Influence of Lactation on Body Weight Regulation Sally Ann Lederman, Ph.D.

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1 July 2004: (II)S112 S119 Influence of Lactation on Body Weight Regulation Sally Ann Lederman, Ph.D. Maternal weight homeostasis during lactation depends on the management of energy resources. Studies indicate that regulatory processes allow for successful lactation in varied conditions of food availability. These processes involve mobilizing fat, increasing food intake, reducing energy expenditure, and changing the composition or volume of milk. Changes in energy efficiency do not seem important. Early in lactation, fat mobilization appears to be physiologic and gradual, even when food is readily available. Later in lactation, dietary intake may decline even though that allows continued loss of body fat when fat has already lowered. Where increasing dietary intake is less possible, or dietary restriction is imposed, reductions in energy expenditure seem to take precedence over an increase in the rate of fat mobilization. The findings reviewed indicate that for lactation to play a major role in the reduction of body fat in the postpartum period, women have to breastfeed fully for a substantial period. Key words: lactation, breastfeeding, energy balance, fat mobilization 2004 International Life Sciences Institute doi: /nr.2004.jul.S112 S119 The relationship of lactation to body weight regulation can be viewed from several different perspectives. In conditions of surfeit, we might focus on whether lactation has a role in the prevention of postpartum obesity, that is, whether lactation can enhance maternal weight loss. In undernutrition, weight regulation during lactation is more easily seen as a problem of maintaining maternal weight and milk output under conditions of energy stress. Several decades ago, lactation by undernourished women was the major focus of studies of lactation. There was concern that milk production would be inadequate if maternal body weight was low, and therefore children of undernourished women might be starved if they were only breastfed. However, since substantial evidence suggested that poorly nourished women were generally able Dr. Lederman is with the Columbia Center for Children s Environmental Health, 100 Haven Avenue, Suite 25F, New York, N.Y , USA. to lactate successfully, some workers were concerned that undernourished women might harm their own health if they did not limit their output during lactation. Thus, much of the literature from 10 or more years ago focused on whether a breastfeeding woman could supply the nutrient needs of a child even when she herself was not optimally nourished and whether, if she could, she could do so without serious impairment of her own health. More recently, there has been a developing interest in characterizing the detailed metabolic adjustments occurring during lactation. By understanding how maternal metabolism is altered and energy balance is regulated during lactation, we may better understand the energy limits within which well-nourished or undernourished women provide adequate breast milk and still provide for their own needs. This interest has joined with concern about the growing epidemic of obesity in developed countries and has directed increased attention to whether lactation can contribute to reducing the large amount of body fat deposited during pregnancy by well-nourished or over-nourished women. A lactating woman has essentially the same physiologic issues in regulating body weight as a woman who isn t lactating, but lactation adds an additional consideration because there is a continuous need for energy output in the form of milk. Further, the synthesis of that milk also requires energy. Thus lactation is reasonably viewed as an energy stress. Such regulatory challenges are not unique to lactation. Heavy physical labor, pregnancy, or professionallevel athletic training may impose similarly severe energy demands. Lactation is unique mostly because of the seeming inflexibility of its timing and its requirement for relentless continuation irrespective of maternal condition. Figure 1 shows the components of energy balance during lactation. Energy needs are determined by the factors listed under output. When output rises, because of the onset of milk production, balance can be reestablished in several ways. One option is to increase dietary energy intake. A second option would be to use maternal fat stores. Although using body fat for energy needs may be considered undesirable, particularly in undernourished women, in the postpartum period this might be considered less undesirable. We might take a less negative view of fat mobilization postpartum, since most S112 Nutrition Reviews, Vol. 62, No. 7

