Paleo lifestyle challenges a lot of CW, especially on diet and fitness regimen. That understood, is BMI still a valid rubric, or are there better ways to understand ratios of bone, muscle, and fat when wanting to improve fitness?
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Wikipedia has a good discussion of the drawbacks and limitations of use of the BMI.
Body Mass Index correlates with body fat percentage quite well across the population however there is a great deal of variability between individuals particularly at the intermediate ranges of the BMI.
This study is discussed in the Wikipedia article: Accuracy of Body Mass Index to Diagnose Obesity In the US Adult Population. The discussion is worth reading.
Their main findings were that for a BMI of above 30 almost everyone had too much body fat. At BMIs below 30 the variability is much greater. As you can see from the graph above many men with a BMI of 25-30 have low body fat while many men with a healthy BMI of 20-25 have unhealthily high levels of body fat.
They state that in men the BMI correlates better with lean body mass than with fat. This could explain the findings that people with a BMI of 25-30 seem to live longer than those with a BMI of 20-25. Having more lean body mass like muscle is a good predictor of health. The BMI also became less accurate as people got older.
This figure below is Figure 3. from the paper shows the variation in body fat percentage among men and women with a BMI of 25. As you can see there is a wide range of measured body fat levels at the same BMI.
You could have a BMI of 28 and have a healthier level of body fat than someone else with a BMI of 22.
Personally I think it is useful as a population measurement. It is the only really simple measure of body fat that can look at trends over time, height and weight measurements have been taken for a long time.
It is understandable that people want a simple measure to put people into categories with but I think using it to measure individuals is arbitrary and can be unhelpful especially when people are miss-classified. Simply lowering your BMI a few points is unlikely in itself to show that you have improved your health unless you know that your body fat % has also lowered and not your lean body mass. I think too much emphasis is placed upon it now with out enough discussion of its limitations.
I'm not sure that this is going to provide any kind of an answer at all -- but from my perspective, whether it's pounds on a scale, or BMI, etc., I think that what is -truly- relevant when considering 'fitness' is whether or not we are able to do the things we want to do in the way that we want to do them.
If your body does what you want it to do both when you're relaxing AND when you're pushing it to the limit, then you've got a good handle on functional fitness.
As much as I hated to admit it, I had to break down a couple of years ago and face the fact that, at the size that I was at, there was NO way that I was going to be able to get out of the mobility chair I was parked in... the ONLY way I was going to be able to do it was to get my body down to a size where I could do things again. I'm not all the way there -- but I can tell you that the scale and the doctor's BMI calculator have been a LOT less informational for me than whether or not I could walk to the end of the hall -- then up a flight of stairs -- then around the block -- then on a real trail-hike.
If your fitness level is such that you're having to measure your improvement by number rather than by increasing capability -- I guess I don't really see where there is any benefit to the measurement. So I guess, for me, BMI is not really a valid measure of fitness, from a primal perspective.
I liked this article, and think it sums up the limitations of BMI pretty well. Do You Believe in Fairies, Unicorns, or the BMI?: http://www.maa.org/devlin/devlin_05_09.html
BMI -- if it has any usefulness -- has applications to groups as a whole. Even comparing between societies and various ethnicities makes BMI, which is nothing more than a glorified height-weight ratio, of dubious meaningfulness. On an individual basis, it's no more useful than your basic bathroom scale and the height-weight charts. Getting an accurate measurement of body fat percentage seems more important.
Dorian Yates in 1993. 5'10", 257lbs. BMI = @37: