Energetic efficiency during pregnancy in mice fed ad libitum or pair-fed to the normal energy intake of unmated animals.

The energetic efficiency of pregnant mice pair-fed to the ad libitum energy intake of unmated mice or given unrestricted access to food was investigated. Mice were divided into three groups of similar mean body weight, and two groups were then mated, with the third serving as unmated controls. After d 19 of pregnancy the energy content of all three groups was determined, together with the energy content of the food and feces. Both groups of pregnant mice showed a considerable increase in body weight but the weight gain of the pair-fed group was less than that of the mice fed ad libitum. There was a substantial difference in energy gain and gross efficiency between the two pregnant groups, both parameters being almost zero in the pair-fed mice. Only the pregnant mice fed ad libitum showed a higher energy gain and a higher gross efficiency than the nonpregnant mice, and approximately half of the extra energy intake of the pregnant mice fed ad libitum was deposited. The difference in energy gain between the ad libitum-fed and pair-fed pregnant groups was entirely accounted for by differences in maternal energy stores; there was no difference in the number of fetuses nor their energy density. It is concluded from the present study that there is no increase in overall energetic efficiency in pregnant mice restricted to the normal energy intake of nonpregnant animals; the increase in gross efficiency seen in pregnant mice fed ad libitum in associated with their increased energy intake.