The effect of dietary betaine supplementation on energy partitioning in growing pigs under energy-restricted dietary conditions was assessed. The effect of betaine on the adaptation in energy metabolism of pigs over time after a change in diet and housing also was studied. Six groups of 14 group-housed barrows were assigned to one of two experimental diets: control or betaine-supplemented (0 or 1.29 g/kg of feed). Diets were corn- and soybean meal-based and were formulated to be limiting in energy content but sufficient in amino acids. The experiment comprised a 3-wk adaptation and a 3-wk experimental period. At the start of the experimental period, initial BW was 46 kg, each group of pigs was housed in a climate-controlled respiration chamber, and all pigs were subjected to a change in diet. During the experimental period, diets were diluted with 10% oat hulls. Pigs were fed at 2.5 times the energy requirements for maintenance, and during the experimental period, heat production, energy, and nitrogen balances were measured weekly. Metabolizibility of energy did not differ (P > 0.10) between diets. Averaged over the experimental period, betaine reduced heat production (P < 0.05) and energy requirements for maintenance (P < 0.10) and consequently increased energy retention (P < 0.10). Moreover, the difference in heat production between diets increased with time (P < 0.05). Similarly, the effect of betaine on the energy requirements for maintenance changed with time (P < 0.05). Maintenance requirements were similar in wk 1 and were decreased by betaine supplementation by 5.5% during wk 3 (477 vs. 452 kJ/[kg(0.75) x d]). Results of this study show that dietary betaine supplementation affects energy partitioning by growing pigs. However, based on the observed time-related changes in energy partitioning, it was concluded that dietary betaine supplementation did not influence adaptation by pigs to a change in housing and diet.