2 Figure 1. Components of energy balance during lactation. women, even women on relatively low intakes, have stored significant fat during pregnancy in preparation for lactation. Maternal body fat is considered a buffer of lactation needs and we might expect body fat to be depleted in a physiologic manner during lactation. A third way that maternal energy needs can be reduced is by decreasing energy expenditure due to activity. Improving the efficiency of energy use for energy-requiring processes, such as basal metabolism or the thermic effect of food, or improving the efficiency of milk synthesis would also lower the increment of energy needed for lactation. Finally, reducing the amount of milk produced would decrease energy needs. Most of these alternatives have been the subject of investigation. To best determine how energy balance is achieved during lactation, studies were needed that concurrently measured energy intake, energy expenditure, milk production, and maternal body composition changes over many months of lactation under different conditions. Measuring any one of these components accurately is difficult, even in research settings. All are extremely difficult to measure well in field settings. Nevertheless, this has been done in a number of environments, with not necessarily every measurement being made in all of them, but with key measurements made concurrently. Considering the possible sources of error, a strikingly coherent picture emerges of the components of energy balance during lactation. Let us first consider dietary intake. Brewer, Bates and Vannoy 1 reported dietary intake determined from 3-day food records completed at both 3 and 6 months postpartum by 56 women in Louisiana (Table 1). Despite the fact that the women were significantly heavier than before pregnancy and all were losing weight after delivery, intake was more than 500 kcal greater in lactating women than in formula feeders, even if breastfeeding was not exclusive. However, weight loss during the first 3 months postpartum was faster in the formula feeders, 8.14 kg, vs kg in the exclusively breastfeeding group and 6.39 kg in the combination feeders. These results are supported by the findings in a study from Salt Lake City using 3-day diet and activity records at 1, 6, and 12 weeks postpartum. 2 In this study, lactating women also lost less weight in the first 12 weeks postpartum (weight loss 4.8 kg vs. 7.4 kg in about 4 months) and they reported food intakes that were more than 500 kcal above those reported by nonlactating women (intake of 1974 kcal vs kcal, respectively). Van Raaij et al. 3 used 5-day weighed food measures to reveal that between 5 and 27 weeks postpartum, lactating Dutch women consumed 2462 to 2672 kcal/day (weekly averages) whereas nonlactators were averaging 1678 kcal/day (almost 800 kcal/day difference). Intake at 12 weeks of pregnancy had averaged 2001 kcal/day. Sadurskis et al. 4 also used weighed food intake (4 days) to determine that lactating Swedish women consumed 2280 kcal/day at 2 months postpartum compared with 2000 kcal/day before pregnancy. In a study of 24 lactating Dutch women, Spaaij et al. 5 reported a smaller increase in energy intake during lactation, from 2299 kcal before pregnancy to 2450 kcal at 10 weeks postpartum. Differences in reported energy intake were also observed in a cross-sectional study in the United States based on the Continuing Surveys of Food Intakes by Individuals, Whereas breastfeeders reported intakes of 1914 kcal before the first 90 days postpartum, declining to 1790 kcal after 90 days postpartum, nonlactating women reported intakes of 1558 and 1643 kcal for the same time periods. The smaller intake differences observed between lactators and nonlactators, particularly after 90 days postpartum, may reflect the inclusion of even token breastfeeders in the lactating group. Similarly, in a group of 45 well-nourished women in Texas, weight loss was slow 0.67 kg/month after the first month and weight remained 5% above prepregnancy weight at 4 months postpartum. 7 Dietary intake provided 2186 kcal/day, declining from 2334 kcal/day in the first month to 2092 kcal/day in the fourth month. Only 156 kcal/day were mobilized from tissue reserves. In these women, dietary intake was correlated with milk production, accounting for 13% of the variance in milk production. In less affluent women, this correlation might be thought to indicate a dependence of milk Table 1. Dietary Intake in Women Using Different Infant Feeding Methods Methods Number of Cases SD Exclusive breastfeeding Combination feeding Formula feeding From reference 1. Nutrition Reviews, Vol. 62, No. 7 S113

3 production on dietary intake, such that higher intakes promote greater milk production. In this population, it likely illustrates that when possible, women who can produce large volumes of milk will increase dietary intake to do so, rather than increase the rate of utilization of fat stores. That is, higher milk production induces increased intake, despite the availability of copious energy stores. Dewey et al. 8 examined weight and intake changes in nondieting breastfeeders and a formula feeding group (which included women who breastfed briefly). This study is especially informative because of its focus on women who were not dieting. Both groups lost weight, but breastfeeders lost weight more quickly than formula feeders beginning at about 2 months postpartum. By 6 months