Measuring and Tracking Body Composition

Continuing from the last blog, let's look at some of the practical aspects involved in both measuring and tracking changes in body composition or body fat percentage (BF%). I will be discussing a number of methods but it's important to realize that none provide more than an estimate. Every method has pros and cons.

From a tracking standpoint, changes are more important than absolute numbers. Since they can also be relevant, I will also be looking at some non-body composition methods of tracking progress in this blog and will make recommendations for combinations of those methods that can be used.

True Body Composition Measurements

The first set of methods I want to discuss are true body composition measurements in that they measure (or estimate) some aspect of actual body composition. They vary in their accuracy, difficulty of use and availability and I will describe them more or less in order from least to most complex.

Body-Mass Index (BMI)

The BMI is a fairly old measurement which relates an individual's body weight to their height. More technically BMI is defined as weight in kilograms divided by height in meters squared. For decades BMI has been used to indicate general health or some kind of ideal weight and insurance companies use some version of it to determine how much to charge you per month.

A high BMI tends to correlate with health risk and a BMI greater than 25 kg/m2 is defined as overweight and a BMI greater than 30 kg/m2 is considered obese. Very low values are equally problematic with a value below 18.5 kg/m2 is considered unhealthy or malnourished (possibly indicating an eating disorder or wasting disease). Between 18.5 and 25 is considered optimal. It's critical to note that these are only averages and it's been established that individuals with a high BMI can be healthy while those in the optimal range may be unhealthy.

Part of the reason for this is that BMI is not strictly speaking a measurement of BF% and doesn't indicate body composition or how much fat or LBM someone is carrying. Two females who are 5'7" tall and who weigh 150 pounds have the same BMI. If one is an athlete with 20% body fat and the other is inactive at 35% body fat, not only is their body composition different but so are their relatively health risks. It's also not uncommon for active individuals, generally males, to be told that they are overweight due to a high BMI score although they are relatively lean and simply carry more muscle mass. This leads to active individuals to often suggest that BMI should be thrown out for being useless but this is an over reaction. BMI was never meant to be used in an athletic population.

In the general public, it is simply not that common to find people with high BMI who also have a low BF% although people with a low BMI often have a fairly high BF% (they are often called skinny fat). It's also possible to have a high BMI and be metabolically healthy or a low BMI and be unhealthy. But no body composition method is perfect and BMI is not useless, its limitations simply have to be acknowledged. For that same group, BMI will give at least a rough indicator of general health risk along with giving a fairly easy way to track changes from diet and exercise (technically since height is not changing, tracking body weight would provide the same information).

And while BMI has primarily been used to track overall health trends, it turns out that it can give a rough estimate of BF%. A calculator to determine BMI can be found here:

That BMI value can be used to estimate BF% here:

While the above links may be inappropriate for athletic individuals, I think it is probably the easiest approach for people first starting out. It's quick and easy, provides a good starting point and can be used to track changes over time. Once someone has been working out consistently for 6+ months, I would not consider BMI to be accurate and they should use another of the described methods.

Tape Measure/Circumference Measurements

Although decidedly low-tech, it is actually possible to get a decent estimate of BF% with nothing more than a tape measure. The military has often developed a lot of these equations since they need to be able to measure a lot of people quickly and easily. There are online calculators that estimate BF% in this fashion that generate results that are at least similar to more complicated methods which can be found here:

Even if they are not used to track body composition per se, tape measure measurements still provide another way to track general progress while dieting (or attempting to gain muscle). During a diet, a decrease in circumference measures (i.e. diameter of the hips or arms) generally indicates body fat loss and it's not uncommon to see this occur even in the absence of much weight loss. Muscle is denser and takes up less space than body fat so someone gaining some muscle while losing fat should still see a reduction in their tape measure measurements. Taking a variety of measurements including arms, bust, waist, abdomen, hip and thighs can provide a general indication of whether fat is being lost and specifically from where. Even a single trouble spot (i.e. arms or thighs) could be tracked in this fashion.

Whether used for BF% estimation or just as a general tracking method, the tape measure is not without problems. First and foremost it's critical to always measure at the same spot, around the largest part of the bust or halfway down the thigh or what have you or the values can't be compared to each other. This is not always easy and even slight differences in where the measurement is taken can make them inaccurate. It's also important to at least try to pull the tape measure to the same tension every time, neither too tight nor too loose. This can also be very difficult to do and there are tape measures with a spring on the end such as the Gulick II which will improve the accuracy of measurements.

Waist/Hip Ratio (WHR)

The waist/hip ratio is exactly what it sounds like, that is the ratio of the waist (measured with a tape measure at the narrowest part) and the hips (measured with a tape measure at the widest part). Technically the WHR ratio is not a measure of body composition but a measure of body fat distribution and health risk. On average, women tend to have a lower WHR than men but WHR can go up with menopause, PCOS/subclinical hyperandrogenism and in obesity. A WHR calculator can be found here:

As with circumference measures, it's important to not overtighten the tape measure.

Skinfold Calipers

Possibly the most commonly used method of BF% measurement are skinfold calipers, a small plastic device that is used to squeeze fat at different parts of the body. The measurements go into an equation that then estimates BF%. A variety of sites ranging from 3 to 7 (or more) can be used and numerous different equations exist. Typical sites for women's measurement are the back of the arm, chest, iliac crest (above the hip) and thigh. Calipers give a BF% estimate that is usually close to much more high-tech methods at least in the hands of a trained user. Trained is a key word here as using calipers correctly takes a good deal of practice and many do not have it (this can be a big problem at commercial gyms with a high employee turnover). My general experience is that most trainers are hesitant to grab as much fat as they should. Many women have a thigh skinfold that is nearly impossible to measure accurately in many cases. This can lead to drastically underestimated BF% values.

While generally accurate when used properly, calipers do have an inherent error of about 2-3% in either direction (note that all methods have some degree of inherent error). This means that they may not be able to pick up smaller changes in BF% to begin with. Fat cells do store water and changes in water retention (that will usually show up as changes in scale weight) can impact on skinfold measurement. The equations can be problematic as well. A host of assumptions are being made about bone density (which differs between women and men, can vary with training, etc.) which can cause them to give some strange values, even if the skinfold measurements are accurate. The equations will occasionally put women well below the 10% lower limit for essential fat, men have been estimated at 1-2% and, due to differences in bone density, black male athletes are occasionally given a negative number. Although it doesn't give an estimate of BF%, some recommend just tracking the skinfold changes. If the thigh skinfold goes down from 25mm to 22mm, fat has been lost.

Other Tracking Methods