measurements. In that analysis, no adjustments were made for the potential influence of infant weight. However, in an analysis of data collected at different times for the same individuals, there was a significant association between increase in maternal body weight and increased milk volume even when the analysis was controlled for infant weight. Curiously, these mothers tended to gain weight during the first 3 months post partum, despite producing an average milk volume of 550 to 690 g/day.

In The Gambia, milk volume during the baseline period (before food supplementation) was positively correlated with maternal weight and height, but not when infant weight was included in the analysis (Prentice et al., 1986). In an earlier study of 16 lactating women in The Gambia, milk volume measured by 12-hour test weighing was inversely related to increases in maternal triceps and subscapular skinfold thicknesses during weeks 6 to 12 of lactation (Paul et al., 1979). This surprising finding was interpreted by the authors to indicate that there is competition between milk synthesis and replenishment of maternal fat stores during lactation.

The findings of other observational studies of maternal weight for height and milk volume are often difficult to interpret because they include infants who are not exclusively breastfed. For example, van Steenbergen et al. (1983) compared lactation performance of low weight-for-height women (70 to 80% of expected) in Kenya with the performance of normal-weight women (90 to 115% of expected) during the first 6 months post partum. In the low weight-for-height group, milk volume was found to be lower (695 compared with 790 g/day), but milk energy density was approximately 12% higher. In both groups, most infants were given supplementary foods, usually beginning in the third month. No data comparing only the exclusively breastfed infants were provided. In studies of mixed-fed infants, one cannot determine retrospectively whether low milk volume leads mothers to supplement early or whether early supplementation leads to low milk volumes.

The complaint of having an insufficient supply of milk for the baby is heard in both well-nourished and poorly nourished populations, but the incidence of this complaint was not related to maternal nutritional status in cross-cultural studies conducted by Tully and Dewey (1985).

Energy Utilization During Lactation

There are several potential mechanisms of energy conservation during lactation in addition to the mobilization and utilization of fat. Prentice and Prentice (1988) described three such "energy-sparing adaptations" that may permit lactation to proceed normally when energy intake is limited, including decreases in basal metabolic rate (BMR), thermogenesis, and physical activity. Whether these types of decreases should be considered adaptations is a matter of considerable debate.

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