postpartum, breastfeeders had lost about 2.8 kg more than the formula feeders. Breastfeeders were at their prepregnancy weight by 1 year postpartum whereas bottle feeders remained about 4% above prepregnant weight through 2 years postpartum, having stopped losing weight by 9 months postpartum. It is likely that most studies that observe greater weight loss in formula feeders are seeing the effect of intentional food restriction. As in other studies, this study found that breastfeeders lost more weight between 9 and 12 months than between 3 and 6 months. The authors posit that the high prolactin levels of early lactation increase maternal intake, if infant demand is high. Later in lactation, when prolactin levels have lowered, high-energy demand will promote fat mobilization. Thus, lactation might promote maternal weight loss only if it is extended beyond several months. Motil et al. 9 studied lactating, nonlactating and nulliparous Texan women every 6 weeks between 6 and 24 weeks postpartum and also at 52 weeks postpartum, or at equivalent times in the nulliparous women (Table 2). Despite a regular loss of weight and fat during the year-long study, at 1 year postpartum body weight remained 5.3% and 7.7% above prepregnancy levels in the lactating and nonlactating groups, respectively. Nonlactators had a lower energy intake than nulliparous women at all visits, whereas lactators maintained a significantly higher intake than both of these groups through 24 weeks postpartum, while continuing to preserve some fat. This suggests that lactation can proceed while the mobilization of fat is controlled, with preferential maintenance of higher intake levels. Women from developed countries adapt to lactation in part by increasing energy intake. But what happens in places where food is very limited? Martinez et al. 10 followed 36 Mexican women on a very low-fat diet (10.8% of calories) through 8 months of lactation. Intake increased only about 250 kcal/day during pregnancy, although mean pregnancy weight gain was low (6.1 kg through the mid third trimester). One might conclude that there was little possibility of increasing intake adequately during lactation, when energy needs are higher. However, energy intake at 6-8 months lactation was about 660 kcal/day above preconception intakes. The findings of this study suggest that lactation promotes increased intake much more than pregnancy does, at least on a very low fat diet. Studies that have examined both food intake and activity are revealing. In 10 affluent British women followed during lactation and after weaning, similar adjustments of intake have been observed during lactation. 11 During lactation at weeks 4 and 12 postpartum, the women showed a dietary intake of 305 to 415 kcal/day more than after weaning. However, total energy expenditure was an average of 164 to 225 kcal/day less at 4, 8 and 12 weeks lactation than after weaning. Under these conditions, weight loss was slow during lactation 0.26 kg between weeks 4 and 12 of lactation (about 4.6 g/day). Women met 56% of the costs of lactation through increased intake and the remainder largely through decreased physical activity. The results of the study of Dutch women mentioned earlier are broadly similar. 5 At 10 weeks postpartum, energy intake was increased about 150 kcal/day while active time was reduced by 45 minutes per day. These well-nourished women responded to the needs of lactation by increasing intake a bit, decreasing activity and mobilizing an estimated 15.7 g of fat per day (during the second month after delivery) with an estimated energy value of 144 kcal. At 9 weeks postpartum they still had 2.3 kg of additional fat mass above their prepregnant values. We see that well-nourished lactating women show Table 2. Dietary Intake During the First 6 Months Postpartum Approximate Times v1 6wk v2 12 wk v3 18 wk v4 24 wk % Above Pre-pregnancy Weight, v4 Lactating MJ/day 5.3% kcal/day Not lactating MJ/day 7.7% kcal/day Nulliparous MJ/day NA kcal/day From reference 9. S114 Nutrition Reviews, Vol. 62, No. 7

4 only low rates of weight loss while having decreased energy expenditure relative to formula feeders or prepregnancy. What would be the effect of intentionally increased energy expenditure during lactation in similar women? Consistent with the above findings, between 6 8 weeks and weeks postpartum, a group of ad libitum fed, breastfeeding, Californian women whose energy expenditure was increased through increased exercise maintained a higher energy intake and achieved the same rate of weight loss as nonexercising, lactating women. 