Nutrition to promote optimal sport performance has improved greatly over the past decade, however, for some athletic groups there are still few good data regarding basic energy requirements. This is paradoxical as the adequate provision of energy is fundamental to successful sport performance. One of the foremost reasons behind this problem was, until recently, the lack of an adequate assessment method that could be used without placing restrictions on the activity of the individual. In the 1980's it became possible to use stable isotopes in humans to answer many of these unresolved energy metabolism' questions. In addition, new stable isotope techniques have emerged allowing other factors that influence sport performance to be investigated.
A unique feature of this thesis is that relevant, sport specific questions will be addressed using stable isotope techniques. These techniques have rarely been used in the sports science setting and as such, many of the data following represents the first time they have been collected worldwide. Within this thesis, stable isotopes were used to investigate energy and fluid requirements during periods of very high activity, energy requirements during daily habitual activity, validate self-reported dietary intake data, and assess the hydration ability of 'sports drink' solutions.
For ultra endurance athletes, training and performance are at the extremes of human physiology. In the event of inadequate energy supply, their performance, and general health, may be compromised. Little is currently known about the potentially high energy requirements of such undertakings. To provide insight into this dilemma, a case study is presented of a 37 year old ultra marathon runner as he ran around the coast of Australia. Total energy expenditure was measured over a two week period using the doubly labelled water technique. The average total energy expenditure of the case subject was 6321 kcal/day and the subject's average daily water turnover was 6.1L over the 14 day period. This information will provide a guide to the energy requirements of ultra endurance running and enable athletes, nutritionists and coaches to optimise performance without compromising the health of the participant.
While it is important to know the energy requirements of short periods of extremely high activity, it is equally important to understand the long term daily habitual energy requirements of sporting persons. For athletes, especially where participation and sporting success is reliant on meeting a weight criterion, it is difficult to manipulate energy intake without accurately knowing requirements. Many such athletes resort to potentially dangerous practices to control body weight, all of which could be avoided by altering energy intake if it could be accurately determined. The aim of this research was therefore to determine the energy expenditure and hence energy requirements of seven lightweight female rowers using the doubly labelled water technique over a 14 day period, and further importantly, to compare this to their self-reported energy intake. The rowers self-reported energy intake was 2214 kcal/day and their total energy expenditure was 3957 kcal/day. After adjusting total energy expenditure for changes in body weight, the comparison between adjusted energy intake and reported energy intake showed a bias to underreporting of 1133 kcal/day or 34% of adjusted energy intake. The bias was not consistent across adjusted energy intakes and two out of the seven subjects over-reported their intake. Due to the underreporting of energy intake in most cases, diet recording may not be an appropriate way of assessing energy requirements in lightweight female rowers. A benefit of accurately determining energy requirements, as with doubly labelled water, is that female lightweight rowers will be able to successfully manipulate their energy intake and achieve the set weight cut-off for participation without compromising their health or performance.
Further, many athletes now look to commercially available 'sports drinks' to supplement their energy intake and hydrate the body, and the two aforementioned groups are no exception. Ultra endurance athletes consume sports drinks to supply energy in the form of exogenous carbohydrate and replace the water that was lost during exercise. Lightweight rowers often ingest sports drinks following 'weighing in' before competition to restore body water lost by dehydration practices they may have used to decrease body weight. Promoters of these 'sports drinks' claim that in addition to providing energy, the solutions will hydrate the body faster than water alone.
While there is little doubt that sports drinks improve performance by providing an exogenous source of carbohydrate and hence, delaying fatigue, the evidence is less clear regarding their effect on hydration, another contributing factor to fatigue. This section of the thesis therefore investigates the hydration ability of three commercially available sports drinks (Isosport, Powerade and Staminade) and water using a deuterium dilution technique at rest and during exercise.
On four separate occasions, the rest group subjects ingested gelatine capsules containing deuterium oxide with one of the test solutions and their saliva was sampled every five minutes for an hour while the subject remained seated. The deuterium was administered as above for the exercise group but sample collection was during one hour of exercise on a treadmill at 55% of the subject's maximum heart rate.
The amount of deuterium in each saliva sample, that is, enrichment, was compared using a basic ANOVA. At rest, significant differences in enrichment were found at 2, 10, 25 and 30 minutes post dose between water and Isosport, water and Staminade, water and Powerade and water and Powerade, respectively. There were no differences with respect to mean peak enrichment or mean peak enrichment time. For the exercise group, no significant differences were found for any parameters. In addition to the comparisons of enrichment, the enrichment data for each subject was mathematically modelled to describe the kinetics of hydration and the parameters obtained were compared across drinks. At rest, significant differences were found for the time when absorption rate was maximum (t1), the time when 50% of the drink had been absorbed (t1/2), and the percent of drink absorbed at t1. The differences between drinks were not significant for time when absorption rate returned to zero, i.e. when 100% of the drink was absorbed (t2), or the maximum absorption rate. For the exercise group, the only significant difference was found between water and the sports drinks at t1.
Labelling with a deuterium tracer is a good measure of the relative rate ingested fluids are absorbed by the body. Due to the lack of differences found at t2, which is indicative of the 100% absorption time, both at rest and during exercise, it may be speculated that compared to water, the sports drinks studied did not hydrate the body at a faster rate. Thus, during exercise that does not require supplementation with exogenous carbohydrate to improve performance, it may therefore not be necessary to consume excess energy in the form of sports drinks because they do not appear to hydrate the body with greater efficiency than plain tap water.
This thesis will conclude with a summary of the main findings of this research and also address the limitation of the studies detailed herein. Finally, future research directions will be suggested regarding energy metabolism and stable isotopes.