|Bodybuilders use carb-ups to look more muscular with a good reason.|
Compared to other techniques such as body impedance analyses and the skin-fold method, which are highly susceptible to changes in the water balance and the expertise of the person taking the measurements, respectively, DXA data is considered a more reliable method to assess lean body mass (LBM) and body fat percentage in clinical research and athletic practice.
DXA is also one of the few technologies that can allegedly reliably determine total and segmental (i.e. trunk, leg and arm fat mass and LBM) body composition, a feature you should keep in mind, because it will be relevant to the evaluation of a recent study from the Université du Québec à Montréal (Rouillier. 2015), which found increases particularly in the legs, yet not in the trunk musculature. Irrespective of the location of the gains, it's a study that puts a question-mark behind the reliability of DXA data certain, not exactly uncommon scenarios.
It is common knowledge that the LBM values you get with DXA represent water, glycogen, proteins and minerals in the lean muscle. In view of the fact that the amount of water in muscle increases with each gram of glycogen that's stored (rule of thumb: 4g of water per 1g of glycogen), scientists have thus long speculated that significant changes in muscle glycogen and the subsequent increases in water may thwart the results you get when you access your own or your subjects's body composition by the means of DXA. Since the muscle glycogen concentrations and water content could markedly change in humans, during very short, acute phases of high-carbohydrate intakes, Pietrobelli et el. advocated research on the impact of changing muscle glycogen levels on body composition measurements, using DXA as early as in 1996 (Pietrobelli. 2015) - research that has not been done in the past ~20 years, though, until... well, until Rouillier et al. took care of it.
Study results w/ important implications for researchers: As Rouillier et al. point out, future studies will probably have to make up for the glyocogen-induced errors by using "a standard preparatory diet for all participants before performing a DXA scan". That carb-depriving or -loading subjects at the end of - let's say - a low carb vs high-carb study, may produce its own problems such as an increased glycogen super-compensation in the low- vs. high carbohydrate group, is yet an issue that's not addressed by the Canadian scientists. Eventually it may thus be necessary to develop a method to correlate DXA values with likewise measured glycogen levels in the muscle and liver to acquire 100% reliable results.In their study, twenty non-obese young men (age 22.7 ± 2.6 years, BMI 23.5 ± 2.1 kg/m²) were subjected to two DXA tests. One before and after another one after being fed a high-carbohydrate diet for 3 days.
To ensure that the subjects did not mess up the results by non-adherence, the participants completed a food diary during the 3-day high-carbohydrate diet to determine the mean percentage of carbohydrates consumed from total kilocalories. In that, the participants were asked to write as much information as possible about the foods they ate (i.e. brand names, percentage of carbohydrates, fats and proteins, how the food was cooked, etc.).
"All participants followed a high-carbohydrate diet ( ≥ 75% of total kilocalories consumed from carbohydrates) for 3 days. Examples of detailed high-carbohydrate meals for the day as well as examples of foods high in carbohydrates, which included the amount of carbohydrate for each food in grams, were given to each subject based on their total energy expenditure.
Post-Workout Glycogen Repletion - The Role of Protein, Leucine, Phenylalanine and Insulin. Plus: Protein & Carbs How Much do You Actually Need After a Workout? Learn more in this SV Classic!
Examples of foods that were recommended to the participants were pasta, cereals, rice, bread, muffins, potatoes, corn, juices and fruits. Participants were allowed to eat carbohydrate rich foods of their choice and no restrictions were given. It should be noted that each participant was instructed not to perform any strenuous exercises during the 3 days of energy expenditure estimation before the diet started and 3 days during the diet" (Rouillier. 2015)
- Pietrobelli, Angelo, et al. "Dual-energy X-ray absorptiometry body composition model: review of physical concepts." American Journal of Physiology-Endocrinology And Metabolism 271.6 (1996): E941-E951.
- Rouillier, M-A., et al. "Effect of an Acute High Carbohydrate Diet on Body Composition Using DXA in Young Men." Annals of Nutrition and Metabolism 66.4 (2015): 233-236.