GEEKLING
Posts: 601
3/9/12 5:29 P

Take two identical boxes.

Fill one with lead, one with feathers.

Which one weights more?

That's what people mean when they use the expression that muscle weights more than fat. Not that a pound of this is heavier than a pound of something else...



WOFTRACKS
Posts: 61
3/9/12 5:11 P

12004Riley..

1 pound of muscle weighs 16 ounces, 1 pound of fat weighs 16 ounces, 1 pound of lead weighs 16 ounces, and 1 pound of feathers weighs 16 ounces.. regardless of statements to the contrary..



UNIDENT
Posts: 33,498
3/9/12 1:41 P

Because it's quick, easy, and does actually apply to populations. Probably more than 85% of people do find they get an accurate result from BMI calculations. If you're particularly muscular, yes it will show you as "obese". Rugby players and body builders frequently laugh at the BMI calculations for doing this to them.

But for most of us average people who aren't intensely muscular, it does work out pretty much about right.

Yes, if you pack on a little bit of muscle, and remove some fat, you can end up with the same BMI but a healthier body. That's why it's only a guideline, not an absolute.

And before anyone chimes in 'correcting' you that muscle doesn't weigh more than fat - muscle DOES weight more than fat in the same way that lead weighs more than aluminum. Cubic volume for cubic volume, which is the only way to use that statement, it is absolutely correct that muscle weighs more than fat.



ARMSPORTS
Posts: 1,310
3/9/12 10:03 A

A pound of muscle weighs the same as a pound of fat. Muscle is more dense so that pound takes up less space than the pound of fat.

Your doctor measuring body mass index. Weight (lbs) x 705 divided by Height (inches) squared. It does not take into account how much of your weight is in the form of either fat or muscle. It is a quick and easy (though not very reliable) indicator of your weight status.





12004RILEY
SparkPoints: (19,434)
Fitness Minutes: (28,248)
Posts: 229
3/9/12 9:44 A

Can someone please explain to me if muscle weighs more than fat--why do they use weight and height to measure body mass index and body fat? When I pinch my fat, I now get more skin or hard to grab fat due to the muscle. I am not losing any more weight--I hover around 143-145. However when I go to the doctor they just tell you what your body fat percentage is based on your height and weight. That doesn't seem right. Someone please clarify!



 
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