If you just can’t say no to premenstrual cravings, even when you are dieting, you have a good reason: Your body doesn’t want you to. Your body wants extra calories and increased nutrition when you have ovulated.

A study at the University of British Columbia demonstrates what many women have suspected for some time: Women eat differently during the second half of their cycles — surprisingly, though, only if they’ve ovulated that month.

Susan Barr, PhD, and her colleagues studied 42 women and compared their daily food intakes with their daily body temperatures. The women who had a rise in body temperature, signaling ovulation, had eaten more calories during the second half of their cycles. The women whose temperatures showed no change, and therefore had not ovulated, did not change their caloric intake.

When an egg is released but isn’t fertilized, the body secretes progesterone to start the menstrual flow. If no egg is released, the surge in progesterone doesn’t occur. Progesterone has a thermogenic effect; in other words, it makes heat. To produce the heat, energy — in the form of calories — is burned.

The women did not know when or if they were ovulating, proving that women subconsciously and automatically eat more to make up for the calorie deficit. The average increase was about 260 calories, but some women ate up to 500 extra calories a day. Women on birth control pills don’t ovulate, so they don’t experience progesterone’s fuel burn.