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2.4. Summary of protein requirements during pregnancy

Accounting for prepregnancy BMI and for the revised recommended gestational weight gains may increase substantially estimates of the additional protein needed during pregnancy. An adequate evaluation of this possibility requires that pregnancy outcomes, gestational weight gains, and maternal BMI be examined in other than US populations. Furthermore, the data base which describes the composition of weight gain during pregnancy for groups of women representative of a wide range of prepregnancy BMI should be improved, and more detailed attention should be given to the efficiency of N utilization during pregnancy.

3. Energy requirements during pregnancy

3.1. Influence of gestational weight gains on energy needs
3.2. Physical activity and pregnancy
3.3. Summary of energy requirements during pregnancy

3.1. Influence of gestational weight gains on energy needs

Energy demands of pregnancy usually are estimated at approximately 77000 kcal (HYTTEN and LEITCH, 1971). Although variations in activity levels account for major differences in the energy needs of individuals, most estimates of the energy demands of pregnancy assume that decreases in activity during pregnancy compensate for the increased costs of moving a heavier body. The energy costs of pregnancy, therefore, are estimated from expected gains in body weight, changes in maternal body composition, and increments in basal metabolism. The most commonly used estimates of energy needs are based on a gestational weight gain of 12.5 kg. This weight gain represents increases in fat and protein of approximately 3.8 kg and 0.9 kg, respectively, and a net increase in basal metabolism through pregnancy equivalent to approximately 36000 kcal. Basal metabolism, therefore, accounts for 47% of the energy cost, fat stores for a similar proportion, and gains in protein for the remaining 6%.

Estimates of increments in basal metabolism during pregnancy, however, are variable and appear to be highly dependent upon maternal nutritional well-being. A recent multicountry study of the energy requirements of pregnancy (DURNIN, 1987) reported values for Scottish and Dutch women (approximately 30000 to 34000 kcal), similar to estimates published earlier by HYTTEN and LEITCH (1971) for Scottish women, but much lower values for Gambian women (approximately 2000 kcal; see Table 6). Increments in basal metabolism, however, rose to approximately 13000 kcal in the Gambian group following food supplementation.

The reduced costs of pregnancy in Gambian women were explained, in part, by rates of basal metabolism in early pregnancy below preconception values in both supplemented and unsupplemented groups (LAWRENCE et al., 1984). While reductions in basal metabolism may be seen as a potentially beneficial adjustment to the nutritional demands of pregnancy in a nutritionally stressed population, the physiologic costs may be substantial. Reductions in basal metabolism, associated with nutritional deprivation, carry significant functional costs, as documented exhaustively in the Minnesota semistarvation studies (KEYS et al., 1950).

Gains in maternal fat also appear to vary widely. In the multicountry study referred to previously, the highest and lowest fat gains were observed in Scottish and Gambian women, 25347 kcal and 6600 kcal, respectively. Initial body mass indices for these groups were moderate, 21.8 for the Scottish women and 20.6 for the Gambian group. The mean weight gain was 11.7 kg for the Scottish group and 7.7 kg for the Gambian group.

Table 6. Energy requirements of pregnancy data from the five-country study¹

¹ DURNIN, 1987.
² At 10 wk post partum.
³ Based on Subcommittee on Nutritional Status and Weight Gain During Pregnancy and Subcommittee on Dietary Intake and Nutrient Supplements During Pregnancy. Nutrition During Pregnancy. Institute of Medicine. National Academy Press, Washington, DC, 1990.
⁴ Contribution of BMR to total cost of pregnancy.

Table 7. Estimated energy requirements (kcal/d) by trimester for pregnant white, non-Hispanic, American married women (BMI¹ < 19.8) delivering live infants

¹ Body mass index (kg/m²).
² Total weight gain percentile.
³ Assumes 4 kcal/g of protein gained.
⁴ Assumes 9.5 kcal/g of fat gained.
⁵ Assumes daily increment of 67 kcal, 108 kcal, and 208 kcal per day across all groups for trimesters 1, 2, and 3, respectively.

Table 8. Estimated energy requirements (kcal/d) by trimester for pregnant white, non-Hispanic, American married women (BMI¹ 19.8-26.0) delivering live infants

¹ Body mass index (kg/m²).
² Total weight gain percentile.
³ Assumes 4 kcal/g of protein gained.
⁴ Assumes 9.5 kcal/g of fat gained.
⁵ Assumes daily increment of 67 kcal, 108 kcal, and 208 kcal per day across all groups for trimesters 1, 2, and 3, respectively.

