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2. Protein needs during pregnancy

2.1. Influence of gestational weight gain on protein needs
2.2. Efficiency of protein utilization during pregnancy
2.3. Influence of dietary energy on protein utilization
2.4. Summary of protein requirements during pregnancy

2.1. Influence of gestational weight gain on protein needs

With those types of relationships in mind, protein needs were estimated for successive trimesters for women grouped by prepregnancy BMI. Table 1 summarizes the 15th, 50th and 85th percentiles of weight gains derived by the Institute of Medicine (IOM) from the 1980 National Natality Study (Subcommittee on Nutritional Status and Weight Gain During Pregnancy, and Subcommittee on Dietary Intake and Nutrient Supplements During Pregnancy, 1990; TAFFEL, 1986). These estimates reflect data from observations of white, non-Hispanic, American women who delivered live infants.

At both the 50th and 85th percentiles, weight gains were above 12.5 kg. Table 2 summarizes expected gains in body protein by trimester for each of the indicated subgroups. The most important characteristic of the estimates in Table 2 is that the additional protein needs of pregnant women with the lowest prepregnancy BMI generally are higher than those of the comparison groups at each of the indicated percentiles.

Table 1. Total weight gain (kg) for pregnant white, non-Hispanic, American married women delivering live infants¹

¹ Adapted from Tables 5-1, (Subcommittee on Nutritional Status and Weight Gain During Pregnancy and Subcommittee on Dietary Intake and Nutrient Supplements During Pregnancy, 1990).
² Body mass index (kg/m²).

Table 2. Expected gain in body protein (g) by trimester¹ for pregnant white, non-Hispanic, American married women delivering live infants

¹ Assumes that 8% of total weight gain is protein and that protein gain occurs in a ratio of 1:5:9 for each successive trimester.
² Body mass index (kg/m²).
³ Total weight gain percentile.

Table 3. Estimated dietary protein needs (g protein/day) 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 Protein gains expressed as g/d. Assumes that 8% of total weight gain is protein and that protein gain occurs in a ratio of 1:5:9 for each successive trimester.
⁴ Assumes efficiency of 0.7 for conversion of dietary protein to fetal and maternal tissues.
⁵ Corrects for both efficiency of dietary protein utilization and the coefficient of variation of birthweight.

Table 4. Estimated dietary protein needs (g protein/day) 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 Protein gains expressed as g/d. Assumes that 8% of total weight gain is protein and that protein gain occurs in a ratio of 1:5:9 for each successive trimester.
⁴ Assumes efficiency of 0.7 for conversion of dietary protein to fetal and maternal tissues.
⁵ Corrects for both efficiency of dietary protein utilization and the coefficient of variation of birthweight.

Table 5. Estimated dietary protein needs (g protein/day) 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 Protein gains expressed as g/d. Assumes that 8% of total weight gain is protein and that protein gain occurs in a ratio of 1:5:9 for each successive trimester.
⁴ Assumes efficiency of 0.7 for conversion of dietary protein to fetal and maternal tissues.
⁵ Corrects for both efficiency of dietary protein utilization and the coefficient of variation of birthweight.

Tables 3-5 summarize the additional daily protein intakes required above the needs of non-pregnant, non-lactating women to support the weight gains indicated in Table 1. Estimates in Tables 2-5 assume a ratio of protein gain throughout pregnancy of 1:5:9 during the first, second, and third trimester (FAO/WHO/UNU, 1985), respectively, and a consistent composition of the weight gained (i.e., water, protein, and fat represent 62%, 8%, and 30%, respectively, of total weight gain) by each of the indicated groups. These Tables include estimates which correspond to gains in protein per day, estimates adjusted for an efficiency of conversion of dietary to tissue protein of 0.7, and estimates adjusted for both conversion efficiency and an additional 30% to account for interindividual variability.

It was assumed that the coefficient of birthweight was the best proxy indicator for the magnitude of interindividual variability (FAO/WHO/UNU, 1985). Projected estimates of the additional protein required during pregancy are approximately 20 to 70% higher for groups at the 85th percentile than are the most recent FAO/WHO/UNU recommendations (1985). The magnitude of the difference depends on whether estimates in Tables 3-5 are adjusted for both conversion efficiency and interindividual variation, or only for conversion efficiency.

The most important assumptions made by these calculations relate to the accuracy and consistency of the composition of weight gained across percentiles of gestational weight gain and prepregnancy BMI, the generalizability of the relationship between gestational weight gain and pregnancy outcomes across populations, the extrapolation of baseline protein needs of non-pregnant, non-lactating women to calculate modified estimates of the total protein requirements of pregnant women, the adequacy of the total diet's nutrient composition, and the accuracy of factors used to adjust for the efficiency of conversion of dietary protein to tissue and interindividual variability. The relative strength of these estimates lies in the implicit acknowledgement that prepregnancy BMI influences relationships among gestational weight gain, a favorable pregnancy outcome, and dietary protein needs during pregnancy.

