The effects of environment, diet and exercise on the reliability and validity of measurements of resting metabolic rate and body composition in athletes

Resting energy expenditure (REE) can be reduced in situations of Low Energy Availability (LEA) in athletes, providing both a diagnostic sign of LEA and a potent risk factor for illness, injury and sub-optimal health. Current protocols regarding pre-measurement standardisation for REE are based on non-athlete populations, often following stringent rest and fasting protocols that would not be practical in a high performance environment. Furthermore, the reliability of measurements derived from these protocols has often been assessed in general and clinical populations and is unknown in an athlete population. Characteristics of the test protocol which alter an athlete's presentation (e.g. the location of the test, the duration of recovery from the last exercise bout) and changes in the athlete's own characteristics (changes in intramuscular solute and water content) were identified as variables that could affect the reliability of measurement of REE. This thesis presents a series of distinct but related studies which examine the how these variables affect the measurement and interpretation of REE in athletes.

Study 1 examined the effect of testing location on REE in 32 elite and sub-elite athletes. REE was measured either at their bedside upon waking (inpatient) and as an outpatient (laboratory) protocol in a cross-over design following 8 h overnight fasts prior to each measurement. The day to day variation and reliability of each protocol was also assessed. There was no difference in REE when measured under the inpatient or outpatient protocols (7302 ± 1272 and 7216 ± 1119 kJ/d respectively). Both protocols showed good day to day reliability (inpatient 96%, outpatient 97%), however, the outpatient protocol was found to have a greater typical error (TE) (478 kJ/d) and to be less sensitive to changes in REE than the inpatient protocol (336kJ/d).

Study 2 was a pilot study that investigated the effect of acute exercise on REE. A cross-over intervention was used in ten male athletes. Measurements were undertaken following training sessions in the morning and afternoon to determine REE approximately 12, 24, 36 and 48 h post exercise. There was a trend for REE to decrease with increasing rest time from exercise, with REE measured 48 h post exercise being significantly lower than REE measured at 12 h. However, the difference of 375 kJ/d was within the typical error determined in Study 1.

Study 3 focused on the reliability of DXA estimates of lean mass (LM), which is important in the interpretation of REE relative to fat free mass (FFM). Intramuscular solutes and fluid were manipulated through a series of glycogen depletion, glycogen loading and creatine loading protocols in 18 male cyclists. Main outcome measures were total body and leg LM measured by dual x-ray absorptiometry (DXA), and total body water (TBW) measured by bioelectrical impedance spectroscopy (BIS). Changes in the mean were considered substantial if they reached the threshold for the smallest worthwhile effect of the treatment. There were substantial increases in TBW (2.3 and 2.5%), total body (2.1 and 3.0%) and leg LM (2.6 and 3.1%) following glycogen loading and the combined glycogen-creatine loading protocols respectively. Glycogen depletion caused a substantial decrease in leg LM (- 1.4%) and trivial decrease in total body LM (-1.3%). Creatine loading resulted in substantial increases in TBW and in trivial increases in LM measures.

Study 4 addressed the potential development of a practical method to determine an athlete's glycogen stores in combination with DXA-derived estimates of LM by investigating the validity of measuring muscle glycogen via a non-invasive ultrasound technique. The same cohort and design involved in Study 2 was used in this investigation, with the ultrasound derived estimates of muscle glycogen concentration and changes in glycogen concentration being compared with results derived from direct (biopsy-derived) measurements. Poor correlations and substantially large or unclear errors were determined for the ultrasound estimates of muscle glycogen compared to muscle biopsy. Therefore, under the conditions employed in the current study design, the ultrasound technique was unable to accurately predict either single measures of muscle glycogen or changes in muscle glycogen stores.

Study 5 applied the findings from Study 3 to investigate how changes in muscle glycogen influence the measurement and interpretation of REE in athletes, with particular interest in its effect on the sensitivity to detect changes in REE over time or as a result of an intervention. The investigation was undertaken within a larger study of the effect of adaptation to a low carbohydrate, high fat (LCHF) diet during a 21 day training camp for endurance athletes. Nineteen elite male race-walkers participated in this study; ten were assigned to the control group (CHO) where they received a diet providing 60% of energy from carbohydrate while nine were in the intervention group in which carbohydrate was restricted to70% of their energy intake for this period.

Before and after the dietary interventions, measurements were made of REE, body composition (DXA) and TBW (BIS). Information derived from Study 3 to distinguish acute changes in TBW associated with changes in intramuscular glycogen and its bound water, from true (chronic) changes in muscle mass, was applied to the baseline and post-intervention measures of LM in all athletes. There was a significant decrease in FFM between Baseline and uncorrected Post-Intervention values FFM (-1.4; 95% CI -2.0, -0.80 kg). Using the uncorrected measures of FFM, we interpreted that no change in relative REE between baseline and post intervention occurred in either group. However, when the correction factor was applied to FFM of the LCHF group, correcting for the artefact of reduced muscle glycogen levels associated with restricted carbohydrate intake, we detected a decrease in relative REE post intervention measurements compared to baseline. The conclusions from this series of studies are; 1) Inpatient and outpatient protocols should not be used interchangeably when tracking changes in REE over time. 2) An 8 h overnight fast has good day to day reliability for both inpatient and outpatient protocols. 3) Rest time from exercise should be kept consistent between measures of REE for longitudinal monitoring. 4) Manipulations of muscle glycogen and creatine supplementation cause an artefact in the DXA which changes the estimate of LM accordingly. 5) Measurement of TBW via BIS is better suited to track changes in muscle glycogen than proprietary ultrasound technology. 6) A reduction in muscle glycogen stores, such as that achieved by the consumption of a LCHF diet, creates an artefact in the DXA-derived measurement of FFM, which could potentially alter the interpretation of relative REE. This knowledge should be integrated into best practice guidelines for the measurement of REE in athletes to enhance the reliability and validity of measurement as well as the interpretation of the results.