Lactation increases the efficiency of energy utilization in rats.

The possibility that lactation increases the efficiency of energy use was investigated by comparing energy expenditure on activity and maintenance in virgin and lactating rats fed either ad libitum or restricted amounts of food. Energy expenditure was determined, by difference, in a nutrient balance study. Lactating rats fed ad libitum expended 34.9% less energy on activity and maintenance than did control animals. Of this change, 82% was specifically associated with lactation, but the remaining 18% was associated with the negative energy balance that occurred during lactation. Lactating rats fed restricted amounts of food expended less energy on activity and maintenance than did virgin rats fed at the same level of restriction, but this change was associated with increased negative energy balance and not specifically with lactation. These results indicate that the efficiency of energy use is increased substantially during lactation in the rat, regardless of the level of energy intake, and that mechanisms associated with negative energy balance play an important role in the change.