Most Accurate Body Fat Measurement? Dexa Vs Bio Impedance


Most Accurate Body Fat Measurement? Dexa Vs Bio Impedance

December 1, 2015 Posted by Andy Campbell in HealthInstructional

When you begin a new nutrition and exercise program it can be motivating to have a body composition scan done at the start and at intervals throughout your program so that you can track the progress you are making and also to hold you accountable. We look at two of the most accurate body composition scans available to the general public Bio-impedance and DEXA scans.

BIO-IMPEDANCE

WHAT IS IT?

Bio-impedance is a way of measuring body composition that is easy to use, quick, non-invasive and relatively inexpensive.

The machine that is used is much smaller than what is used for a DEXA scan and produces relatively accurate results making it popular in gyms and health clinics.

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HOW DOES IT WORK?

 

 

There are a number of different bio-impedance machines available. The user can either chose a hand held machine, stand on a platform while holding adjustable hand grips or can be lying down with electrodes attached to various points of the body.

The participant’s height, age, gender, weight and often other physical characteristics such as body type, physical activity level and ethnicity are then entered into the computer.

DIFFERENT TYPES OF MACHINES

SINGLE FREQUENCY BIO-IMPEDANCE ANALYSIS (SF-BIA)

 

This is the most basic type of bio-impedance where a single 50 kHz current is passed between the various electrodes placed on the body.

This type of BIA is low cost, portable and easy to handle and is commercially available.

SF-BIA does not strictly measure total body water, instead it uses prediction equations and algorithms to make an estimate of fat-free mass and total body water.

These algorithms have been devised from generally healthy subjects so the one algorithm is not applicable for all subjects and there will be a choice of body types to choose from on the machine.

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MULTIPLE FREQUENCY BIO-IMPEDANCE ANALYSIS (MF-BIA)

Multi-frequency BIA (MF-BIA) uses a range of seven different frequencies, to estimate fat-free mass, total body water, inta-cellular water and extra-cellular water.

Generally multi-frequency impedance does not give a more accurate overall body composition compared to single-frequency impedance, but it is does more accurately measure TBW and ECW because of the greater range of frequencies used.

BIO-IMPEDANCE SPECTROSCOPY (BIS)

Bio impedance Spectroscopy uses 256 frequencies and has the capacity to more accurately measure the difference between ICW and ECW, giving a more accurate estimate of body cell mass.

This means Bio-impedance Spectroscopy does not need to rely on statistically derived, population-specific prediction equations like the other Bio-impedance models do.

HOW ACCURATE IS IT?

BIA is only accurate so long as the correct details are entered for the participant’s age, gender, level of physical activity, relative level of body fat and ethnicity.

Other factors that may affect the results of a bio-impedance test include eating, alcohol and fluid intake and physical activity performed on the day of the evaluation.

BIA results will also be inaccurate if the participant being tested is dehydrated, has excess water retention or is using diuretics.

So for these reasons participants are advised to avoid alcohol for at least 8 hours before the test and to not eat or drink any water for 4 to 6 hours beforehand.

Participants are also advised to avoid high or moderate physical activity on the day of the test.

The accuracy of some of the more commercially available devices is sometimes questionable so while they are great for tracking progress in body composition over time they are not the most accurate single measurement machines.

COST?

$30 – $60

TIME?

5 minutes

DEXA

WHAT IS IT?

A DEXA scan or Dual energy X-ray absorptiometry is a way of measuring bone mineral density where two xray beams with different levels of energy are aimed at the participant.

A DEXA scan is also the most accurate method for determining total body composition and body fat percentage.

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HOW DOES IT WORK?

The participant lies down on a flat surface and the machine passes over them. The DEXA scan machine sends two different x-rays through the participant’s body and the density of the body part being measured influences the amount of X-rays that pass through it to a detector. This information is then sent to a computer which creates an image of the body being scanned and determines which parts of the body are bone, fat, water, muscle ect. Depending on their density.

EFFECTIVENESS?

A DEXA scan is the most accurate measure for determining body composition. Participants are advised to fast overnight and avoid drinking any water before the scan as this can impact on the results.

COST?

Start around $80

TIME?

Around 10-15 minutes for a scan

Written by Andy Campbell

REFERENCES:

  1. Ling C, de Craen A, Slagboom P et al. Accuracy of direct segmental multi-frequency bioimpedance analysis in the assessment of total body and segmental body composition in middle-aged adult population. Clinical Nutrition. 2011;30(5):610-615. doi:10.1016/j.clnu.2011.04.001.
  2. Donadio C, Halim AB, Caprio F, Grassi G, Khedr B, Mazzantini M. Single- and multi-frequency bioelectrical impedance analyses to analyse body composition in maintenance haemodialysis patients: comparison with dual-energy x-ray absorptiometry. Physiol Meas 2008; 29: S517-S524.
  3. Dehghan M, Merchant A. Is bioelectrical impedance accurate for use in large epidemiological studies?. Nutrition Journal. 2008;7(1):26. doi:10.1186/1475-2891-7-26.
  4. Van Loan MD, et al. Use of bioimpedance spectroscopy (BIS) to determine extracellular fluid (ECF), intracellular fluid (ICF), total body water (TBW), and fatfree mass (FFM). pp. 6770. In Ellis, K. (ed.) Human Body Composition: In Vivo Measurement and Studies. Plenum Publishing Co., New York. 1993.

 

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