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Body Mass Index

  1. Aug 30, 2009 #1

    G01

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    According to the BMI, I am obese...

    The truth is, I am not obese. I am in better physical condition that most of my friends. Since moving to Boston, I don't use a car. I bike and walk everywhere I need to go sometimes with the help of the T. I also train in Judo ~3 times a week, which is one of the best full-body workouts I have ever participated in.

    I am in good shape and I eat healthy. My blood pressure is well within the normal range. I don't have six pack abs, but I don't have a beer gut either!

    I admit I am a big guy (6ft, 215lbs). I won't be starring in an xkcd comic strip anytime soon. However, I have a higher proportion of my body weight in muscle mass. People who have tried to guess my weight usually guess around 185-195lbs by the way I look.

    Can someone in the medical or bio community tell me why the BMI is used when it only accurately describes people who are of average build and do no physical activity whatsoever?
     
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  3. Aug 30, 2009 #2
    Because it is used to describe the average POPULATION of people... possibly? By the measurements you gave me you're not obese assuming you're exacly 6' you're BMI is 29.2. Obesity starts above 29.9.

    Indication that you are obese is your body fat percentage. If you have over 25% body fat then THAT puts in you the obese range. Obesity is when the body fat accumlated on your body affects your health negatively.

    Although just, a small point, because you THINK it's healthy and you're in perfect shape means nothing. Having low body fat percentage is still unhealthy and dangerous.

    The BMI is not used to make these judgements. It's used to PROXY. You'll never find a doctor take your BMI and claim you're obese...
     
  4. Aug 30, 2009 #3
    I don't actually think we have the medical community to thank for BMI. I think I remember reading that it was once suggested as a hard and fast (and really rough) indicator of obesity and then the popular nutrition movement and media just ran with it. I don't know if it's actually used in the medical community in a serious way at all. It's more fad diet books and stuff which don't have a clue what they're talking about anyways (and government nutritional prescriptions which are the same difference). Could be wrong though. I'm in a quasi-similar boat. I definitely have a bit of fat but I'm also just very large. Like I have very broad shoulders and thick arms (which are pretty muscular). According to BMI I'm on the high end of obese but my body fat index isn't nearly that high.
     
  5. Aug 30, 2009 #4

    f95toli

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    But as Sorry! has already pointed out: No doctor would ever use BMI to determine whether or not you are overweight and need to loose weight.
    BMI is certainly useful in that a high BMI is ONE risk factor for many medical conditions (e.g. type II diabetes) and is easy to calculate. However, all a high BMI means to YOU is that it might be worth talking to your doctor.
    As far as I know the only time BMI is used "scientifically" is for epidemiological studies; i.e. to correlate obesity with certain medical conditions.

    There are certainly people that have BMI much higher/lower than "normal" that are still healthy (professional athletes would be an obvious example) and for those BMI is a useless measure, but for the vast majority of us BMI is still a useful indicator.
     
  6. Aug 30, 2009 #5

    Astronuc

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    At my heaviest, the BMI would have indicated that I was over-weight. However, my fat to body content was low (~5-6%) because I ran long distance (and sprints), played soccer, rode a bicycle for transportation, and did weight-lifting/training. The BMI for a lean muscular person is the same for someone of the same height and mass but higher body fat content.

    Sparring or a full-body workout in any of the martial arts is a great workout.
     
  7. Aug 30, 2009 #6
    My body fat percentage is less than 10% and it tells me I'm overweight.
    I don't see the point of it, if it can be so wrong for so many people. Body fat percentage however, isn't. Why use anything else?

    I like the old fashioned method of finding out if you're overweight; looking in the mirror.
     
  8. Aug 30, 2009 #7

    Moonbear

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    Sorry! has it right. BMI is used on a population basis, where those who are leaner than it predicts and those who are fatter than it predicts balance out. It's more of an epidemiological tool than useful on an individual basis. That it is useful to tell you anything about an individual is a myth perpetuated by websites with BMI calculators. It's probably useful to some extent for those who really are obese and in denial about their weight, but if you work out and are generally lean and muscular, you can safely ignore it.
     
  9. Aug 30, 2009 #8

    G01

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    That's makes sense. Though, I still think some people give the BMI more credit than it's worth.

    The reason I calculated my BMI in the first place was because a local school is going to start keeping track of BMI to determine whether a student is overweight or obese and inform and guide the parents based on this data. I wanted to see what it would say about me. Unfortunately, this leads me to think that a bigger, yet athletic student may wrongly be told that they are overweight or obese and needs to change his or her body. I think this is probably the last think we want to do to adolescents and children who already are overly concerned with how they look.

    Do you guys think the BMI should be used in a situation like this? I think if the school really wanted to positively enforce a healthy lifestyle they would do so through lunch offerings, P/E classes, and by monitoring more accurate measures of one's health. It seems to me the school wants to say they support healthy living while avoiding putting more money into the cafeteria and P/E.
     
    Last edited: Aug 30, 2009
  10. Aug 30, 2009 #9

    Astronuc

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    No. If the school is promoting a healthy lifestyle, using the BMI is the wrong criterion. It should be based on cardiac efficiency, HDL/LDL levels, triglycerides, blood pressure, pulse rate when resting, and perhaps other factors.