12 Over time, these exercising women eventually lowered the energy expended in daily activities (and lowered intake) so that their total energy expenditure at the end of the study was no longer greater than the nonexercising group. Thus, dietary intake and total energy expenditure were altered in preference to increasing fat mobilization during lactation. This study of exercise during lactation, 12 although done in a prosperous population, raises an interesting question regarding women in more meager circumstances, where increased intake might not be feasible. It was long known that dietary intake traditionally fell during certain seasons in subsistence economies, because of the lack of readily available food. Thus, it was assumed that undernourished women could not freely choose to increase intake when they were breastfeeding. Would marginally nourished women therefore reduce energy expenditure? It is commonly thought that subsistence farmers would have little opportunity to cut back on energy output because much of their energy is expended gathering wood, or carrying water, or producing food, and it seemed unlikely that those essential activities could be cut back without harm. A study of 32 Gambian agricultural women 13 addressed this question. Detailed activity records were combined with data on the energy cost of activities to determine energy expenditure during pregnancy and lactation. Total energy expenditure ranged from a mean of 2300 kcal/day to 2700 kcal/day in the different seasons, as agricultural workload varied. When the effects of season were removed, total energy expenditure of lactating women was similar to that in early pregnancy. It is interesting to see how this was possible, despite the energy costs of milk production. In these women, energy expenditure was reduced during late pregnancy and early postpartum by reducing the number of days in the fields. The fields were relatively far from the village and, on field days, about an hour would be spent walking to and from them, with an additional hour or so spent at agricultural work. Women would go to the fields less often when they were lactating. For example, women in early pregnancy would go to the fields 77% to 84% of days in the early and late wet seasons, whereas women in late pregnancy or early lactation would go 50% to 67% of days in the same seasons. These more strenuous field activities were replaced by an increase in the hours of sleep and in other very light activities. Thus, decreased energy output is used to achieve energy balance during lactation, even in a society where women s work is crucial to food production. A study of energy expenditure of women in the Northwest Amazon is consistent. 14 These women were responsible for a great deal of agricultural labor in fields that were several kilometers from the village. Measurements of energy expenditure, anthropometric changes, and activity patterns and costs lead to conclusions similar to those suggested by the Gambian study. Total energy expenditure was lower on days spent in the village compared with days spent gardening or collecting food (1782 kcal/day vs kcal/day). Clearly an increase in the proportion of days spent in the village could result in significant energy savings. In past decades, when the consensus of opinion was that increased intake or decreased energy expenditure during lactation were unlikely in subsistence economies, the possibility of improvements in the efficiency of energy utilization was often raised. If activity, basal metabolism, energy metabolism, milk production or synthesis, or the processing of ingested nutrients (reflected in the thermic effect of food) were carried out more efficiently, the energy saving would help the mother provide breast milk and maintain energy balance, without additional energy intake. Changes in efficiency could then explain how women could produce about calories of milk a day in conditions of chronically low energy intakes. The studies discussed above suggest that increased intake and decreased energy expenditure may in fact explain how even subsistence farming women adapt to lactation. Nevertheless, increased efficiency could play a supportive role. Studies looking at improved efficiency have provided some insights. It seems clear now that early assumptions about the energy cost of certain physical activities were wrong. For example, in the study of women in the Amazon, 14 it was found that efficiency for routine activities can be very high when people are very practiced at them. These women, who averaged only 49.6 kg (109 lb) and were only cm (4 10 ) tall, were able to carry burdens of 30 kg, about 60% of their own weight, with minimal increase in energy expenditure. The way they carried objects that they carried all the time involved terrific efficiency of movement, making these seemingly very strenuous activities insignificant in determining energy needs. Thus, there was little chance to save energy by increased efficiency of movement. Another possible source of improved efficiency of energy use during lactation, namely lowering of the BMR, has been examined. In a study of 24 lactating Nutrition Reviews, Vol. 62, No. 7 S115

