On a human mission to Mars, every kilogram of payload will count. According to Kate Greene, sending women instead of men will lower the amount of food necessary for such an expedition.

Greene took part in a four-month study in which six “crew-members” simulated a Mars expedition. The subjects conducted mock experiments and wore spacesuits any time they ventured outside of the camp housing, which was located on the side of the Mauna Loa volcano in Hawaii. Greene documented her experience for Slate.

While on the mock expedition, Greene documented the crew’s sleeping habits and estimated each member’s caloric expenditure. She concluded that women tended to burn, at most, 2,000 calories in a day while males typically burned 3,000 or more calories per day, even when daily exertion was about the same for everyone. Male crew members generally ate more and complained that they had difficulty maintaining weight.

“At mealtime, the women took smaller portions than the men, who often went back for seconds,” said Greene.

Planning a space mission involves substantial consideration for crew diet. Greene explained, “the more food a person needs to maintain her weight on a long space journey, the more food should launch with her. The more food launched, the heavier the payload. The heavier the payload, the more fuel required to blast it into orbit and beyond. The more fuel required, the heavier the rocket becomes, which it in turn requires more fuel to launch.”

In the 1960s, NASA spent time training female astronauts but eventually dismissed them out of public relations concerns. Having a female astronaut fatality was not something NASA was ready for, even with female Russian cosmonaut Valentina Tereshkova going into space in 1963.

NASA researcher Harry Jones, who has published a paper on the gender difference in caloric demand, says that “the issues are all about crew performance including group dynamics, individual psychology, etc.“