The objective of this study was to determine the protein turnover and heat production rate of sows at day 30, 45 and 105 of gestation. Gravid sows (n=4) were fed 2.4 ± 0.1 kg of a barley-wheat-SBM diet of 12.5 MJ ME/kg, 0.65% total lysine, and 15% crude protein twice daily throughout gestation. Heat production was measured by indirect calorimetry and a primed-constant infusion of L-[1-13C]leucine (1.0 mg/kg/h) was simultaneously delivered intravenously over 24 h. All measurements were made at d 30, 45, and 105 of gestation. Respiratory quotient (RQ) was calculated to determine nutrient utilization. Energy retention (intake minus heat production) (MJ/d) was greatest (P<0.01) on d 45 (6.1 ± 0.9) compared to d 30 (3.7 ± 0.9) or d 105 (1.4 ± 1.5) of gestation. The RQ was greater than 1 and highest on d 45 indicating lipogenesis. Leucine flux, appearance from breakdown, and incorporation into protein were highest (P<0.0001) on d 45 compared to days 30 or 105 of gestation. However, as a result of the significant increase in protein breakdown (9.3 vs. 1.0 g/d) protein gain was lower (P<0.05) for sows on d 45 (165.2 g/d) compared to d 30 (172.4 g/d and). Therefore, energy was preferentially stored by these sows on d 45 at the expense of protein. The correct utilization of phase feeding would improve productivity and provide positive economic returns.