There is conflicting evidence regarding whether BMR increases, decreases, or stays constant during lactation. Since the energy cost of milk synthesis is included within the BMR, one might predict BMR to increase slightly during lactation. Sadurskis et al. (1988) calculated an approximately 5.6% increase in BMR over prepregnancy values in the same women at 2 months of lactation, correcting for changes in body composition. Earlier studies in India also showed higher BMRs among lactating women than among nonlactating, "normal" Indian women (Khan and Belavady, 1973; Venkatachalam and Gopalan, 1960). In contrast, other investigators found lower BMRs during lactation than expected on the basis of weight and height (Blackburn and Calloway, 1985; Lovelady et al., 1990; Prentice and Whitehead, 1987; van Raaij et al., 1987) or no difference between lactating and nonlactating women (Illingworth et al., 1986; Schutz et al., 1980).
A 30% reduction in postprandial thermogenesis during lactation was observed in one study (Illingworth et al., 1986). This has little overall impact, however, since postprandial thermogenesis represents only about 10% of total energy expenditure, and thus, only about 60 kcal is saved per day.
There is considerable variation in the degree to which energy expenditure for activity could be reduced during lactation. During the postpartum period, the demands of feeding and caring for an infant take up considerable time but may require either more or less energy than the woman's former activities. Total activity levels of lactating mothers may be constrained by being housebound with a new baby. Data from small samples of relatively sedentary lactating women in the United States indicate that total energy expenditure (not including milk production) averages only 1,800 to 1,900 kcal/day (Blackburn and Calloway, 1976; Lovelady et al., 1990); the estimated expenditure for nonlactating women, assuming light to moderate activity, is 2,200 kcal/day (NRC, 1989). In contrast, lactating women exercising on a regular basis expended an average of 2,631 kcal/day, not including energy output in milk (Lovelady et al., 1990). Women who must care for several children or who are employed in physically demanding jobs may also have high activity levels. Mothers who do not have access to adequate food cannot always decrease their workload to reduce their energy deficit (Singh et al., 1989). If these mothers decrease other day-to-day activities as an energy-sparing mechanism, there may be adverse effects on their quality of life.
Studies in lactating rats (Jansen and Monte, 1977; Naismith et al., 1982; Sampson and Jansen, 1985) and in swine (Mahan, 1977) indicate that protein intake can increase milk volume independently of total energy intake. Early studies in humans by Gopalan (1958) and Edozien et al. (1976) suggest the same relationship: milk output of women in India and Nigeria increased when