Understanding the DEXA results

“Persistence can change failure into extraordinary achievement.”

– Marv Levy

Understanding the DEXA results

How a DEXA works
The DEXA is used to measure your body composition The repeatability for percent fat measures is typically better than 1% (standard deviation) and 2% (coefficient of variation) for total fat and lean mass measures. (Leonard et all 2009)

The radiation dose of a DEXA whole-body scan is very low, and very small in comparison to other X-ray imaging modalities. One whole-body DEXA is <5 μSv (microsieverts). This is less radiation than you will experience on a flight from Sydney to Melbourne or from walking around in the city (8 μSv = 1 day’s background radiation). (Snijder et all 2002)

As safe as it is, please note, it is not recommended having a scan while pregnant. Having a scan while breastfeeding does not have any adverse effects.

Effects of food and fluid intake
DEXA technology assumes that soft tissues are normally hydrated for accurate partitioning into fat and lean tissues (Laskey,1996; Plank, 2005). It assumes that there is a constant and uniform fat-free mass hydration of 73% (Pietrobelli et al.,1998) as X-Ray’s cannot scan for fluid.

The intake of food and fluid prior to the scan has been shown not to alter bone mineral density and fat mass measurements. (Vilaca et al., 2009).

Studies do show that there can be small variations of lean tissue with diet modifications:

A low or no CHO diet decreases lean mass analysed through the scan (likely related to glycogen depletion)
total energy/calorie intake of less than the RMR may result in a decrease in lean mass due to a decline in RMR and a starvation response
Small changes in fat free mass (this is everything in your body that is not fat) can be affected by the fluid content within the body through sweat loss or drinking water (and in the case of exercise, the fluid shifts between body compartments)
In regards to females, the timing of the menstrual cycle will not affect the results.

Effects of training
Training has been found to have trivial changes in the total mass and lean mass results. However, there have been some studies which show regional changes in lean tissue reflected from the shifts in blood volume to the working muscle as a result of exercise.
(Montain, 1992; Rowell, 2004).

What it all means
To maximise the reliability and repeatability of whole body scans, follow-up scans should be performed with similar pre-testing conditions:

same time of day
food / fluid intake (amount, composition and timing relative to the scan)
training intensity / duration / timing

LUKE MICHAEL
BAPPSC(EX. & SP.SC)
accredited exercise physiologist

References

Nana A, Slater G, Stewart A, Burke L (2014) Methodology Review: Using Dual-Energy X-Ray

Absorptiometry (DXA) for the Assessment of Body Composition in Athletes and Active People. International Journal of Sport Nutrition and Exercise Metabolism, 24, 198 -215

Laskey, M.A. (1996). Dual-energy X-ray absorptiometry and body composition. Nutrition (Burbank, Los Angeles County, Calif.),12, 45–51

Leonard CM, Roza MA, Barr RD, Webber CE. Reproducibility of DXA measurements of bone mineral density and body composition in children. Pediatr Radiol. 2009; 39(2):148–154. [PubMed: 19052738]

Montain, S. J. C., E. F. (1992). Fluid ingestion during exercise increases skin blood flow independent of increases in blood volume. Journal of Applied Physiology (Bethesda, Md.),73, 903–910.

Pietrobelli, A., Wang, Z., Formica, C., & Heymsfield, S.B. (1998). Dual-energy X-ray absorptiometry: fat estimation errors due to variation in soft tissue hydration. The American Journal of Physiology, 274, E808–E816

Plank, L.D. (2005). Dual-energy X-ray absorptiometry and body composition. Current Opinion in Clinical Nutrition and Metabolic Care, 8, 305–309

Snijder MB, Visser M, Dekker JM, et al. The prediction of visceral fat by dual-energy X-ray absorptiometry in the elderly: a comparison with computed tomography and anthropometry. Int J Obes Relat Metab Disord. 2002; 26(7):984–993. [PubMed: 12080454]

Vilaca, K.H., Ferriolli, E., Lima, N.K., Paula, F.J., & Moriguti, J.C. (2009). Effect of fluid and food intake on the body composition evaluation of elderly persons. The Journal of Nutrition, Health & Aging, 13, 183–186

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