Particularly relevant issues to this discussion are the adequacy of available estimates of body composition during pregnancy and the possibility that metabolic responses during pregnancy may influence the efficiency of protein utilization. Estimates of body composition have been derived from observations which rely significantly on indirect methods, dependent on assumptions originally validated in non-pregnant models. Of particular concern to this point is the magnitude of nitrogen retention measured in most balance studies in the latter half of pregnancy. These are in the range of 1.2 g N/d (corrected for miscellaneous losses of nitrogen; KING et al., 1987). That figure is approximately 25 % greater than expected from the usual predictions of N retention by the fetus, placenta, and various gains in maternal tissues related to pregnancy (e.g., uterus, mammary tissue, and blood) if a mean weight gain of 12.5 kg is assumed.

Retentions of 1.2 g N/d, however, are not necessarily excessive at weight gains above the 50th percentile during the third trimester. Nonetheless, the higher than expected estimates of N retention have led some investigators to suggest that maternal 'stores' of nitrogen are increased during pregnancy. The only corroborative evidence from human studies for increased maternal 'stores' of N during pregnancy are serial measurements of 40K during pregnancy reported in the last trimester by PIPE et al. in 1979 (2.5 meq/d), EMERSON et al. in 1975 (4.2 meq/d), and KING et al. in 1973 (3.4 meq/d).

In these reports, gains in total body potassium (TBK) were generally greater than predicted by the expected gains in nitrogen, but the magnitude of total body protein predicted by serial measurements of TBK were not sufficiently large to account fully for the N retention observed in balance studies. This view, however, is tempered by suggestions that intracellular concentrations in human muscle may be lower during pregnancy than in the non-pregnant state (CHEEK et al., 1985).

FORSUM et al., (1988) published a report of TBK in pregnancy that includes prepregnancy values. If these prepregnancy values are included in the analysis, changes in total body potassium are smaller than those reported by others. These smaller changes are a consequence of lower values measured in early pregnancy compared with prepregnancy measurements. If prepregnancy values are ignored, changes in TBK during pregnancy reported by FORSUM et al. (1988) are similar to other published values.

TBK changes in the last trimester (5.2 meq/d) were higher than reported by others, but the distribution of measurements was within the range of previous reports. Serial estimates of total body potassium and balance estimates, therefore, suggest that fat stores may be less than currently estimated and/or that the water content of the fat-free compartment may be less than predicted. However, skinfold measurements tend to support the accumulation of significant fat stores during pregnancy. Data from rats suggest that the water content of the lean tissue compartment increases during pregnancy (NAISMITH, 1966).

2.2. Efficiency of protein utilization during pregnancy

There is no experimental evidence from human studies that the efficiency of nitrogen utilization is enhanced during pregnancy. CALLOWAY (1974) has summarized data from numerous balance studies. The slope of the line relating nitrogen balance and nitrogen intake was 0.26, significantly less than the 70% assumed in most calculations. The inclusion of nitrogen intakes which resulted in retentions 2 to 8 times the expected value, however, is likely to account for some, if not all, of the dampening of the slope relating N intake to N retention. Unfortunately, data for non-pregnant controls at similar planes of nitrogen intake were not reported.

In contrast to those indicators of possible decreased efficiency, KALHAN et al., (1982) reported 40% reductions in urea synthesis during pregnancy. Together with expected expansion in total body water, these reductions appear consistent with 60% decreases in plasma urea concentrations which are normally observed during pregnancy. Mechanisms which regulate the reduction in urea synthesis are not well described, although a series of animal studies by NAISMITH (1977) suggests that metabolic responses, responsible for reduced urea synthesis, are hormonally mediated.

Total body protein turnover studies have been reported for normal pregnant women (FITCH and KING, 1987; DE BENOIST et al., 1985). Fitch and King found no differences between women studied at 30-36 weeks gestation and non-pregnant controls. De Benoist and coworkers studied three groups of pregnant women and a group of non-pregnant controls. The highest turnover rate was reported for women at 12 weeks gestation. Turnover rates fell as pregnancy progressed. It is not clear if the highest turnover rates reflect a redistribution of nitrogen. The mean turnover rate of women at 33 weeks was only slightly higher than the value observed in non-pregnant controls. These findings are surprising, because the highest rates of turnover were measured during the period with the lowest rate of nitrogen retention. The increasing rate of nitrogen retention and decreasing rate of protein synthesis suggest an improving efficiency in protein metabolism as gestation progresses, i.e., grams of protein retained per gram of protein synthesized rise. No studies, however, were found which investigated the possible modulating role of diverse levels of energy consumption or maternal weight gain on rates of protein turnover.

2.3. Influence of dietary energy on protein utilization

CALLOWAY (1974) also examined the relationship between N retention and energy intake. The magnitude of 'nitrogen sparing' by surfeit energy was similar to values reported in studies of adult men (CALLOWAY, 1975; GARZA et al., 1976). It is not clear, however, that this relationship was evaluated at relative levels of energy 'excess' required to sustain gestational weight gains presently recommended by the Institute of Medicine (Subcommittee on Nutritional Status and Weight Gain During Pregnancy, 1990).

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