Having too much body fat puts you at risk for chronic disease and endangers your health. Your doctor may use body mass index, or BMI, which is a relationship between your height and weight, to estimate whether you're at a healthy weight or you're likely overweight or obese. This measure is crude, however, and not always a useful metric for highly athletic people. Alternatives exist, and they range from very accurate clinical procedures that may not be accessible to the average person to more imprecise handheld devices and body-fat scales.
Measuring Body Composition
A measure of your body composition distinguishes between fat mass and all other body mass, including bones, muscle, connective tissue and internal organs. Fat exists subcutaneously -- just under the skin; viscerally -- inside your belly and around your internal organs; and, essentially, in the bone marrow, central nervous system and organs, such as the heart and kidneys.
BMI tables only look at weight, not what type of weight you're carrying. A waist circumference measure helps you determine if you've got too much belly fat, which is a high risk factor for chronic disease, but it won't give you a precise body fat percentage.
DEXA or DXA stands for dual-emission X-ray absorptiometry. It's usually used for bone density testing, but it's also considered one of the most accurate ways to measure body fat. A DEXA scan is like getting a full-body X-ray, so it does deliver a small degree of radiation. You generally need a prescription and must visit a special clinic to have one done, and the whole work may cost hundreds or even thousands of dollars. For extremely overweight people, DEXA scans may be off by 3 to 5 percent.
Hydrostatic weighing involves submerging your body in water to determine your body density. Fat is less dense than muscle; thus, a person with a higher body fat will have a lower body density. You get into a dunk tank or pool, sit on the specially designed stool, and expel as much air as possible from your system. Many people find exhaling explosively to expel all their stored air challenging, so the results may be skewed slightly -- up to 2.7 percent. You must visit a special facility to have the test performed; it's quite involved and expensive.
Air Displacement Plethysmography
Air displacement plethysmography, available commercially via a machine known as the BodPod, uses some of the same principles of hydrostatic weighing, but measures displaced air rather than water. As long ago as 1999, researchers declared air displacement plethysmography as an accurate way to measure body composition. A later study published in a 2006 issue of Nutrition and Metabolism compared this method of measuring body composition against the DEXA scan and found that it measures changes in percentage of body fat similarly when someone is trying to lose weight.
Availability of the method is limited, though, as machines are expensive. Professional sports teams and athletes are usually the primary users.
Alternative Accurate Methods
Some methods are exceptionally accurate, but not practical for most people. MRI and CT scans, for example, can distinguish between different types of tissue fairly readily, but they aren't usually necessary, or feasible, outside the clinical setting. They can even identify different types of fat -- specifically visceral and subcutaneous fat.
CT scans may deliver a high dose of radiation, though. The equipment used for both types of test is quite expensive and may not be able to hold people who are massively overweight.
Fair Ways to Quickly Assess Body Fat
Skinfold calipers pinch certain sites, often the triceps, thigh, illiac crest and upper back, to estimate overall body fat, but they are subject to the skill of the person measuring and the quality of the calipers. Caliper testing is convenient and inexpensive, however, and a viable alternative to clinical measures.
Bioelectrical impedance scales, available commercially at retail stores, send a mild electrical current through your feet to measure your body's amount of lean mass, water and fat. Handheld bioelectrical impedance scales are also available. This method, though, is notoriously inaccurate, since your results can vary wildly according to your level of hydration.
More sophisticated devices send the current through your hands as well as your feet -- increasing the accuracy of the reading. Commercial machines, such as the InBody, are expensive, but nowhere near the cost of an MRI or CT scan machine. You simply step on the machine and hold the handles to get a reading, so it's quite noninvasive and may be more practical than more expensive methods for measuring body fat.
- American Council on Exercise: What are the Guidelines for Percentage of Body Fat Lost?
- Shape: The Best and Worst Ways to Measure Body Fat
- University of New Mexico: Understanding Body Composition
- CNN: Which Test Should I Trust When Measuring My Body Fat?
- Nutrition and Metabolism: Validity of Air-Displacement Plethysmography in the Assessment of Body Composition Changes in a 16-Month Weight Loss Program
- American Journal of Clinical Nutrition: Comparison of Air-Displacement Plethysmography with Hydrostatic Weighing and Bioelectrical Impedance Analysis for the Assessment of Body Composition in Healthy Adults
- Harvard School of Public Health: Measuring Obesity
- InBody: InBody 570