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Recommendations for pregnancy

We recommend one of two possible methods of expressing the energy requirements of pregnancy. The first is a new proposal which extends the philosophy of using PAL values into pregnancy. The second is in line with the 1985 recommendations which simply refer to fixed increments, with optional reductions if activity is reduced.

Method 1: Expressing the energy requirements of pregnancy in terms of PAL values

The 1985 report treats pregnant women as non pregnant women with special incremental needs for energy deposition, and then makes some crude assumptions as to whether or not physical activity might alter. It could be argued that this misses an opportunity since the whole concept of recommendations based on PALs was designed to allow for differences in activity between groups of people and could logically be extended into pregnancy. There would be several theoretical advantages to unifying the methods of expression in this way, particularly as any assumptions about behavioural decreases in physical activity during the reproductive cycle could then be incorporated into the recommendations in a more formalised and explicit manner. We explored the possibility of using such an approach and recommend that its merits be considered.

We commented above that the Schofield equations cannot be used to accurately predict BMR during later pregnancy, but that fixed increments to the NPNL BMR could be proposed on the basis of the marked similarity between changes in BMR observed in well nourished women (see Figure 1). Additions of 0.2, 0.4 and 1.1 MJ/day were proposed for each trimester.

The use of PALs is somewhat complicated during pregnancy because BMR, the denominator in the calculation, increases over time. This means that a woman expending the same amount of energy on activity (TEE-BMR-DIT) throughout pregnancy would have a progressively decreasing PAL. If we take an example of a woman who performed exactly the same number of activities for the same duration and at the same intensity, PAL values would decline. This is because the energy cost of activity would only increase by about 10% in the last trimester (due to the mixed influence of weight-bearing and non-weight-bearing activity), whereas BMR would increase by 20%. The decline in PAL would also be greater in a more active woman. These changes are illustrated in Table 6 for hypothetical women and show that these theoretical objections to using PALs in pregnancy are not very significant in practice. Analogous objections can be raised to their use in obese subjects and these have been tolerated by previous expert committees.

Table 6 Examples to illustrate how PAL declines during pregnancy even if activity remains constant

^a Assumes that the net cost of activity remains unchanged.
^b Assumes a 10% increase in net cost of activity.

If this approach is adopted it would need to be combined with a statement indicating that PALs do appear to decrease by about 0.1-0.2 units, at least in affluent women, as pregnancy progresses. For women in developing countries the obligatory nature of agricultural labour may remove opportunities for energy-sparing decreases in activity and PALs may remain constant (cf. Singh et al, 1987). The approach suggested here would readily accommodate such differences in behaviour and would highlight the fact that the energy requirements of pregnancy are only low if physical activity is reduced.

A complete rationalisation of the energy requirements of pregnancy in terms of PALs would require that the energy deposited be expressed as a PAL increment (i.e. as a multiple of BMR). This would only be possible if the required energy for deposition was known to be proportional to BMR, which is not the case. Such a procedure would be particularly flawed if the US recommendations (Nutritional Status and Weight Gain During Pregnancy, 1990) for differential weight gains according to initial BMI are adopted, because we would be recommending that thin women (with low BMRs) should gain more tissue (with a greater cost of deposition). This would require a whole set of PAL increments and thus be impractical. We recommend the use of fixed increments for energy deposition of 0.4, 0.7 and 0.5 MJ/day as discussed earlier.

Examples of the proposed approach. Table 7 illustrates two examples of how energy requirements in pregnancy can be estimated using PALs. The great advantage of such an approach is that it separates out the obligatory costs of pregnancy from possible energy-sparing adjustments, and therefore emphasises the fact that the energy requirements of an optimal pregnancy are high and must be met either from the diet or by decreasing activity.

Method 2: expressing energy requirements of pregnancy in terms of fixed increments

This approach is the same as employed in the 1985 report, but quantitatively updated to incorporate the large amount of new information available.

Figure 13 combines all the data from well-nourished women reviewed in previous sections to build up a picture of the energy needs of an average optimal pregnancy by summing the costs of maintenance, energy deposition, increased DIT and the passive increase in the cost of weight-bearing activity. This figure assumes that there is no change in activity pattern from NPNL. The average costs by trimester are 0.40, 1.10 and 1.80 MJ/day (96, 265 and 430 kcal/day), respectively. The total cost is 308 MJ (73 600 kcal). This is very similar to the estimate used in the 1985 recommendations.

Table 7 Examples of how to calculate energy requirements in pregnancy from pregnancy PAL values

^a All values are hypothetical and for illustrative purposes.
^b Calculated by applying Schofield formula (1985) to average NPNL body weight for populations in question.

Figure 14 uses the same data as in Figure 13, but plots them relative to the average observed increase in EI in affluent pregnant women. The average costs by trimester for such pregnancies are, by definition, equal to the observed increase in intake and are 0.14, 0.38 and 0.42 MJ/day, respectively, or 0.3 MJ/day over the whole of pregnancy. The total cost would be 84 MJ (or 140 MJ if we allowed for potential progressive under-reporting in longitudinal studies). Another way of arriving at this figure is to add up the total costs of each component over the whole of pregnancy as:

S E_{CONCEPTUS +} SE_FAT + SE_MAINTENANCE where SE_CONCEPTUS = 42 MJ or 10 000 kcal (includes 400 g fetal at); SE_FAT = 2.5 kg × 39 kJ/g = 98 MJ (23400 kcal); and SE_MAINTENANCE = 160 MJ (38 000 kcal).

Figure 13 Aggregated energy costs of pregnancy if physical activity remains unchanged. Clear area, energy deposited, shaded area BMR dark area, DIT; hatched area, physical activity: area 1 = PAL of 1.55, areas 1 + 2 PAL of 2.00.

Figure 14 Aggregated energy costs of pregnancy plotted in relation to observed increases in energy intake in well-nourished women. Hatched area, energy deposited; shaded area, BMR; dark area, DIT; horizontal stripes, physical activity if NPNL PAL (1.55) remains unchanged; stippled area, physical activity if NPNL PAL (2.00) remains unchanged.

This gives a total of 300 MJ (71 700 kcal) equivalent to 1.07 MJ/day (255 kcal/day) (rounded to 1.1 MJ/day).

Both of these values are considerably less even than the lower 1985 recommendation which is conditional on women reducing their activity. The lower edge of the hatched area in Figure 14 indicates the extent to which physical activity must be reduced in order to accommodate the obligatory costs of pregnancy within such a small increase in intake.

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