5 Dutch women, Spaaij et al. 5 compared resting metabolic rate (RMR) before pregnancy and during lactation at 10 weeks postpartum. They observed an increase of 0.17 kj/min during lactation. They also measured postprandial energy expenditure to determine if some energy was saved by reduction in the thermic effects of food (TEF) and found no savings at all. Postprandial energy expenditure was also higher in lactating women, by 0.17 kj/min. Thus, savings did not accrue due to reductions in either RMR or the TEF. It is possible that increased efficiency will be seen in metabolic rate or thermic effects of food only in conditions of extreme and persistent undernutrition coupled with high energy expenditure, but not in other circumstances where women are under less severe energetic/nutritional stress. With respect to the efficiency of milk production, careful work suggests that milk is produced with a high efficiency. For example, based on milk production and energy content and on the increase in resting metabolic rate during lactation, Spaaij et al. 5 estimated that milk was produced with an efficiency exceeding 90%. Early estimates of the efficiency of milk production were much lower and the derived energy intake recommendations err on the side of caution, allowing for very low efficiencies. If efficiency in milk production is very high, there is little leeway for marked improvements as a way to conserve energy. In general, improvements in efficiency do not seem to be a significant contributor to achieving energy balance during lactation. Can energy balance be achieved during lactation, in part by reducing the quality and the quantity of milk during energy-restricted conditions? If a lactating woman has inadequate energy resources, reducing milk energy output will reduce the problem. Studies that have examined both the quality and quantity of milk under different nutritional conditions indicate that maternal fat stores and dietary energy intake influence the composition of milk. For example, women with higher levels of body fat produce higher fat milk. However, when the fat concentration of the milk is higher, the volume produced is lower. 15,16 Generally, the total energy transferred in milk is comparable between leaner women and fatter women, particularly when they are both well fed. However, there probably is a threshold of energy availability below which milk volume is reduced. That is, if milk fat content is very low and the mother s reserves themselves are very low, and if adjustments are not made in energy intake or expenditure to bring her reasonably close to energy balance, milk volume would likely decline. In such circumstances, it is also likely that energy would be deficient during pregnancy. If so, part of its effect would be to decrease birth weight. A lowerweight baby has lower energy requirements and thus, low birthweight decreases the total calorie needs of the newborn. Lower birth weight would thus be another (suboptimal) way that maternal milk energy output could be brought in line with energy availability. Are there conditions under which maternal reserves would be depleted? A longitudinal study examined the components of energy balance during lactation in San Mateo Capulhuac in Mexico. 15 Otomi women from this location had been studied previously and negative energy balance was expected based partly on the prior demonstration of faltering of infant growth at 4 6 months postpartum during exclusive breastfeeding. Enrolled women were divided by BMI into two groups (BMI 23 or 23). Between 2 weeks and 6 months postpartum, body fat decreased slowly ( 1.4 kg) in the lower BMI women, whereas it increased ( 1.1 kg) in the higher BMI group. Although milk fat was higher in the higher BMI group, increased milk volume in the lower BMI group equalized the total energy transferred in milk. Infant growth was not affected by this difference and growth faltering was not observed. Although the lower BMI group may have been less well fed, they were able to provide for lactation needs without high rates of fat mobilization. The authors suggest that secular lifestyle changes have resulted in reduced energy expenditure in this population, enabling more successful lactation performance, even in low weight women. Although lactation proceeds without problem for most women, there are conditions in which lactation may contribute to depletion of maternal energy reserves. Adair and Popkin 17 reported a 2-year study of Filipino women from Cebu. They found that lactation had a negative effect on the weight of urban women with a greater effect for full breastfeeding than partial breastfeeding. The effect was greater if lactation continued longer. In rural women, a negative effect was not seen unless lactation was extended beyond 6 months. Women with an initial low BMI ( 18.5) were the ones least likely to recover, that is, to attain a BMI of 18.5 or more during the 2-year study or before the next pregnancy. As one might have expected, net weight loss by 2 years postpartum was increased by longer lactation, lower energy intake, higher energy expenditure, and lower initial weight. This study may show what can occur if lactating women do not have the opportunity to increase energy intake or decrease energy expenditure. Eventually, tissue mobilization can deplete maternal stores, even if the rate of mobilization is low. If an acute shortage of food occurred only during lactation, and compensatory decreases in energy expenditure were not possible (as might be the case in displaced persons during wartime), milk energy output might be decreased. Having seen that virtually every way of ensuring energy balance during lactation is used when it is needed, it seems likely that if milk energy transfer S116 Nutrition Reviews, Vol. 62, No. 7

