(introduction...)

Objective
Experimental details
Summary of main results
Conclusions and comments

R G. Whitehead, A.A. Paul, A.E. Black, and S.J. Wiles
Medical Research Council Dunn Nutrition Unit, Cambridge, England

Objective

To obtain information on the dietary energy intakes of British women during pregnancy and lactation, since estimates of the dietary intakes of individuals living in the United Kingdom have shown a consistent downward trend in recent years.

Experimental details

1. Subjects
Twenty-five women were recruited near the beginning of the second trimester of pregnancy through the antenatal clinic of the Cambridge Maternity Hospital. They were 21 to 35 years old (mean 29) and belonged to social grades I, II, and III. Twelve were primiparous and worked during most of their pregnancy, mainly in a clerical capacity. None of the multiparous mothers had outside jobs. Their mean height was 161.7 cm (147.5 to 172.5), and the stated pre-pregnant weight was 56.2 (43.5 to 71.7). The validity of the latter measurement was verified by comparison with the initial weight found on recruitment.

2. Dietary Intake
Energy and nutrient intakes were measured over four consecutive days each month throughout pregnancy and lactation by the mother herself, after instruction, weighing the food and drink she consumed. The food intake measurements were interpreted using food composition tables.

3. Weight Changes and Stored Energy
A number of anthropometric measurements were made, including weight, at monthly intervals throughout pregnancy, at two weeks after delivery, and then once again at monthly intervals. Energy stored as fat during pregnancy was estimated from the difference in body weight between two weeks postpartum and the pre-pregnant weight, making the assumption that adipose tissue provides, during lactation, 6.5 kcal/g body-weight change (Thomson et al., Brit,J. Nutr., 24: 565 [1970] ).

4. Duration of Pregnancy and Birth Weights
Birth weights, which were all over 2.6 kg, were obtained by the maternity hospital staff. Mean gestational age was 39 completed weeks, range 36 to 43 weeks.

5. Breast-milk Production
Breast-milk intake was also measured by the mother on four consecutive days each month by test weighing, using Salter Baby weigher Model 40 Scales. The test weighing measurements in a number of subjects were checked by the recently developed deuterium oxide method (Coward et al., Lancet, ii: 13 [1979], the milk intakes showing good agreement between the two procedures.

Summary of main results

1. Pregnancy
a. The principal pregnancy data are given in table 1. There was no significant difference in energy intake between the second and third trimesters of pregnancy, and both values agreed closely with those for the first trimester reported by

Smithells in Leeds (Brit. J. Nutr., 38:497 11977] ).

TABLE 1. Energy Intakes and Body-Weight Changes of 25 Mothers during the Second and Third Trimesters of Pregnancy (Mean ± S.D.)

Energy intake, 2nd trimester	(kcal/day)	1,950 ± 380
3rd trimester	(kcal/day)	2,005 ± 345
2nd & 3rd trimester	(kcal/day)	1,978 ± 350
Weight gain during pregnancy*	(kg)	12.6 ± 4 .0
Estimated maternal energy store	(kcal)	38,662 ± 28,570
Birth weight	(kg)	3.31 ± 0.35

* To the 36th week.

TABLE 2. Energy Intake and Milk Output of 17 Mothers at Different Stages of Lactation (Mean + S.D.)

Month of lactation	Energy intake (kcal/day)	Breast-milk output (9/24 hr)
2	2,278 ± 458	715 ± 148
3	2,300 ± 470	773 ± 140
4	2,380 ± 408	755 ± 136

b. Among the Cambridge mothers, there was no correlation between energy intake in the last trimester of pregnancy and birth weight (r = 0.01).

2. Lactation
a. Of the 25 mothers, 4 breast-fed for only 2 to 14 days, and another 4 for less than three months. Seventeen breast-fed at least up to the beginning of the fifth month, 11 exclusively, but the remaining 6 had by then introduced small amounts of other foods, which supplied only an average of 18 per cent of the babies' total energy intake.

b. The basic lactation data are summarized in table 2. Lactation was associated with an increase in food intake, but daily energy consumption was still 450 kcal less than the United Kingdom's DHSS Recommended Daily Amount. In the fourth month, there was little difference in energy intake among the 11 mothers who were exclusively breast-feeding (2,278 ± 431 kcal [mean ± S.D.] ) and the six who were not (2,363 ± 402 kcal), although the mean milk output of the former, 791 ± 93 g/day, was higher than that of the mothers who were using mixed feeding, 688 ± 186 g/day.

c. Figure 1 shows the relationship between dietary energy intake and milk output. The line of best fit (r = 0.76, p < 0.001) was significant(y curvilinear (p < 0.01). Mean milk outputs were not significantly different in mothers with energy intakes of 2,000 to 2,400 kcal/day and in those with intakes over 2,400 kcal-768 ± 63 to 780 ± 148 g/day, respectively. Energy intakes below 2,000 kcal were, however, associated with significantly lower milk outputs: 455 ± 227 g/day (t-4.2, p < 0.001). Three of the mothers who could not breast-feed for more than two months had intakes below 1,720 kcal/day, and they had the three lowest milk outputs. Data for the fourth mother were omitted because she had been complying with advice to eat beyond her appetite-3,338 kcal/day-in an unsuccessful attempt to boost her milk output.

FIG 1. Relation between Maternal Energy Intake and Breast-Milk Output.

Most points are the mean of dietary energy and milk output measurements over 12 days during the second, third, and fourth months of lactation.

d. There was no important correlation between the average amount of milk that the individual mother produced during the first four months and her corresponding loss of weight (r = 0.20 NS). There was a significant correlation between weight loss and overall energy intake during lactation (r = 0.56, p < 0.02), and the relationship was even stronger (r = 0.78, p < 0.001) when weight change was related to the increase in energy intake that occurred when the mothers passed from pregnancy into lactation.

FIG. 2. Relation between Lactation Dietary Energy Increment and Weight Loss over the First Four Months Post Partum

Figure 2 predicts that at the DHSS recommended daily energy increment for lactation-600 kcal-mothers would lose none of their excess fat, while at the mean increment for the group studied, 281 kcal, the mothers lost weight at an average rate of 570 g/month. e. Figure 3 partly explains why some mothers ate so little extra during lactation. There was a high(y significant negative relationship (r = 0.73, p. < 0.001) between the extra food energy consumed during lactation and the amount of weight a mother had retained after her pregnancy.

FIG. 2. Relation between Weight retainrd after Pregnancy and Lactation Dietary Energy Increment

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.