Energy requirements during pregnancy: old questions and new findings

The question of how much extra energy a pregnant woman needs to obtain through her diet is closely linked to the question of the amount of weight she ought to gain during her pregnancy. A short but useful description of recommendations for weight gain during pregnancy, published over the past 50 y, is provided by Abrams et al (1). It has long been recommended that pregnant women restrict their food intake to avoid excessive weight gain to prevent toxemia, difficult births, and maternal obesity. However, this policy was challenged in the 1960s when the significance of low birth weight for infant health began to be recognized. Consequently, in 1970 the US Food and Nutrition Board increased the formal recommendation for weight gain during pregnancy and stated that severe caloric restrictions and routine limitations for weight gain should be avoided during pregnancy (2). In Britain, Hytten and Leitch (3) used data from 3800 primigravidae who were eating without restriction to establish the physiologic norms for weight gain during pregnancy and the weight gain associated with the best reproductive performance. These authors found that a total gestational weight gain of 12.5 kg was associated with the best reproductive outcome in terms of infant birth weight, infant survival, and incidence of preeclampsia (3). In 1990 a report from the Institute of Medicine (IOM;4) reassessed the relation between pregnancy weight gain and various maternal and fetal outcomes. An important point in this report was that the nutritional status of a woman before conception should be taken into account when assessing optimal weight gain during pregnancy. Thus, it was recommended that women with a prepregnancy body mass index (BMI; in kg/m) 19.8 should gain as much as 12.5–18 kg during the entire pregnancy, whereas a woman with a BMI between 19.8 and 26 before gestation needed to gain only 11.5–16 kg. For overweight women, the recommended weight gain was even smaller. These total weight gain ranges, recommended on the basis of prepregnancy BMIs and known as the IOM recommendations, have been evaluated in several studies. These studies confirmed that these recommendations represent a sound approach to addressing the question of optimal weight gain during pregnancy (1). Hytten and Leitch (3) also studied the maintenance energy metabolism of pregnant women but were unable to measure the physical activity component of energy expenditure during pregnancy. Nevertheless, they stated that “The energy cost of pregnancy could be met without increase of food intake by economy of activity, and economy of activity is characteristic of pregnancy in both animals and man.” They did, however, assess the energy cost of the weight gained during pregnancy because, after much effort, they were able to conclude that 3 of the 12.5-kg recommended weight gain consisted of fat retained in the maternal body. Considering the methods available at that time, this finding was indeed a major achievement. In 1980, Hytten and Chamberlain (5) published their estimate of the total energy cost of pregnancy, ie, 80 000 kcal, based on an estimated fat retention of 3.8 kg and an increase in maintenance energy metabolism corresponding to 36 000 kcal. Other authors thereafter published studies regarding the energy cost of pregnancy in women living in different countries under different nutritional conditions. The results indicated that average gestational fat retention varies considerably among different populations of women and that the increase in maintenance energy metabolism during pregnancy varies in relation to the prepregnant body fat content of the women and in relation to energy intake during pregnancy (6). The studies by Butte et al reported in this issue of the Journal (7) and in the American Journal of Obstetrics and Gynecology (8) represent a logical continuation of the studies cited above, because these authors measured the energy cost of pregnancy in women classified before pregnancy into different BMI groups defined by the IOM (4). The results confirmed earlier observations that the increase in maintenance energy metabolism, one of the main components of the energy cost of pregnancy, varies in response to prepregnant body fat content. The study also showed that weight gain above the IOM recommendations tends to consist of fat, which confirms that such weight gain is undesirable. Furthermore, the study included assessment of total energy expenditure with the doubly labeled water method, which made it possible to conclude that, for the population under study, decreases in the amount of energy spent on physical activity were not sufficient to counterbalance the energy cost of pregnancy because of fat retention and increases in maintenance energy metabolism. Consequently, although the statement by Hytten and Leitch that “the energy cost of pregnancy could be met without increase of food intake by economy of activity” may well be true, this was not the case for this healthy, moderately active