“Reproducing a human is a massive biological effort. Energy-wise, it's on par with the limits of the most extreme sports, for example, running 5,000 km (3,000 miles) over 120 days or cycling the Tour de France.” PregnancyChildbearingReproductive Biology Book:Blood: The Science, Medicine, and Mythology of Menstruation Source: Blood: The Science, Medicine, and Mythology of Menstruation