Background:The study objective was to test the hypothesis that 1) lowering dietary crude protein (CP) increases dietary energetic efficiency and reduces metabolic heat associated with lactation,and 2) excessive dietary leucine (Leu) supplementation in a low CP diet decreases dietary energetic efficiency and increases metabolic heat associated with lactation.Methods:Fifty-four lactating multiparous Yorkshire sows were allotted to 1 of 3 isocaloric diets (10.80 MJ/kg net energy):1) control (CON;18.75％ CP),2) reduced CP with a near ideal or optimal AA profile (OPT;13.75％ CP) and 3) diet OPT with excessive Leu (OPTLEU;14.25％ CP).Sow body weight and backfat were recorded on day 1 and 21 of lactation and piglets were weighed on day 1,4,8,14,18,and 21 of lactation.Energy balance was measured on sows during early (day 4 to 8) and peak (day 14 to18) lactation,and milk was sampled on day 8 and 18.Results:Over 21-day lactation,sows fed OPT lost body weight and body lipid (P＜ 0.05).In peak lactation,sows fed OPT had higher milk energy output (P ＜ 0.05) than CON.Sows fed OPTLEU tended (P =0.07) to have less milk energy output than OPT and did not differ from CON.Matal energy retention was dower (P ＜ 0.05) in OPT and OPTLEU compared to CON sows,and did not differ between OPTLEU and OPT sows.Sows fed OPT had higher (P ＜ 0.05) apparent energy efficiency for milk production compared to CON.Heat production associated with lactation was lower (P ＜ 0.05) or tended to be lower (P =0.082),respectively,in OPT and OPTLEU compared to CON sows.Conclusion:The OPT diet,in peak lactation,improved dietary energy utilization for lactation due to less urinary energy and metabolic heat loss,and triggered dietary energy deposition into milk at the expense of matal lipid mobilization.Leucine supplementation above requirement may reduce dietary energy utilization for lactation by decreasing the energy partitioning towards milk,partially explaining the effectiveness of OPT diet over CON diets.