PT on the Net Research

Body Fat Assessment


I have a client who is 5'7 and 170 pounds. We've been working out for three months now, she's lost six pounds and is making excellent improvements in her cardio and resistant training. The problem is my body fat scale shows she's at 43 percent body fat. I know dehydration affects this, but I was thinking she's more in the 30 percent range. Any ideas what could be causing a discrepancy such as this? She actually has muscle tone and runs three to five miles now, so I'm a confused and she's frustrated!! Help!


Most practical methods of body composition assessment have a three to four percent error in their prediction of body fat. The method of body composition testing that is referenced in the question, bioelectrical impedance analysis (BIA), may have more error depending on some factors that will be outlined below.

The concept of BIA is based on the fact that the body contains intracellular and extracellular fluids that have electrical conductivity. A low level current flows through these fluids on a daily basis. Fat free mass contains much of the body's water and electrolytes, so it serves as a better conductor of the electrical current. Fat mass contains less water, and thus, it is not as good a conductor. The BIA technique is essentially an index of total body water, from which fat free mass is estimated.

Interestingly, studies that altered hydration status (hypo- and hyper-hydrated) have shown that electrolyte balance influences BIA measurements independent of fluid changes. These effects can be difficult to predict since fluid and electrolyte levels fluctuate throughout the day, and these also affect the ratio of intra- to extracellular water, which influences resistivity. The extracellular to intracellular water ratio is a factor that is known to limit the applicability of predictive equations. BIA does not accurately assess total body water and extracellular water when body water compartments are undergoing acute changes.

In addition, one study found (and others support) that BIA overestimates body fat when body fat percentage are less than 15 percent in men and less than 25 percent in women and underestimates body fat when body fat percentages are greater than 25 percent in men and greater than 33 percent in women. Using BIA to measure body fat percentage should be carefully interpreted when used on lean, overweight or obese individuals as well as when there are fluctuations in total body electrolytes and water.

When using BIA assessment, using a standard protocol that is repeated for each test is important to minimize error as much as possible. The following are guidelines to follow for BIA testing:

  1. Do not eat or drink within four hours of the test.
  2. Do not exercise within 12 hours of the test.
  3. Do urinate within 30 minutes of the test.
  4. Do not drink alcohol within 48 hours of the test.
  5. Do not take diuretics within seven days of the test.


  1. Sun, G. et al. (2005). Comparison of multifrequency bioelectrical impedance analysis with dual-energy X-ray absorptiometry for assessment of percentage body fat in a large, healthy population. Am J Clin Nutr; 81: 74-78.
  2. Kyle, U.G. et al. (2004). Bioelectrical impedance analysis-part I: review of principles and methods. Clinical Nutrition; 23: 1226-1243.