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Interestingly enough, I've heard that BMI mostly under-predicts problems. There are a lot more people at a higher body-fat percentage now than when BMI was deve

by likpok 9y ago

Interestingly enough, I've heard that BMI mostly under-predicts problems. There are a lot more people at a higher body-fat percentage now than when BMI was developed, meaning that BMI assumes that the average person is leaner than they are.

grzm 9y ago
I'm not sure I follow. BMI is a objective (if arguably problematic) measurement. How would changes in population demographics change it's predictive usefulness?
- likpok 9y ago
  BMI's predictive usefulness is based on people looking like when it was developed (it's an index, so you have to choose values for "normal", "overweight", etc). Back then, people in the "normal" band had more muscle and less fat than they do today. From what I understand, body-fat % is a much better metric for individual health outcomes, and BMI is a rough proxy to that. Taking two populations of the same BMI, one from say 40 years ago and one now, the one now will be fatter and less healthy. The core problem is that BMI is a proxy for what you're actually trying to measure: body composition. Height and weight gets you there on average, if you make some assumptions about the average composition. If average composition gets worse, BMI becomes more inaccurate.
- TheCoelacanth 9y ago
  BMI is objective, but it is used as a crude proxy for amount of body fat. If the average amount of lean body mass for a given height changes, it would change the correlation between BMI and body fat percentage, which would make it a less accurate proxy for amount of body fat when comparing to old BMIs.
  - grzm 9y ago
    I see what you're getting at. I don't see this as being driven by a change in population over time; this is just bad interpretation. As I mention down thread (and I'm sure you're aware), body builders often have BMI values that fall into the "morbidly obese" range. This is just due to the measure itself, not a change in population averages.
    - likpok 9y ago
      Bodybuilders often have BMIs that wrongly predict their health outcomes. They know this, though, and it's pretty obvious that they're not actually overweight. The problem is the average person, because BMI will list them as incorrectly healthy. If you're a "normal" weight, but 25% bodyfat, you're not in good shape. But you may think "oh, I'm okay" rather than "I really need to adjust". It seems obvious to me that BMI will tend to error by understating the problem. That means that if average BMI is increasing, the problem is getting worse much more rapidly than you might think.
  - takingflac 9y ago
    Why not just use body fat % measurements?
    - grzm 9y ago
      Primarily because they're hard to take accurately. Using calipers takes training; the bathroom scales which report body fat percentage are using an approximation based on conductivity which have varying results. BMI is quick, and I agree not a very good measure, particularly if only used on their own. For example, lean, muscular people can have BMI values which, outside of context, place them as morbidly obese. But, because they're expedient, they've continued to be used.

by likpok 9y ago

grzm 9y ago

I'm not sure I follow. BMI is a objective (if arguably problematic) measurement. How would changes in population demographics change it's predictive usefulness?

likpok 9y ago
BMI's predictive usefulness is based on people looking like when it was developed (it's an index, so you have to choose values for "normal", "overweight", etc). Back then, people in the "normal" band had more muscle and less fat than they do today. From what I understand, body-fat % is a much better metric for individual health outcomes, and BMI is a rough proxy to that. Taking two populations of the same BMI, one from say 40 years ago and one now, the one now will be fatter and less healthy. The core problem is that BMI is a proxy for what you're actually trying to measure: body composition. Height and weight gets you there on average, if you make some assumptions about the average composition. If average composition gets worse, BMI becomes more inaccurate.
TheCoelacanth 9y ago
BMI is objective, but it is used as a crude proxy for amount of body fat. If the average amount of lean body mass for a given height changes, it would change the correlation between BMI and body fat percentage, which would make it a less accurate proxy for amount of body fat when comparing to old BMIs.
- grzm 9y ago
  I see what you're getting at. I don't see this as being driven by a change in population over time; this is just bad interpretation. As I mention down thread (and I'm sure you're aware), body builders often have BMI values that fall into the "morbidly obese" range. This is just due to the measure itself, not a change in population averages.
  - likpok 9y ago
    Bodybuilders often have BMIs that wrongly predict their health outcomes. They know this, though, and it's pretty obvious that they're not actually overweight. The problem is the average person, because BMI will list them as incorrectly healthy. If you're a "normal" weight, but 25% bodyfat, you're not in good shape. But you may think "oh, I'm okay" rather than "I really need to adjust". It seems obvious to me that BMI will tend to error by understating the problem. That means that if average BMI is increasing, the problem is getting worse much more rapidly than you might think.
- takingflac 9y ago
  Why not just use body fat % measurements?
  - grzm 9y ago
    Primarily because they're hard to take accurately. Using calipers takes training; the bathroom scales which report body fat percentage are using an approximation based on conductivity which have varying results. BMI is quick, and I agree not a very good measure, particularly if only used on their own. For example, lean, muscular people can have BMI values which, outside of context, place them as morbidly obese. But, because they're expedient, they've continued to be used.