BBC News reported that human endurance has a limit. Researchers looked at marathon runners who race over a period of hours, and Tour de France cyclists who race over weeks, to extreme athletes in the 140-day Race Across the USA and those who trek Antarctica. James Gallagher writes, “the cap was 2.5 times the body’s resting metabolic rate, or 4,000 calories a day for an average person.” A healthy person may be able to sustain effort within that daily limit. The body just can’t produce more energy than that over the long haul.

Alongside the extreme athletics, the article added:

The research, by Duke University, also showed pregnant women were endurance specialists, living at nearly the limit of what the human body can cope with.… During pregnancy, women’s energy use peaks at 2.2 times their resting metabolic rate….

Women in pregnancy approach the limit of what a body can do. This amazes and terrifies me. My wife is pregnant and enduring it well in one of the hottest, most polluted cities right now. There’s no break. The doctor says that she’s not getting enough energy. I can’t carry the baby, but I want to help her cross the finish line. She deserves a dozen more milkshakes and respect.