1. The energy metabolism of young pigs offered low-protein diets to appetite or a normal starter (diet 1, 225 g crude protein (nitrogen x 6·25; CP)/kg dry matter (DM)) to maintain zero energy balance was studied using indirect calorimetry and slaughter. The treatments were: T1, diet 1; T2, 46 g CP/kg DM; T3, 21 g CP/kg DM; T4, 17 g CP/kg DM; T5, 21 g CP/kg DM and no supplemental thiamin.
2. Daily heat production (kJ/d per kg body-weight (W)0·75) declined on all treatments from a pre-experimental value of 613. The greatest decline was observed in T1 pigs where the final value was 375.
3. There was a progressive reduction in energy intake from 5·4 MJ/d for T2 pigs to 3·0 MJ/d for T4 and T5 pigs (P < 0·001). The mean intake of T1 pigs was 2·3 MJ/d.
4. There were highly significant treatment differences (P < 0·001) in gain of live-weight and in the gain of carcass DM, CP, fat and energy.
5. There were marked treatment effects on body composition. T1 pigs showed a decrease in the proportion of carcass fat and an increase in the proportion of carcass CP during the experiment while the other four treatments caused large increases (P < 0·001) in carcass DM and fat content and no change in CP content. These changes resulted in large differences in the energy content of the gain, the mean values (MJ/kg) being 0·6, 35·5 and 88 for T1, T2 and T3 pigs respectively. T4 and T5 caused energy gain coupled with weight loss.
6. A linear regression of energy retention (ER; MJ/d per kg W0·75) on metabolizable energy (ME) intake yielded the equation
ER = 0·78 ME-0·365, r0·96.
7. These results indicate that heat production declines when the growth rate of young pigs is reduced either by restriction of energy or of protein intake. They do not support the suggestion that low-protein diets give rise to increased ‘diet-induced’ thermogenesis.