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Protein-energy relationships in pregnancy and lactation

C. GARZA* and K.J. MOTIL**

* Division of Nutritional Sciences, Savage Hall, Cornell University, Ithaca, NY 14853-6301, U.S.A.

** USDA/ARS Children's Nutrition Research Center, 1100 Bates Street, Houston TX 77030, U.S.A.

1. Influence of gestational weight gain on pregnancy outcomes
2. Protein needs during pregnancy
3. Energy requirements during pregnancy
4. Protein needs during lactation
5. Energy needs during lactation
6. Conclusions


The demands of pregnancy and lactation for dietary protein and energy have been estimated by balance methods, factorial approaches, and estimates of energy and protein consumed by presumably well-nourished women. The limitations of these approaches have been reviewed by various committees and individual investigators. Briefly, balance methods are often too short in duration for the assessment of long-term well-being, and they are plagued by the likely overestimation of net balance, because the intakes are more likely to be overestimated and the losses underestimated than the converse. Factorial approaches generally also do not account for nutrient interactions, ignore possible changes in baseline needs during pregnancy and lactation, and are not designed to consider physiologic costs associated with adjustments to diverse planes of intake. Estimating usual intakes of free-living populations represents significant methodological challenges, and there are no sound reasons to expect that customary nutrient intakes are necessarily consistent with good health, especially if long-term outcomes are of concern.

To the extent possible, therefore, present recommended intakes of protein and energy during pregnancy and lactation are derived from informed judgements that rely on data obtained by all three approaches and give renewed emphasis to the principle that nutrient intakes be consistent with long-term health and allow for the maintenance of "economically necessary and socially desirable physical activity" (FAO/WHO/UNU, 1985). In considering pregnancy and lactation, the need to promote the well-being of both mother and fetus, or infant, complicates the task.

1. Influence of gestational weight gain on pregnancy outcomes

The interpretation of results from balance studies, factorial calculations, and dietary surveys for estimating protein and energy needs during pregnancy is dependent upon a definition of the desired pregnancy outcome. The recent report, Nutrition During Pregnancy, of the Subcommittee on Nutritional Status and Weight Gain During Pregnancy convened by the Institute of Medicine (Subcommittee on Nutritional Status and Weight Gain During Pregnancy and Subcommittee on Dietary Intake and Nutrient Supplements During Pregnancy, 1990) adopted a gestational duration of 39 to 41 weeks and live birthweights of 3 to 4 kg as an operational definition of a favorable pregnancy outcome. The 15th and 85th percentiles of weight gain were 7.7 and 18.6 kg, respectively, for women of body mass index (BMI, weight/height2) between 19.8 and 26, who experienced favorable pregnancy outcomes.

After weight gain during pregnancy, prepregnancy weight-for-height was the principal physiologic determinant of the relationship between gestational weight gain and infant birthweight i.e., after adjusting for total maternal gestational weight gain, the birthweights of infants of women with low BMIs at conception were lower than those of infants of women with higher BMIs. Maternal energy intake was not a strong predictor of gestational weight gain, but variations in nutrient stores, physical activity, the nutrient adequacy of sources of dietary energy, and difficulties in accurately estimating food consumption of free-living populations cloud the interpretation of those analyses.

Generally, women who were overweight at conception (BMI > 26) and who experienced a favorable pregnancy outcome, tended to gain less weight than did women of lower BMI. After controlling for various possible confounding and modifying socioeconomic, behavioral, and demographic factors, black women and women of ethnic groups of small average body size tended to have lower gestational weight gains. Age did not seem to affect the relationship between weight gain and birthweight, except among very young women (defined as less than two years postmenarche). They tended to have smaller infants compared with older women who experienced similar gestational weight gains.

Other consequences of gestational weight gain were also reviewed. Increases in maternal fat, lean tissue, and water were associated individually with increased fetal growth. The timing of gestational weight gain was of particular significance to fetal growth, i.e., weight gain in the second and third trimesters was much more important to fetal growth than was weight gain in the first trimester. While not conclusive, low rates of gestational weight gain appeared to increase the risk of premature delivery, but the rate of weight gain was not an important predictor of spontaneous abortion or congenital anomalies. Very high gestational weight gains were associated with increased risk of birth trauma, asphyxia, and mortality, particularly in infants of shorter women (< 152 cm height).

From a maternal health standpoint, very high gestational weight gains were associated with an increased risk of fetopelvic disproportion, particularly among shorter women, but high gestational weight gains were not associated with an increased risk of maternal mortality or pregnancy-induced hypertension and preeclampsia.

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