An energy deficit can also occur when energy expenditure is unusually high (in excess of energy intake). Lovelady and colleagues (1990) compared the lactation performance of eight physically fit, exercising women and eight sedentary controls. There were no significant differences in milk volume or composition despite wide group differences in energy intake and expenditure. Exercising subjects compensated for higher energy expenditure by increasing their energy intake,2 so there was no net difference in energy deficit between groups.
Studies in rats have been conducted to investigate the effects of dietary supplementation on milk production. Rolls et al. (1980) fed rats a high-energy, low-protein supplement in addition to their usual diet during lactation. They found that although maternal food intake increased among the supplemented compared with the control animals, litter growth rate was reduced, indicating a reduction in milk output. In contrast, Roberts and Coward (1985) provided adequately fed rats with a supplement with the same protein-to-energy ratio as that of their usual diet and observed a mean increase of 31% in milk output. On average, the rats in the latter study increased both their protein and energy intakes by 20%; therefore, it is not possible to distinguish between the effects of energy and those of protein on milk volume. In a study that is more analogous to energy supplementation trials in humans, Kliewer and Rasmussen (1987) restricted rats to 50% of usual energy intake before and during pregnancy and then allowed them to feed ad libitum during lactation. Milk volume and litter growth in this group were equivalent to those of control animals.
Findings from energy supplementation studies in humans are not conclusive. Many efforts have been made over the years to ''feed the nursing mother, thereby the infant" (Sosa et al., 1976, p. 668) with mixed success. In a review of such studies up to 1980, Whitehead (1983) concludes that the results "have not been inspiring" (p. 44-45). The studies presented here are in chronologic order.
Rural Mexico. Several major investigations were conducted in Latin America and Asia during the 1960s and 1970s, but only one—a 2-year longitudinal study in a rural area of Mexico—included actual measurements of milk volume (Chávez and Martinez, 1980). In the first year of the study, one group of 17 mothers consuming their usual diet was followed throughout lactation. Milk