6 were reduced, the infant also would have a range of adaptations available so that the effects on growth and development would be moderated. Nevertheless, faltering of infant growth is likely to be one of the manifestations of declining milk output in undernourished women. The research to date indicates that the problem of maintaining maternal weight homeostasis during lactation in conditions of nutritional stress is solved through regulation of available resources in a logical, priority order. These regulatory processes go beyond merely mobilizing fat or taking in more food, or reducing energy expenditure, or changing the composition or volume of milk to increase or slow the rate of maternal weight loss. The data suggest that all of these methods can contribute to ensuring what is the most important aspect of lactation, which is not the maintenance of maternal body weight, nor the full mobilization of stored fat to prevent maternal obesity development, but rather the optimization of the health of the mother/infant dyad. Such regulatory systems as have evolved may not optimize the health of every individual mother-infant pair under all conditions. What do the findings suggest for obese women who may be considering breastfeeding? First, we should be clear that breastfeeding has developed to be the rather fine and precise process it is in circumstances where obesity was unlikely to be common and where the prevention of obesity was unlikely to be a high physiologic priority. In fact, the way in which weight changes spontaneously during lactation in well-nourished women illustrates this point. In early lactation, a slow loss of body weight appears to be physiologic and natural. That is, the majority of lactating women will spontaneously lose weight without consciously dieting. But the usual rate of weight loss is slow (Table 3). Typically, women on average lose about kg/month during the first six months, excluding the immediate postpartum period. 1,5,7,8,11 During the next six months, weight loss is slower, about kg/month. 8,17,18 Table 3. Weight Loss Postpartum in Women Using Different Infant Feeding Methods Group Postpartum Months Exclusive breastfeeders 6.75 kg 1.29 kg 8.30 kg Combination feeders Formula feeders Difference by group NS P 0.05 From reference 1. If food were restricted during lactation, what adjustments would the obese woman make? Based on studies of women with differing access to food, it does not seem that the rate of weight loss will spontaneously rise while milk production and physical activity continue unabated. Instead, restricted women seem to spontaneously cut back on activity. Thus, on lowered energy intake, milk production could remain unaltered but the rate of weight loss would not be noticeably increased due to reduced energy expenditure. Women may act on the basis of feeling fatigued and wanting rest, or they may simply make small changes in their usual activities, but the net result is that they are likely to unconsciously adjust to a reduction in available food energy by lowering energy expenditure. Later in lactation, physiologic priorities may change. If a woman still has more fat than she did before pregnancy, fat mobilization is no longer as risky as it was earlier in lactation. We have seen that dietary intake may decline relative to early lactation, even though that allows continued loss of fat when body fat has already declined. Brewer et al. 1 (Table 3) observed only small differences in weight loss during the first 3 months postpartum among women following different infant feeding practices, but greater weight loss was observed in exclusive breastfeeders than in combination feeders or formula feeders during the next 3 months. Overall, formula feeders lost as much as exclusive breastfeeders. The somewhat lesser weight loss of the combination feeders may be the result of a physiologically slower fat mobilization in early lactation combined with a reduced energy need at 3 6 months postpartum in women not fully lactating. The picture that emerges makes a great deal of sense from an evolutionary perspective. In the first months after delivery, it would be wise for women to be physiologically driven to eat more if it were available, and be driven to be less active if more food were not available. They should rest, if they could, when they could. They should maintain some of the body stores that they had managed to gain during pregnancy because lactation is designed to be a long process and it is more energy costly than pregnancy. Over the long period of lactation, if later circumstances required a decrease in food intake or forced an increase in energy expenditure, both the woman and the baby would be protected if she had not used her fat stores at a fast rate early on, but rather had cut back on activity, or increased intake when that was possible. As lactation continues and the baby gets to a more independent age, it would be more reasonable to expect maternal activity to increase, or dietary intake to decline, even if this allowed depletion of pregnancy fat stores for the maintenance of milk production. As the end of Nutrition Reviews, Vol. 62, No. 7 S117

