Measuring and Optimising Rehydration

Simon Van Rosendal (2010). Measuring and Optimising Rehydration PhD Thesis, School of Human Movement Studies, The University of Queensland.

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Author Simon Van Rosendal
Thesis Title Measuring and Optimising Rehydration
School, Centre or Institute School of Human Movement Studies
Institution The University of Queensland
Publication date 2010-04
Thesis type PhD Thesis
Total pages 324
Total black and white pages 324
Subjects 11 Medical and Health Sciences
Abstract/Summary Exercise performance is usually degraded when athletes are dehydrated by more than approximately 2% of their bodyweight. Thus, rehydrating between exercise sessions is typically vital for recovery and preparation for a subsequent exercise bout. The combination of IV fluid and oral glycerol is one method that could prove efficacious in the rapid rehydration of athletes, especially when large volumes of fluid are required with minimal duration for rehydration. Monitoring hydration status in athletes is recognised to be vital for determining how much fluid should be consumed for optimal recovery. The aims of this PhD thesis were (1) to assess whether rehydration can be enhanced by using a protocol that combines intravenous (IV) fluids and oral glycerol, as previously used by a professional sporting team in Australia and (2) to investigate quick and simple methods of detecting hydration status in athletes. Section one of the thesis contains the major study of the PhD which aimed to determine an optimal rehydration protocol for athletes who have a short recovery period before another exercise bout. Four different rehydration protocols (100% oral fluid [oral], 100% oral fluid with glycerol [oral with glycerol]; 50% oral fluid, 50% IV fluid [IV] and 50% oral fluid, 50% IV fluid with oral glycerol [IV with oral glycerol]) were assessed for their effects on cardiovascular, thermoregulatory, endocrine, metabolic and performance variables. Oral with glycerol, IV and IV with oral glycerol all significantly (P < 0.05) improved performance (by 3.7%, 3.5% and 4.1%, respectively) during a 40 km time trial, compared to oral rehydration. However, the exact mechanism underpinning these performance benefits was not definitive. Performance improvements coincided with the following findings: (1) plasma volume restoration was highest in IV with oral glycerol, then IV, then oral with glycerol, then oral (P < 0.05 for all of these comparisons), (2) urine volume was significantly (P < 0.01) reduced in both IV trials compared to oral rehydration (3) IV and IV with oral glycerol resulted in significantly (P < 0.05) lower aldosterone levels during rehydration and performance, and lower cortisol levels during rehydration, and (4) glucose was significantly (P < 0.05) higher at the end of the performance test in the oral with glycerol and IV with oral glycerol conditions. It was therefore suggested that a combination of these factors allowed the subjects to maintain a higher work rate in the oral with glycerol, IV and IV with oral glycerol trials. Thus, athletes who require large volumes of fluid for rehydration in short time frames will benefit from IV fluid and the inclusion of oral glycerol will further enhance rehydration. Section two of the thesis focuses on methods used to measure hydration status. In the first of these studies, bioelectrical impedance spectroscopy (BIS) was assessed as a rapid, non-invasive method to measure the body’s major fluid compartment volumes (total body water, TBW; extracellular water, ECW; and intracellular water, ICW) at rest. The BIS derived volumes were compared to these volumes measured by criterion tracer methods (deuterium oxide for TBW and sodium bromide for ECW). The BIS was then used to predict changes to these compartments during dehydration and rehydration. The technique requires assessment of the body’s resistance to current flow, as electrical current is conducted by water. We found that BIS erroneously predicted an increase for TBW and ECW following low level dehydration (~1% bodyweight) and decreases to these volumes following rehydration. It was determined that exercise-induced alterations to physiological variables (such as core temperature and plasma electrolytes) change the inherent resistivity of the body and affect the resistance to current flow, independent of fluid volume changes. From this study we were unable to elucidate the specific variables affecting current flow through the body, or whether BIS may be more accurate following larger perturbations to the body’s water compartments. The BIS technique was then applied to the larger dehydration study detailed in section 1. On this occasion we found that the BIS predicted fluid shifts that were in the expected direction (i.e. decreased TBW and ECW following dehydration and increases to these compartments following rehydration). However, the technique displayed a high variability between subjects and was not sensitive enough to measure volume changes that were significantly different to the baseline volumes. Again, we were unable to determine which haemodynamic and thermoregulatory factors contributed the greatest effect to the BIS because there were no significant correlations between resistance changes and changes to factors known to affect resistance (e.g. core and skin temperatures and plasma electrolytes). Together, these two studies indicate that BIS is not a suitable method to measure hydration status in athletes following exercise. The final study used a second instrument to assess hydration status via local changes in skin and subcutaneous tissue moisture content (during the major experiment detailed in section 1), using a measurement called the dielectric constant. The principle of measurement is that water molecules present in the skin and subcutaneous tissues absorb ultra high frequency energy. When an electromagnetic wave is delivered to the skin, the reflected wave contains information on the water content. If large numbers of water molecules are present (i.e. the tissues are more hydrated) then more of the energy of the electromagnetic wave is absorbed and less energy is reflected. Changes in skin and subcutaneous tissue water content were associated with changes in hydration status. However, the direction of these associations was opposite to that expected, indicating that, like BIS, the assessment of skin and subcutaneous tissue water using the dielectric constant is not a good predictor of dehydration and rehydration in athletes.
Keyword Glycerol
Bioelectrical impedance spectroscopy
Additional Notes Landscape pages: 53, 54, 55, 56, 57, 58, 59, 77, 78, 79, 80, 81, 82, 84, 85, 107, 108, 114, 115, 184, 187, 190, 217, 220, 237

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Created: Mon, 15 Nov 2010, 13:41:57 EST by Mr Simon Van Rosendal on behalf of Library - Information Access Service