Feeding a diet high in fat and sucrose (HFS) during pregnancy and lactation is known to increase susceptibility to develop metabolic derangements later in life. A trait for increased behavioral activity may oppose these effects, since this would drain energy from milk produced to be made available to the offspring. To investigate these interactions, we assessed several components of behavioral energetics during lactation in control mice (C) and in mice of two lines selectively bred for high wheel-running activity (S1, S2) subjected to a HFS diet or a low-fat (LF) diet. Energy intake, litter growth, and milk energy output at peak lactation (MEO; assessed by subtracting maternal metabolic rate from energy intake) were elevated in HFS-feeding dams across all lines compared to the LF condition, an effect that was particularly evident in the S dams. This effect was not preceded by improved lactation behaviors assessed between PND1-7. In fact, S1 dams had less high-quality nursing and S2 dams showed poorer pup-retrieval than C dams during PND1-7, and S dams had generally higher levels of physical activity at peak lactation. These data demonstrate that HFS-feeding increases MEO underlying increased litter and pup growth, particularly in mice with a trait for increased behavioral physical activity.