My own personal philosophy is to
1) get the BMI below 30, and then
2) focus on dropping body fat.
The problem is, how do you actually *measure* body fat? It's elusive.
Here's a nice synopsis on Wikipedia:
Here's an interactive web tutorial on some methods:
(Calipers aren't mentioned in this one.)
Leigh Peele wrote a very nice post about the topic:
The pictures in there are especially helpful.
James Krieger wrote a really really in-depth series of 7 columns about the relative pros and cons of different methods for figuring out % body fat:
The Pitfalls of Body Fat Measurement: Part 1
The Pitfalls of Body Fat Measurement: Part 2
"The bottom line is that underwater weighing can give good results when looking at group averages, but not so good results when looking at individuals. The sad thing is that underwater weighing is actually the best method out of the 2-compartment models. Other methods, including the Bod Pod, BIA, and skinfolds, are significantly worse."
The Pitfalls of Body Fat Measurement, Part 3: Bod Pod
"The Bod Pod does OK when looking at group averages, with some studies showing error rates of around 2%; however, other studies have indicated average error rates of over 5%. The individual error rate for the Bod Pod can be unacceptably high in some individuals, and the Bod Pod is horrible for tracking change over time. For these reasons I would recommend against using the Bod Pod as a body composition assessment tool. Hydrostatic weighing, despite some of its problems, is much more reliable."
BIOELECTRICAL IMPEDANCE (BIA)
The Pitfalls of Bodyfat Measurement, Part 4: Bioelectrical Impedance (BIA)
"BIA can be problematic because itâ€™s a prediction based off of a prediction, so the error gets compounded. When you look at group averages for BIA measurements, there tends to be bias, with BIA often underpredicting how much fat you have. As with other techniques, the individual error rates can get high, with some research showing error rates of around 8-9%. In fact, BIA doesnâ€™t do much better than BMI at predicting body fat in some cases. When it comes to measuring change over time, BIA can often underpredict the amount of fat loss, and the estimated change can be off by up to 8%.
For all of these reasons, I am not a fan of BIA for measuring body composition in individuals. If you are going to use BIA for tracking body composition over time, I recommend very long time intervals between measurements (at least 3 months, but 6 months is probably better), as the error rate for BIA can be larger than the changes in body fat in you see. Whatever numbers you do get using BIA, always remember they are very rough predictionsâ€¦.and I emphasize very rough."
The Pitfalls of Body Fat Measurement, Part 5: Skinfolds
"Like BIA, skinfolds can be way off when it comes to determining body fat percentage in individuals. When it comes to tracking change over time in groups, then skinfolds do pretty well. However, errors for tracking change in individuals over time can be up to 3-5%. Thus, if you are going to use skinfolds for tracking a single person over time, I recommend very long time intervals between measurements (minimum of 3 months but 6 months is better); otherwise, the error rate is higher than the change that you can see. In fact, I recommend against even calculating a body fat percentage. If skinfold thicknesses are going down, then you are likely losing fat."
DUAL-ENERGY X-RAY ABSORBANCE (DXA / DEXA)
The Pitfalls of Body Fat â€œMeasurement, Part 6: Dual-Energy X-Ray
"Despite the fact that DEXA represents a 3-compartment model, its error rates are no better than hydrostatic weighing, and in some cases is worse. Like other techniques, DEXA does well when looking at group averages, but not so well when looking at individuals. Individual error rates tend to hover around 5%, although some studies have shown error rates as high as 10%. When looking at change over time in individuals, error rates have hovered around 5% in some research, although other research has indicated DEXA to perform much more poorly. For these reasons, I do not recommend DEXA for tracking change over time in individuals. If you do use DEXA for tracking change over time, I recommend very long time periods between measurements (a minimum of 3-6 months), as you will need a minimum of a 5% change in body fat to reliably detect a true change in body fat in most people."
The Pitfalls of Body Fat Measurement, The Final Chapter
"Body fat testing isnâ€™t useless, but you do need to be careful in how you interpret the results." "For extremely obese people, I recommend simple body weight and circumference measurements."
According to him, hydrostatic weighing (NOT Bod Pod) and DXA are the most accurate. For following trends in % body fat he recommends calipers or if you're extremely obese, a plain old measuring tape.
For those of us who have weighed over 300 lbs, our bones are likely to be extremely dense (we're weight-lifting all the time just by walking around), so DXA is probably a better bet than hydrostatic weighing if you want to go either of those routes. I had a DXA scan done Jan '10 and at that point they said my bones were 25% denser than average.
I checked my Tanita BIA scale against the DXA % body fat measurement and discovered that if the scale was set to "athlete" mode the readings matched exactly.
So for now I'm just going with what my scale says, although BIA is not supposed to be very accurate. I'm averaging the daily measurements over at physicsdiet.com.
Although DXA is the technology used on the Biggest Loser to assess % body fat, many people still do not know about it. Here is a nice PDF article by Mary Oates summarizing its use, with pictures:
So, where can you get a DXA scan done?
If you live west of the Continental Divide you can probably find a sports medicine or wellness center that will do it, if you just use Google Maps and search for "DXA" and "DEXA."
If you live in the East, it seems to generally be a bit trickier. You'll need to contact the radiology or imaging departments at a few local hospitals to find out if they 1) have the right machines (GE Lunar or Hologic) and 2) if they will even do a whole body scan (many places just use DXA for bone density testing and either don't know they can or don't choose to provide whole body composition analyses).
Once you've found a place that knows what you are talking about, you can bring the above Oats PDF article to your physician and ask for an order for the scan.
There aren't any lists anywhere of facilities that provide DXA body composition analysis, so I've started compiling one. The list of places that have responded so far is here:
If you know of a place that does it, or any lists where people collect that kind of information, please have them fill out this form here, so we can find more places: