Conclusions and comments

1. The energy intake of women did not increase during pregnancy, and it was comparable to that of nonpregnant and non-(actating Cambridge women of similar age and social background: 2,029 ± 392 kcal/day (Nelson et al., unpublished).
2. In spite of these low intakes, mean weight gain during pregnancy coincided with the conventional standard of 12.5 kg. Mean birth weight was also normal, as was the weight retained by the mothers after parturition and hence the estimation of their energy stores for lactation.
3. Since the pregnant women were not demonstrably less active, it would appear that they must have been able to satisfy at least part of the additional needs of pregnancy by subtle changes in activity or by an enhanced efficiency of metabolism.
4. Milk outputs did not increase with dietary intakes above 2,000 kcal per day, but they decreased with lower levels of energy intake.
5. The maintenance of substantial amounts of milk production at relatively low levels of energy intake cannot be explained just on the basis of an increased utilization of the subcutaneous fat stored during pregnancy. At the mean dietary energy increment of 281 kcal/day observed during lactation in the present study, it can be calculated from figure 2 that only 124 additional kcal/day would be made available from the body weight changes (a loss of 19 9 body fat/day), resulting in a net gain of 405 kcal/day. Yet the mean volume of milk produced, 747 g/day, would contain an average 515 kcal. Conventional estimates of the efficiency of conversion of dietary energy for human milk production have clearly been misleading. The present results show that calculated efficiency factors vary with the level of energy intake. At a dietary increment of 800 kcal/day during lactation, the calculated efficiency of conversion would be the generally accepted value of 80 per cent, but at the mean measured increment of 281 kcal/day, as just calculated, the apparent efficiency is 127 per cent -at first sight a nonsense factor. The anomaly can be resolved, however, if it is accepted that there have been substantial compensatory alterations in the non lactational component of the mother's physiological efficiency.
6. We have produced circumstantial evidence for energy-conserving adaptations during lactation as well as pregnancy that ensure foetal development and subsequent milk production without the need for excessively high energy intakes or, alternatively, drastic changes in the body composition of the mother. The data presented indicate that, for the type of population studied, current recommended daily amounts of dietary energy during pregnancy and lactation are unnecessarily high: lower mean values of 2,000 kcal and 2,400 kcal, respectively, could be safely adopted. The suggested value for lactation would also enable the mother to attain her pre-pregnancy weight within a more acceptable time.

Acknowledgements

We thank Professor C. Douglas, Mr. R.E. Robinson, and Dr. N.R.C. Roberton and the staff of the Cambridge Maternity Hospital for their collaboration, and Miss M.J. Whichelow for her help in recruiting the subjects. This study was financially supported by the Department of Health and Social Security.