Since the emergence of Kenyan endurance runners on the world stage at the 1968 Mexico Olympics, where they won 8 medals ranging from 400m relay to the 10 000m, Kenyan success has grown year on year. The staggering success of a country that compromises just 0.5 % of the world population has triggered a number of explanations. Heavily cited explanations are genetic superiority and environmental factors. Despite a number of investigations, genetic superiority remains to be determined, what is clear though is that the environmental factors that interact with each genetic element leading to world-class performance are particularly important. Aims and objectives: Given the importance environmental factors may have on the process leading to world class performance, the main aims of the following research were: 1) to determine the composition of elite Kenyan endurance runners diet and assess their energy balance status prior to major competition using "gold standard" methods; 2) to establish lifestyle practices of elite Kenyan endurance runners prior to major competition that will allow an insight in to the preparation of some of the best athletes in the world; 3) to ascertain the hydration status of elite Kenyan endurance runners during an important training period and directly compare these results to traditional paradigms and current thinking on optimal fluid intake for superior endurance running performance; 4) to investigate the training process leading to world class performance by quantifying training load in the lead up to major competition; 5) to determine the fluid intake behaviours of the world s best marathon runners during racing. This will allow an insight into current practices of elite runners that will act as a benchmark and comparison of current fluid intake guidelines; and 6) to validate and combine existing technologies of heart rate and accelerometry for quantifying energy expenditure during free living conditions. Methods: Chapters 2 and 3 detail extensively the diet, hydration, lifestyle and training practices of a group of highly successful elite Kenyan endurance runners during important training periods based at a high altitude camp in Kenya. Chapter 4 explores the significance of the hydration practices reported in Chapters 2 and 3 (i.e., ad libitum fluid intake) have on elite marathon running performance and the wider implications for fluid intake recommendations for elite marathon running. Chapter 5 investigates novel technology (i.e., the combined use of accelerometry and heart rate) that may further enhance our understanding of the physical activity patterns and training practices of elite Kenyan endurance runners on a day-to-day basis. Results and discussion: Chapter 2 reported elite Kenyan endurance runners are in negative energy balance prior to major competition as assessed by the gold standard doubly labeled water method (Energy intake: 13.2 +/- 1.3 MJ/d: vs. Energy expenditure: 14.6 +/- 1.0 MJ/d; p < 0.005). Considering the relatively high carbohydrate content of their diet (e.g., 67.3 +/- 7.8 %, 9.8 g/kg/bm) it is hypothesised the caloric deficit may not have a direct impact on their training performance. In fact the performance implications of reducing body mass as a result of energy deficiency is that the athletes will be lighter for competition and may thus be at an advantage as the energy cost per unit distance increases in direct proportion to the added load expressed as a percentage of body mass. Measured physical activity patterns (i.e., Physical Activity Ratio (PAR) and accelerometry) of elite Kenyan endurance runners strongly suggest rest between running training sessions is an important lifestyle factor as it was found time spent relaxing, in light activity, slow running (8.0-13.6 km/h), moderate running (13.7-17.3 km/h), and fast running (> 17.4 km/h) as estimated using the PAR method was 82 +/- 6 %, 8 +/- 6 %, 3 +/- 1 %, 5 +/- 1 %, 2 +/- 1 %, respectively. The reported time spent in light, moderate, hard and very hard activity as determined by accelerometry was 82 +/- 3 %, 11 +/- 2 %, 6 +/- 3 %, and 1 +/- 1 % respectively. A further striking finding in Chapter 2 was the relatively low daily fluid intake that consisted of primarily water (0.9 +/- 0.5 L/d) and milky tea (0.9 +/- 0.3 L/d). Chapter 3 found athletes remained hydrated day-to-day drinking ad libitum despite this relatively low daily fluid intake that corroborated prevailing fluid intake recommendations. This was evidenced by mean total body water and pre training body mass being maintained day-to-day throughout the recording period (p = 0.194 and p = 0.302, respectively). Furthermore, there was no significant difference between the osmolality of the morning urine sample and the evening sample (p = 0.685). It was also found that athletes remained in electrolyte balance (Na+ intake: 3245 +/- 901 vs. Na+ loss: 3254 +/- 1070 mg/d; p = 0.975) day-to-day thus negating the need for further supplementation.