I was interested in finding out the impact breastfeeding may have on RMR.
There doesn't appear to be any evidence that breastfeeding changes basal metabolic rate (BMR). However, it has been my experience that nature may, in her wisdom, ensure that women retain significant energy (fat) stores until the breast feeding period is finished. Whether this occurs by reduction in BMR, I can't say for sure, but I believe there may be some sort of protective mechanism operating to ensure that the woman can continue to breastfeed even if the "crops fail." I've noticed that women who are optimally nourished and eating appropriately may remain several pounds heavier than their pre-pregnancy weight until after their baby weans.
Below is one of many published studies on the topic:
- Publication: Journal Of Nutrition
- Publication Request Approval Date: July 27, 2000
Scientists generally assess the energy needs of the lactation period by adding the cost of producing milk to the basic energy requirement of the non-pregnant woman, allowing for energy mobilization from tissue stores. We took an alternative approach and measured total energy expenditure (TEE), milk energy output and energy mobilization from tissue stores, avoiding assumptions regarding energy efficiency or changes in physical activity an body fat. Our goal was to determine a woman's energy needs during lactation by measuring total energy expenditure, milk production and composition, body weight and composition and to compare these estimates to a woman's energy needs in the non-pregnant, non-lactating state. We also wanted to test for energetic adaptations in basal metabolic rate (BMR) and physical activity during the lactation process. Among other techniques, we used doubly labeled water to track what happened in the body.
Our subjects were 24 well-nourished women who were tested while they were exclusively breastfeeding at three months after giving birth and after they stopped breastfeeding at 18 or 24 months after birth. We found the TEE, BMR and physical activity level did not differ between the lactating and non-lactating state. Total energy needs were greater during lactation than afterward. Adaptations in basal metabolism and physical activity were not evident in our well-nourished subjects, which means their energy needs during lactation were met partly by mobilizing tissue stores but mostly from their diet. These findings contribute greatly toward our understanding of the actual energy needs of lactating women, so we will know how to prescribe the best care and diet for them and their babies.