Does anyone actually know?

Coincidentally, I was reading an article yesterday about the pivotal role of serotonin in appetite regulation. The author had written a book called The Serotonin Diet, or something. She cited some practical examples of the relationship of serotonin and weight gain/loss in everyday life, using her theory to explain why weight gain is so common when people go on antidepressants (SSRIs like Prozac) AND when women have their periods!!

Okay, I found a quote [http://www.womenshealthmag.com/health/pms-diet]:

"Premenstrual mood swings and crazy cravings come down to a shortage of the brain chemical serotonin, says Judith Wurtman, Ph.D., coauthor of The Serotonin Power Diet. "The only thing that helps—and it helps instantly—is to eat carbs, because that's the only way the brain makes new serotonin," says Wurtman, who adds that the serotonin boosters need to be eaten straight with no fat or protein."

...and here's another on the subject [http://www.webmd.com/diet/features/is-pms-sabotaging-your-diet]:

The hormonal ebbs and spikes that occur throughout a woman's cycle are the major culprits in PMS. As levels of estrogen go up and down, so do levels of the stress hormone cortisol. . . . Whether a woman will crave sweets or croissants, though, depends on yet another player: the brain chemical serotonin. Most women with PMS experience a drop in serotonin levels, which triggers cravings for carbs because the body uses carbs to make serotonin. If cortisol is high and serotonin is low, you'll seek carbs and fats, but really heavy duty on the simple carbs -- sugar-based sweets like chocolate bars. The reason: Simple sugars are metabolized more quickly than complex carbs, so they offer a quick serotonin fix. If cortisol is way up but serotonin is relatively normal, a woman is more likely to crave a fat-carb combo without a huge sweet component, such as a bagel laden with cream cheese. Other research has linked PMS to a state of low blood sugar, or hypoglycemia, in the second half of the menstrual cycle, says Susan M. Lark, MD, a clinician in Los Altos, Calif., and author of Premenstrual Syndrome Self-Help Book: A Woman's Guide to Feeling Good All Month. "Women in these studies experienced a significant drop in blood sugar after eating, accompanied by edginess and irritability," Lark says. "Then within an hour or two, they are hungry again and craving more food."

...so the unfortunate conclusion, it appears, is that the urge to eat more during PMS and/or menstruation is not rooted in burning more calories. Love you, mother nature.

There is a slight increase in metabolism in the two weeks prior to your menstrual period, not during. There is also a slight decline in the two weeks after.
It balances out.