    BMI is just an easy number to use - but its application here would be incorrect for the reasons stated.

    My current BMI is 25.7, but I'm not overweight, although the BMI says I'm slightly overweight. I just had a physical and the HDL/LDL numbers and triglycerides were fine, and my blood pressure was ~110/70, which it has been for 40+ years.
     
  11. Aug 30, 2009 #10
    When I took Kinesiology we learnt abit about BMI. It is useful for your average sedentary person. Any person who takes part in atheletics or even is just gifted genetically with high muscle mass it will be skewed. It originally was not meant to be used for individuals although as I mentioned before it CAN be useful for the AVERAGE sedentary person.

    Doctors do use it to measure your growth as a child comparative to other children but thats a different BMI. The one you are using is for Adults only. When you get a physical by your doctor you can see them mark down the points on a chart.

    Anyways you can ask your doctor or go to a gym to measure your body fat percentage. Some methods are more accurate than others obviously but I would say that none are way off. Most just mess up if you have to low a body fat percentage. For me my body fat percentage was 11.6%. Since I used to play baseball and run alot.
     
  12. Aug 30, 2009 #11
    At the peak of fitness, running 5 miles per day and lifting weights, my BMI never dropped below 25.5 - about 30 now.
     
  13. Aug 30, 2009 #12

    chroot

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    It is widely known that many professional athletes are considered "obese" by the BMI. As others have said, it is really only useful as a population statistic, and perhaps as a "evidence-based" starting point for discussion between a doctor and patient.

    - Warren
     
  14. Aug 30, 2009 #13

    Moonbear

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    Given the limitations of what a school nurse can likely evaluate, using BMI is probably better than saying, "we looked at your kid and think he's fat" as a starting point for educating parents about their child's health and nutrition. As long as the letters sent explain that it is not foolproof, but rather worth considering a conversation with one's personal physician, it's probably better than any other measure they could have used. You don't get too many body builders in high school, and even the athletes still tend to be lanky at that age, so other than a few kids on the football or wrestling teams, it's probably going to do a decent job of identifying at-risk kids in that age group.
     
  15. Aug 30, 2009 #14
    Is there another more discerning indicator of obesity that relies only on basic measurements (not requiring special equipment) such as the BMI?
     
  16. Aug 31, 2009 #15
    you could measure skin folds with calipers and run it through the equation to get %BF. you need somebody with experience doing it though, and maybe keep that job to the nurse.

    still not sure how well it'd work with kids, i think all the equations i've seen were for adults. also, they all need to go through a bit of a chubby phase near puberty to spur its onset. somewhere around 11 or so, i was eating a pack of crackers a day after school and put on some chub. after going full hormonal, i was skeletor again. stuff like this has to be well thought out. you don't really want them fat in elementary school because it will help spur early onset of puberty. and then you probably don't want to keep them skinny forever to delay it.
     
  17. Aug 31, 2009 #16

    Moonbear

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    And I'm not really sure how accurate that method is either.

    Yes, boys tend to get a little chubby just before that first big pubertal growth spurt. However, even so, that little chubbiness is still not even as overweight as a lot of the kids I see walking around every day who are not peri-pubertal (or at least shouldn't or wouldn't be if they weren't so overweight).

    Again, it highlights the importance of parents being informed to consult a physician. Any kind of health screening in school is just that, a screening. It's not much different than using the vision and hearing tests they use in schools, which may indicate a problem, but may not indicate a vision problem (i.e., a dyslexic child might also have trouble reading an eye chart in the right order), or it might indicate the child is just having a bad day following instructions. With all of them, it's better to include more false positives than false negatives and then have a professional make the final determination. Basically, it's some way to open up a dialogue with the parents about their children's health.

    Of course, I would hope it would all go along with teaching healthy habits at school too...serve nutritionally balanced, proper serving-size lunches at school, teach children what serving sizes look like, and reinstate phys. ed. classes if they have been eliminated.
     
  18. Sep 1, 2009 #17
    CDC sounds like they're OK with it

    http://www.cdc.gov/healthyweight/assessing/bmi/childrens_BMI/about_childrens_BMI.html
     
  19. Sep 1, 2009 #18
    Academy of Pediatrics. Children are completely different than adults in all terms.

    The skin-fold method is slightly accurate but I wouldn't use it as an adult unless BF% is <5%. During my kinesiology regiment I set up for myself we just used hand-held devices that shoot electrical impulses through your body and measure how fast it comes back. These can be very accurate depending on the quality of the device. If you want to know without a doubt in your mind you can always do the submerging in water where they make you exhale ALL your air and take measurements that way. Or you can use something call DEXA where they take X-Ray images and measure that way. The last 2 methods are extremely expensive to have the equipment so normally only universities have them. However I'm sure you can call up a local university and ask if they do fitness testing and go in.
     
    Last edited: Sep 1, 2009
  20. Sep 1, 2009 #19
  21. Sep 1, 2009 #20
    That's correct muscle weighs more than fat per cubic inch, or I forgot how they compare the too.

    I've heard that bringing in waist measurement into the formula helps. I've heard that underwater weighing is more effective. Then if you're really serious, opening you up after you die is the most effective :rolleyes:
     
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