between pregnancies in rats reduces both of these indices of maternal obesity. Major differences between rats and humans in the relative energy costs of lactation, however, limit the inferences that can be made from studies in rats about the long-term health consequences in humans.
Studies of adipose tissue in lactating women (Lafontan et al., 1979; Rebuffe-Scrive et al., 1985) indicate that there are site-specific changes in the metabolism of energy stores during lactation. Basal rates of fat breakdown (lipolysis) are similar in femoral and abdominal adipose tissues in nonpregnant women, but are significantly higher in the femoral depot of lactating women. The lipolytic effect of noradrenaline administration is similar in both tissue sites during lactation but is much less in the femoral region of nonpregnant women and of women during early pregnancy.
In femoral adipose tissue, lipoprotein lipase activity decreases in lactating women; in abdominal adipose tissue, it remains about the same. Levels of adenosine, a locally acting insulin-like effector, have been reported to be lower in femoral than in abdominal adipocytes in lactating women; the lower levels may promote greater lipid mobilization from the femoral site (Stoneham et al., 1988). In nonpregnant and pregnant women, lipid assimilation appears to be favored in femoral sites over abdominal depots, and during lactation, lipid mobilization is favored in femoral adipose tissue.
Several investigators have followed anthropometric characteristics of well-nourished women (Brewer et al., 1989; Butte et al., 1984; Manning-Dalton and Allen, 1983; Morse et al., 1975; Naismith and Richie, 1975) and marginally nourished women (Adair et al., 1984; Brown et al., 1986; Harrison et al., 1975) during lactation (see also Chapter 4). In a few of those studies, fat stores were estimated. In general, anthropometric changes during lactation were minor. The range of mean daily energy deficits is reported to be 110 to 343 kcal/day in presumably well-nourished women living at home and followed longitudinally during lactation for 4 to 6 months (Brewer et al., 1989; Butte et al., 1984). These deficits could be expected to result in loss of approximately 2.6 to 7.9 kg (˜6 to 17 lb) of fat over 6 months.
The wide range of reported values for postpartum weight change among lactating women may be attributable to differences in baseline weight measurements. In some studies (e.g., Butte et al., 1984), baseline weights were obtained at approximately 35 days post partum, whereas others (e.g., Brewer et al., 1989) used maternal body weight on the first postpartum day as the basis for subsequent comparisons. The marked changes in fluid compartments in the early postpartum period most likely result in an overestimation of the net energy deficit when based on early postpartum weights. If the average energy deficit during lactation is closer to 110 than to 343 kcal/day (see above), it is unlikely