Table 9. Estimated energy requirements (kcal/d) by trimester for pregnant white, non-Hispanic, American married women (BMI¹ 26.1-29.0) delivering live infants

¹ Body mass index (kg/m²).
² Total weight gain percentile.
³ Assumes 4 kcal/g of protein gained.
⁴ Assumes 9.5 kcal/g of fat gained.
⁵ Assumes daily increment of 67 kcal, 108 kcal, and 208 kcal per day across all groups for trimesters 1, 2, and 3, respectively.

The Scottish group's gestational weight gain was below the 50th percentile value calculated by the IOM pregnancy report (Subcommittee on Nutritional Status and Weight Gain During Pregnancy and Subcommittee on Dietary Intake and Nutrient Supplements During Pregnancy, 1990) for women of moderate BMIs; the mean gestational weight gain for the Gambian women was below the 15th percentile. It is difficult, therefore, to interpret the public health significance of the range of energy costs of pregnancy (18682 to 67300 kcal) calculated by those studies, without additional measures of maternal and infant outcomes in a larger population sample.

Theoretical estimates of the energy costs of pregnancy, which are based on gestational weight gains associated with good pregnancy outcomes as defined by the IOM subcommittee (Subcommittee on Nutritional Status and Weight Gain During Pregnancy and Subcommittee on Dietary Intake and Nutrient Supplements During Pregnancy, 1990), are found in Tables 7-9. These estimates are based on the same assumptions regarding body composition which were used to derive estimates of protein needs and also assume a ratio of fat accumulation of 1:2.3:0.8 for each respective trimester of pregnancy. On that basis, theoretical estimates of energy needs range from 60100 to 95000 kcal. The lower estimate corresponds to women with moderate BMI who gained weight at the 15th percentile and the higher estimate to women with low BMI who gained weight at the 85th percentile, respectively.

Gestational weight gains at the 50th percentile for women with low, moderate, and high BMI ranged from 75000 to 79000 kcal. Basal metabolic costs were estimated from measurements reported by HYTTEN and LEITCH (1971) and were assumed to be similar across all groups. The need for more detailed observations of body composition and BMR in groups characterized by a relatively wide range of prepregnancy BMI is made clear by the assumptions required for these calculations.

3.2. Physical activity and pregnancy

The influence of diet and physical activity on pregnancy was evaluated recently by a subcommittee organized under the Food and Nutrition Board's Committee on International Nutrition Programs (Subcommittee on Diet, Physical Activity, and Pregnancy Outcome, 1989). Pregnant women appear to decrease the intensity of work, i.e., the same amount of work may be done as before pregnancy, but it is accomplished over a longer period. The efficiency of work performance does not appear to be improved in non-weight-bearing activities during pregnancy, but there is conflicting evidence regarding the efficiency of weight-bearing activities. There were few studies available that include combined measurements of diet, physical activity, and pregnancy outcomes, especially outcomes that focused on maternal well-being.

Data from human studies suggest that mild physical activity does not adversely influence pregnancy outcomes in well-nourished populations, but evidence from animal studies suggests that severe, acute exercise and severe undernutrition decrease uterine blood flow and may thereby compromise the fetus. There is also suggestive epidemiological evidence that strenuous exercise and prolonged mild exertion adversely affect gestational duration. The paucity of data and the public health significance of physical activity during pregnancy, however, point out a strong need to evaluate the relationships among energy expenditure, nutritional status, and pregnancy outcome among pregnant women who perform moderate to severe physical work, especially among groups at risk of malnutrition. No studies of pregnant women were found which investigated protein-energy relationships at diverse planes of physical activity.

3.3. Summary of energy requirements during pregnancy

Variations in weight gain and basal metabolism significantly influence the energy needs of pregnancy. Caution, however, is necessary when evaluating reductions in basal metabolism which are reversed by food supplementation. Significant functional costs to the mother may be associated with diminished rates of basal metabolism. The effects of pregnancy on the costs of physical activity have not been studied well. Available data suggest that net costs are not changed, and increased energy needs, expected on the basis of greater body weight, are compensated for by a decreased intensity of physical activity. No consistent data base is available regarding efficiency of work performance during pregnancy.

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