High energy intakes during an Ironman Triathlon help males athletes to faster finishing times but have the opposite effect in females. That is the surprise finding of study investigating energy balance in 10 male and eight female participants in the 1997 New Zealand Ironman Triathlon, comprising a 3.8k swim, 180k cycle ride and 42.2k marathon run.

Few studies have described energy balance in these situations, and the researchers’ aims were to examine gender differences in the following variables during the Ironman:

Total energy, food and fluid, macronutrient and sodium intake;
Energy expenditure for each stage of the event;
Energy balance for the event;
Relationships between energy and carbohydrate intake, energy balance and finishing times.
Their theory was that athletes would be in substantial negative energy balance (EB) after completing the Ironman and that carbohydrate ingestion would be related to improved performance in both male and female competitors.

They were certainly right in their first supposition: mean energy expenditure (EE) was significantly greater than mean energy intake (EI), with a substantial mean energy deficit after the event of 5,123 and 5,973kcal for women and men respectively.

‘These results reveal,’ comment the researchers, ‘that subjects obtained a high proportion (59%) of their energy from endogenous fuel stores.’

They also illustrate ‘the importance of consuming a high [carbohydrate] diet prior to ultradistance events to maximise endogenous fuel stores’.

But the researchers were surprised to find themselves wrong in their second hypothesis – that energy intake would be positively correlated with performance for both men and women.



Energy intake during the Ironman was monitored by the athletes themselves and passed on to the research team during in-race interviews followed up by telephone interviews a few days later.

Mean total energy intake during the cycle and run portions of the event was 3,115 kcal for women and 3,940 for men, with all subjects consuming significantly more energy during the cycle section than the run. Women obtained significantly more energy from food than fluid during the cycle and run sections – a finding that did not apply to the men. However, the women consumed significantly more water than men and (non-significantly) less sports drink and Coca Cola.

But the most interesting differences between the sexes was this: for women, total energy intake and energy consumed during the cycle section showed significant positive relationship with finishing time: in other words, the more they ate, the slower their times.

For men, the opposite tended to be true, with a significant inverse relationship between relative carbohydrate intake during the run and finishing time: in other words, the more carbohydrate they ate while running, the faster their times.

Acknowledging that the finding about women is difficult to explain, the researchers offer the following possible explanations:

The longer average finishing time for women in this study may have provided more opportunity for energy consumption: in other words the energy consumption was an effect rather than a cause of the slower times;
Difficulties associated with digesting and absorbing large amounts of energy and carbs, particularly in the form of solid food, may have contributed to longer finishing times among the women;
Females may be less reliant than men on energy from carbohydrates because of an enhanced ability to mobilise lipid stores.
The researchers conclude that increasing carbohydrate ingestion during the run portion may be a useful strategy for improving Ironman performance in male triathletes.

International Journal of Sport Nutrition and Exercise Metabolism, 2002, 12, 47-62

Isabel Walker