The research began by studying the extreme athletes participating in the Race Across the USA, a 3,080 mile run from California to Washington D.C. that lasts 140 days. Athletes run 6 marathons a week for five months. The team of researchers studied the number of calories the athletes burned per day and then plotted their data over time.
They also studied urine samples taken on the first and last day of the Race Across the USA. Using these samples, scientists were able to determine that the athletes were burning roughly 600 calories less than what they should have been at the end of the race.
The team continued to research other extreme endurance competitions, such as the Tour de France and Antarctic treks. The pattern they found in each event was the same: the body’s energy expenditure started high and then flattened out to 2.5 times their resting metabolic rate.
While expecting mothers may not be running thousands of miles, they are also pushing their body to the extreme and often come close to the limit of 2.5 times their basal metabolic rate during the 9 months. Scientists speculate that perhaps the physiological limits that are preventing athletes from breaking records could also limit the size that a baby can grow to in the womb.
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