7 lactation approaches, the need to maintain body fat for help in milk production would be lessened and mobilizing the fat for use would be consistent with maintaining maternal health. These observations lead to a general conclusion as to the role of lactation in body weight regulation. They suggest that if lactation is to be of any benefit in terms of reducing body fat in women who have stored significant fat during pregnancy, it is likely that women will have to breastfeed for a substantial period and not in the pattern that is common in the United States. In the United States, a large portion of women do not breastfeed at all; a large portion of those who do breastfeed combine formula feeding with breastfeeding even within the first week or so after delivery, and another large portion do only token breastfeeding. Overall, few women breastfeed beyond 6 months. Unfortunately, even those who breastfeed for 6 months may actually be token breastfeeding for much of that time. Exclusive breastfeeding to 1 year, where other foods that are introduced complement, rather than substitute for breast milk, is extremely unusual. Under these conditions, lactation will not play a major role in returning most women to a healthy non-pregnant weight. If spontaneous weight loss during lactation tends to be gradual and physiologically designed to occur over a long period of time, we can ask if there is something that we can do to assist that weight loss. A few studies provide limited information. It is possible for women desiring to lose weight to accelerate the rate of weight loss to a fairly significant degree (0.5 to 1 kg a week) by dieting during lactation once lactation is fully established. In a study where an energy deficit of 35% was imposed, milk production and infant growth were not affected, although weight loss was accelerated. 19 However, the period studied was short (11 days). When increased weight loss was achieved just by dieting, however, a part of the loss was lean tissue. 19 This was not the case when increased exercise contributed to the energy deficit. Thus, a less restrictive diet combined with increased physical activity is probably more healthful for the mother than an equal calorie deficit achieved by dieting only. However, other work 12 suggests that the mother may need to exert conscious control of intake and energy output to prevent a compensatory decline in general energy expenditure. The safety of a longer period of milder food restriction has been the subject of another study. 20 The goal of the study was to reduce energy intake by 25% for 10 weeks. Measured intake was reduced by more than 500 kcal per day. The rate of weight loss was increased to 0.45 kg per week with no observed effects on infant growth or milk production. Another consideration is suggested by the study that compared breastfeeding and bottle-feeding women who were not dieting. 8 The 3.2-kg greater weight loss observed in the breastfeeders at a year postpartum compared with formula feeders should be a real encouragement for women to breastfeed. The data shown above demonstrated that breastfeeders can consume much more food than bottlefeeders and still lose more weight if breastfeeding is prolonged. For those in whom weight loss is difficult, or who entered pregnancy overweight or obese, or who do not diet when weight rises, the possibility of achieving a more desirable weight without consciously restricting intake should not only be appealing, but may actually be the factor that makes it possible to lose residual pregnancy weight. There are two caveats regarding the application of these findings in an intervention. First, it is doubtful that most women could follow the necessary diet and energy expenditure protocols without professional help. In addition, individual women vary tremendously in their responses to energy stress during lactation. Some women might not be able to adequately adapt to such interventions by increasing fat mobilization and in these women milk production might falter. Certainly this concern would be greater the more marked the energy restriction. These findings indicate that it may be difficult to prevent permanent weight retention after pregnancy in overweight women simply by promoting breastfeeding. Even more disheartening is the observation that obese women are less likely than nonobese women to initiate breastfeeding and to continue breastfeeding successfully. 