Your body maintains your weight through a balance of calories you eat and calories you use. Even at rest, your body is using calories in order to maintain your organs, for cellular functions, and to keep your heart beating. When your body is menstruating, it prepares for this by using extra calories.
Your Basal Metabolic Rate, BMR, is the amount of daily calories needed in order to keep you alive. If you did nothing but lie in bed throughout the day, your body still requires calories. The Journal of Nutritional Science and Vitaminology tells us that BMR is significantly affected by the menstrual cycle. This is not a surprise since the cells are more active during this time which requires additional energy.
Woman's Day Magazine reports that for the week before your period, your body will require 100 to 300 more calories a day. While it is tempting to think of these as free calories, still make healthy food selections such as fruits and vegetables from which to take those calories.
Dr. Anelise Engel, from Manhattan, measures variations in metabolic rates. She reports that your menstrual cycle can require up to a 359 calorie difference. Your lower calorie range being one week before ovulation and your higher range just before menstruation.