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Which test should I trust when measuring my body fat?

By Khanjan, Atlanta

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I am a 24-year-old female who is a fitness enthusiast. I have been trying to lose some weight. My weight is 113 pounds ( it was 120 and I lost 7). Recently I got my BMI, lipid profile, cholesterol, etc., tests done and all of them are normal. I have a normal BMI of around 22. However, when I took a body fat test online, it says I have body fat of 38% and I am obese. I am extremely confused, as to which metric to trust and what should be my ideal weight (I have a small frame). Should I be around 100 pounds?

Expert Bio Picture

Diet and Fitness Expert Dr. Melina Jampolis Physician Nutrition Specialist

Expert answer

This is a very good question.

First let me say that body fat is a much better indicator of health than BMI (body mass index), which is simply a ratio of weight (in kilograms) divided by height (in meters) squared and does not take into account what our bodies are actually made of, specifically: lean body mass like muscle, bone and water versus fat. Online body fat calculators vary, so without knowing which specific calculator you used, I will comment on them in general as well as go over several other methods of body fat testing.

Most online body fat calculators include a measurement of waist size, which can be very valuable when it comes to assessing overall health. A waist size of great than 35 inches in women and 40 inches in men suggests excess belly fat and could put you at risk for diabetes. The online calculators I found also include other measurements throughout the body including hip, neck, arms, thighs and wrists. These measurements, along with your gender, height and weight, can provide a general idea of body fat measurement but they are not an actual measurement.

As with BMI, they do not take into account body composition. In your case, since all of your blood tests look good and you are a fitness enthusiast, it may be worth you investing in a more direct and accurate measure of body fat if you want to set a realistic and healthy goal (although it is important to note except for autopsy all methods utilize some computation but they are based on more specific measurements). According to the American Council on Exercise (ACE), healthy female athletes body fat percentage range from 14-20, fit women 21-24, average women are 25-31, and greater than 32% body fat is considered obese.

Here are four different types of tests that you might consider.

1. DXA Scan (Dual-emission X-ray absorptiometry) --This dual X-ray system (known widely for its use in bone density testing) is considered the preferred method for body composition testing (the most accurate, which other tests are compared to). It is one of the most direct BC testing methods but it can have limitations in the obese as many machines are much less accurate in thicker people (obese) and can have a 3-5% precision error. The iDXA by GE Healthcare was designed to perform obesity research and is one of the most accurate and precise, particularly in thicker, heavy people. It is also the only DXA scanner FDA-approved to quantify dangerous deep belly fat, which is important for true health assessment. Limitations to DXA Scan testing for body fat assessment are the need for a doctor's prescription to get a DXA scan done, less availability than other testing methods discussed below, and expense - you will have to pay out of pocket costs ranging from $59 to $399.

2. Bioelectrical impedance - The principal behind this measurement is that electricity flows differently through fat and lean body mass. By measuring the resistance to the flow of a very small current of electricity through different parts of your body (most commonly the arms, legs or both), body fat percentage is estimated. The problem is that many of these devices don't measure belly fat, which is the most dangerous. So if a man has thin legs and a big belly, their body fat measurement will not be accurate. In addition, as with all body composition testing devices, they are sensitive to hydration status. So if you measure your body fat after a workout (when most people are dehydrated) or first thing in the morning, the test will be less accurate and will overestimate your body fat. The most accurate time of day for testing with these units (and any other body composition testing methods) is late morning and before you exercise. Hydration levels may also be affected by excess alcohol intake, medication, and PMS.

The benefits of this testing method are that many of these scales are very inexpensive, do not require special training, and can provide a fairly good week-to-week comparison as long as measurements are done the same time of day and not after exercise. Tanita, a leading manufacturer of bio-impedance scales, says on its website that "independent research has found their body fat monitors to be accurate within +/- 5 % of DXA and repeatable to within +/- 1% when used under consistent conditions."

If accuracy is critical, you may want to consider tracking down a more accurate impedance device. GE Healthcare's InBody bio-impedance products are one of the only devices that measure leg, arm and belly fat. They are currently being used by federal government agencies, the armed forces, and Division 1 collegiate athletic programs. According to Greg Dudra, metabolic health specialist, GE Healthcare Lunar's line of devices take measurements via eight locations on the body, which ensures precision and they use multiple frequencies which contributes to accuracy.

3. Underwater weighing (also known as hydrostatic weighing) - This was considered the gold standard before DXA scanning and is based on the determination of body density by measuring the weight of the body outside the water, the weight of the body under water after exhaling completely, and the density of the water. The resulting body density is then put into an equation to determine body fat percentage. The limitations of this test which can affect the accuracy include the following: Many people have difficulty exhaling completely before being dunked under water, people can hold up to 4 liters of gas in their system at any given time, and bone mineral density varies considerably from person to person (big-boned people may have more air in their bones, which can affect density measurements). This test is fairly expensive and requires considerable equipment to perform. The measurement of error of this test (an indicator of accuracy) is approximately 2.7% according to the American Council on Exercise.

4. Skinfold caliper testing - This type of testing is generally the least expensive and is widely available. It involves pinching specific areas of skin (and fat) throughout the body and converting the results into an estimate to body fat percentage. While it does provide regional body fat assessment, it does not measure deep belly fat, which is very important in terms of health. There is considerable variability in general as to where people carry extra fat, and it is more difficult to get accurate measurements in the extremely obese. In addition, the test is very dependent on the person doing the testing and the quality of the calipers used. In the hands of a qualified person, the test can provide a good relative measure of body fat percentage (for example, when you are starting an exercise program and after 6-8 weeks of training). ACE has good tips to improve the accuracy of this test.

Other newer methods not discussed in detail for body fat testing exist including Bod Pod (air displacement plethysmography - accurate, limited availability), MRI (expensive, accurate), CT scan (accurate and quantifies deep belly fat, expensive, higher doses of radiation exposure) and ultrasound (newer promising method with less research).

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