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Working Group: LH Allen,1 JGAJ Hautvast2 and AM Prentice3
1Department of Nutrition, University of California, Davis, CA 95616-8669, USA; 2Department of Human Nutrition, Agricultural University, 6700 EV Wageningen, The Netherlands; 3Dunn Clinical Nutrition Centre, Hills Road, Cambridge CB2 2DH, UK
Descriptors: pregnancy,
lactation, energy requirements
1. Two approaches to estimating energy needs were
reviewed: (A) summation of measured or estimated energy costs, and (B) observation of
usual energy intakes in well-nourished pregnant women achieving a successful outcome of
pregnancy. Method A usually results in higher estimates of needs than Method B. Each is
backed by a substantial amount of good quality data. Changes in maternal physical activity
help to resolve the paradoxical estimates.
2. Approach A summates as follows: S Edep (conceptus) + S Edep(fat) + S maintenance where S Edep(conceptus) = 42 MJ (includes 400 g fetal fat); S Edep(fat) = 2.5 kg × 39 kJ/g = 98 MJ and maintenance costs= 160 MJ. This gives a total of 300 MJ equivalent to 1.07 MJ/ day (rounded to 1.1 MJ/day).
3. The full extent of these costs should be made explicitly apparent by use of Figure 13 from the paper by Prentice et al.
4. Maternal energy needs will be estimated as for non-pregnant women using the BMR × PAL system. BMR (in groups of women) should be calculated as NPNL BMR with increments of 0.2, 0.4 and 1.1 MJ/day for each trimester.
5. Extension of the BMR × PAL system to pregnancy allows potential changes in physical activity in pregnancy to be judged on a population by population basis. A potential artifact caused by increases in BMR, the denominator for PAL calculations, in pregnancy is relatively trivial. There is evidence that physical activity decreases in most pregnant women by about 0.15 PAL by term (about 0.07 PAL over all pregnancy). In many populations it appears reasonable to incorporate this decrease into calculations of requirements. However, it must be stressed that for certain groups of women (e.g. in developing countries, in paid manual employment, or in women who already have very low levels of activity) such decreases in activity may be impossible.
6. Allowing for physical activity to decrease as above reduces total needs by 110 MJ to 190 MJ or 0.7 MJ/day. This could be adopted as a subsidiary recommendation as in the 1985 report.
7. Examples of how to calculate pregnancy requirements are shown in Table 7 of the paper by Prentice et al.
8. The Group recognised that, in practice, there are wide variations in the maintenance costs of pregnancy in different individuals and populations. These appear to be related to maternal energy status in a manner which allows successful pregnancies over a wide range of intakes. However, these should not be used to modify recommendations because the long-term consequences of the need to adapt are unknown.
9. The possibility of developing different prescriptive energy intake recommendations for women of different BMIs was considered, but rejected for lack of information. However, the group endorsed the US Institute of Medicine recommendations on differential optimal weight gains.
10. The final recommendations should be 300 MJ or 1.1 MJ/day throughout pregnancy but may be reduced to 200 MJ (0.7 MJ/day) in women who have the option of reducing activity in pregnancy.
11. Approach B indicates that well-nourished women appear to increase energy intake by an average of only 0.3 MJ/day (totalling 84 MJ) in pregnancy. There is some reason to believe that this figure may underestimate the true increment but by only 0.2 MJ/day. This would raise the total increment to 140 MJ.
Correspondence to: AM Prentice
Future research needs
1. There is a need to understand the physiological basis and potential consequences of alterations in maintenance needs during pregnancy.
2. Research is needed on the relationship between pre-pregnant BMI and maternal energy needs.
3. Research is needed on the influence of diet on prevalence of low birthweight.
4. Research is needed on the causes and consequences of extremes of weight gain.
5. Research is needed on the consequences of gestational nutrition for lactation.
6. Research is needed on the influence of fat: carbohydrate ratios of diets to maternal weight gain.
7. Further consideration is needed of how micronutrient requirements are met in pregnancy.
1. It was agreed that the 1985 principles of calculating energy needs of lactation were appropriate:
Maternal needs (BMR × PAL) ± D body fat + energy transfer in milk2. Any changes in activity are accounted for in the BMR × PAL calculation.
3. The agreed assumptions are: milk volumes as in Table 12 of the paper by Prentice et al; energy density = 2.8 kJ/g (with revised justification) and efficiency = 80% (with revised justification).
4. The primary recommendations should make no allowance for possible post-partum weight loss. A secondary recommendation should make an allowance of 500 g/month fat loss over the first 6 months post partum. Recommendations after 3 months will differ for women whose infants continue to receive breast milk as the only source of milk. The final recommendations are listed in Table 12 of the paper by Prentice et al.
Future research needs
1. Future research could usefully concentrate on special situations (e.g. working and exercising women).
2. The effects of supplementation on lactational performance remain controversial. In particular, the effect of fat intake on milk fat content is of interest.
3. The relationship between maternal BMI and energy intake and weight changes in lactation need further investigation.
4. As for other groups, there is a need for more information on PAL values for women around the world.