Submitted to: Journal of Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/22/2000
Publication Date: N/A
Citation: Interpretive Summary: Scientists generally assess the energy needs of the lactation period by adding the cost of producing milk to the basic energy requirement of the nonpregnant 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 nonpregnant, nonlactating 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 3 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 nonlactating 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.
Technical Abstract: Instead of using an incremental approach to assess the energy requirements of lactation, a more comprehensive approach may be taken by measuring total energy expenditure (TEE), milk energy output, and energy mobilization from tissue stores. The purpose of this study was to assess the energy requirements of lactation; to compare these estimates to energy requirements in the nonpregnant, nonlactating state; and to test for energetic adaptations in basal metabolic rate (BMR) and physical activity. Milk production and composition, body weight and composition, TEE, BMR and physical activity levels were measured in 24 women at 3 mo postpartum and at 18 or 24 mo postpartum after breastfeeding. TEE was measured by the doubly labeled water, milk production by 3-d test-weighing, milk energy by bomb calorimetry, body composition by dual-energy x-ray absorptiometry, and BMR by room respiration calorimetry. TEE, BMR, and physical activity level did not differ between the lactating and nonlactating state (TEE, 10.0 +/- 1.5 vs. 10.6 +/- 2.1 MJ/d). Mean milk energy output was 2.02 +/- 0.33 MJ/d. Total energy requirements were greater during lactation than afterward (12.0 +/- 1.4 vs. 10.6 +/- 2.1 MJ/d, P=0.002). Energy mobilization from tissue stores (-0.65 +/- 0.97 MJ/d) resulted in net energy requirements during lactation of 11.4 +/- 1.8 MJ/d. Because adaptations in basal metabolism and physical activity were not evident in these well-nourished women, energy requirements during lactation were met primarily from the diet, and only partially by mobilization of tissue stores.