The New Zealand ironman triathlon: an analysis of nutrient intake, energy balance, substrate utilisation and performance in male and female ultra-endurance triathletes

Abstract

This research examined selected nutritional and physiological aspects of triathletes training for, and competing in, the 1997 New Zealand (NZ) Ironman triathlon (226 km). Eighteen (ten male and eight female) subjects who estimated their finishing time to be between 10.5 and 13.5 hours volunteered to participate in this study. Nutrient intake was analysed using four-day food records three weeks and one week prior to the Ironman. Energy balance during the race was estimated from monitoring energy intake and energy expenditure whilst athletes were competing. In preparing for the NZ Ironman, male and female athletes consumed sufficient energy to meet recommended levels, and obtained a high proportion of total energy intake (an average of 62 to 66%) from carbohydrates. Vitamin and mineral intakes exceeded estimated daily requirements, although supplementation accounted for a high proportion of the vitamin intakes. Nutrient intake was therefore deemed adequate to meet the high physiological demands experienced during prolonged training prior to Ironman competition.

During the NZ Ironman, energy replacement was almost exclusively from carbohydrate (a mean 94% of total energy intake), obtained from food and fluid supplements. Male and female athletes showed a similarly high energy intake deficit in relation to caloric expenditure, indicating that Ironman athletes obtain approximately 40% of their expended energy from nutrients consumed during the race. Cycling was identified as the most important discipline for energy replacement during the Ironman.

To test a hypothesis that female finishing time is faster than performance-matched male triathletes in an ultra-endurance Ironman event, ten subjects (five males and five females) were performance-matched by comparing the time to complete a Half Ironman (113.1 km), training histories, cycle and run maximal oxygen consumption (VO₂max) and lactate profiles. All performance-matching criteria were similar except females averaged a significantly (P < 0.05) greater cycling distance per week than males. Venous blood samples for the analysis of metabolites and hormones were obtained 1 to 1.5 hours prior to the race start, within 10 minutes of finishing the race, and at 0800h the following morning from all matched athletes except one female. During Ironman, the average female velocity (18.68 ± 0.71 km.h⁻¹ ) was significantly (P < 0.05) faster than their male counterparts (18.11 ± 0.57 km.h⁻¹). The faster performance by females during the NZ Ironman could not be attributed to sustaining a greater percentage of their V0₂max or higher rates of energy expenditure than males during the cycle and run.

Furthermore, the greater endurance capacity of female than of male ultra-endurance triathletes was not related to changes in serum glucose, urea, triglycerides, free fatty acids and cortisol, or plasma growth hormone, insulin and catecholamines. However, the higher quantity of endurance training by females may have resulted in their superior performance during the Ironman.