Energy requirements of lactating women derived from doubly labeled water and milk energy output

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 latter approach avoids assumptions regarding energetic e fficiency and changes in physical activity and adiposity. The purpose of th is study was threefold: to assess the energy requirements of lactation; to compare these estimates with energy requirements in the nonpregnant, nonlac tating state and to test for energetic adaptations in basal metabolic rate (BMR) and physical activity during the energy-demanding process of lactatio n. Milk production and composition, body weight and composition, TEE, BMR a nd physical activity levels were measured in 24 well-nourished women during exclusive breastfeeding at 3 mo postpartum and after the cessation of brea stfeeding at 18 or 24 mo postpartum. TEE was measured by the doubly labeled water method, milk production by 3-d test-weighing, milk energy by bomb ca lorimetry on a 24-h milk sample, body composition by dual-energy x-ray abso rptiometry and BMR by room respiration calorimetry. TEE, BMR and physical a ctivity level (physical activity level = TEE/BMR) did not differ between th e lactating and nonlactating state (TEE 10.0 +/- 1.5 versus 10.6 +/- 2.1 MJ /d). Mean milk energy output was equivalent to 2.02 +/- 0.33 MJ/d. Total en ergy requirements were greater during lactation than afterward (12.0 +/- 1. 4 versus 10.6 +/- 2.1 MJ/d, P = 0.002). Energy mobilization from tissue sto res (-0.65 +/- 0.97 MJ/d) resulted in net energy requirements during lactat ion of 11.4 +/- 1.8 MJ/d. Because adaptations in basal metabolism and physi cal activity were not evident in these well-nourished women, energy require ments during lactation were met primarily from the diet and only partially by mobilization of tissue stores.