21,22 This reduced success at breastfeeding may be due to metabolic differences in the obese that directly affect the mammary gland during pregnancy or after delivery. Alternatively, it may be that postpartum behaviors or biological conditions important for successful breastfeeding differ in obese women. Future research will be needed to elucidate the processes involved. In the meantime, obese women may need more support and direction when they begin breastfeeding. In summary, the current state of knowledge suggests several actions that health professionals can take to decrease obesity development in reproductive age women. First, attention should focus on preventing excessive pregnancy weight gain, which is a very serious, common occurrence in U.S. women. In most recent reports, more than a third of pregnant women gain excessively and excessive gains are even more frequent in the overweight and obese. Second, more needs to be done to assist women to avoid weight gain during the postpartum year, and to remain on a path of gradual weight loss. Too many women gain weight during this period. Third, breastfeeding, especially breastfeeding beyond 6 months, should be promoted and supported because of its salutary effects on maternal weight, its support of a more natural level of maternal food intake, as well as its benefits for the child. S118 Nutrition Reviews, Vol. 62, No. 7

8 Although some women can lose weight if they do not breastfeed, in environments where food is available continuously and in excess, the possibility of losing weight while consuming additional calories should encourage weight loss in many more women. Finally, women who have demonstrated difficulty in weight control, namely those who were overweight or obese before pregnancy or who gained excessively during pregnancy, should be identified as less likely to lose adequate weight postpartum, or at risk of gaining additional weight. Special focus on these women might be most efficient for reducing the contribution of reproduction to increased obesity rates. Without such efforts, many women will permanently increase their weight with each pregnancy. 1. Brewer MM, Bates MR, Vannoy LP. Postpartum changes in maternal weight and body fat depots in lactating vs nonlactating women. Am J Clin Nutr. 1989;49: Chou TW, Chan GM, Moyer-Mileur L. Postpartum body composition changes in lactating and nonlactating primiparas. Nutrition. 1999;15: Van Raaij JMA, Schonk CM, Vermaat-Miedema SH, et al. Energy cost of lactation, and energy balances of well-nourished Dutch lactating women: reappraisal of the extra energy requirements of lactation. Am J Clin Nutr. 1991;53: Sadurskis A, Kabir N, Wager J, Forsum E. Energy metabolism, body composition, and milk production in healthy Swedish women during lactation. Am J Clin Nutr. 1988;48: Spaaij CJK, van Raaij JMA, de Groot LCPGM, et al. Effect of lactation on resting metabolic rate and on diet- and work-induced thermogenesis. Am J Clin Nutr. 1994;59: Murphy SP, Abrams BF. Changes in energy intakes during pregnancy and lactation in a national sample of US women. Am J Public Health. 1993;83: Butte NF, Garza C, Stuff JE. Effect of maternal diet and body composition on lactational performance. Am J Clin Nutr. 1984;39: Dewey KG, Heinig MJ, Nommsen LA. Maternal weight-loss patterns during prolonged lactation. Am J Clin Nutr. 1993;58: Motil KJ, Sheng HP, Kertz BL, et al. Lean body mass of well-nourished women is preserved during lactation. Am J Clin Nutr. 1998;67: Martinez H, Allen LH, Lung aho M, et al. Maternal fatness in Mexican women predicts body composition changes in pregnancy and lactation. Adv Exp Med Biol. 1994;352: Goldberg GR, Prentice AM, Coward WA, et al. Longitudinal assessment of the components of energy balance in well-nourished lactating women. Am J Clin Nutr. 1991;54: Lovelady CA, Nommsen-Rivers LA, McCrory MA, Dewey KG. Effects of exercise on plasma lipids and metabolism of lactating women. Med Sci Sports Exerc. 1995;27: Lawrence M, Whitehead RG. Physical activity and total energy expenditure of child-bearing Gambian village women. Eur J Clin Nutr. 1988;42: Dufour DL. The time and energy expenditure of indigenous women horticulturalists in the northwest Amazon. Am J Phys Anthropol. 1984;65: Barbosa L, Butte NF, Villalpando S, et al. Maternal energy balance and lactation performance of Mesoamerindians as a function of body mass index. Am J Clin Nutr. 1997;66: Perez-Escamilla R, Cohen RJ, Brown KH, et al. Maternal anthropometric status and lactation performance in a low-income Honduran population: evidence for the role of infants. Am J Clin Nutr. 1995;61: Adair LS, Popkin BM. Prolonged lactation contributes to depletion of maternal energy reserves in Filipino women. J Nutr. 1992;122: Janney CA, Zhang D, Sowers MF. Lactation and weight retention. Am J Clin Nutr. 1997;66: McCrory MA, Nommsen-Rivers LA, Mole PA, et al. Randomized trial of the short-term effects of dieting compared with dieting plus aerobic exercise on lactation performance. Am J Clin Nutr. 1999;69: Dusdieker LB, Hemingway DL, Stumbo PJ. Is milk production impaired by dieting during lactation? Am J Clin Nutr. 1994;59: Hilson JA, Rasmussen KM, Kjolhede CL. Maternal obesity and breast-feeding success in a rural population of white women. Am J Clin Nutr. 1997;66: Rutishauser IHE, Carlin JB. Body mass index and duration of breastfeeding: a survival analysis during the first six months of life. J Epidemiol Commun Health. 1992;46: Nutrition Reviews, Vol. 62, No. 7 S119