Protein metabolism during pregnancy.

Protein metabolism is altered during pregnancy to allow for an increased rate of tissue synthesis. These adjustments seem to be mediated primarily by hormonal changes. The observed nitrogen, and therefore protein, retention is in the order of 1.1 gm N per day during the last half of pregnancy. The theoretical gain of nitrogen in fetal and maternal accessory tissues is about 0.8 gm per day during the same period. Animal data and a few human studies suggest that the difference may be accounted for by maternal gain of fat-free tissue. The rate of nitrogen retention appears to increase by a factor of 3 from early to late pregnancy. The current NRC protein recommendation for pregnancy is 30 gm additional protein or 1.3 gm per kg pregnant body weight. This allowance takes into consideration the low efficiency of protein utilization in the practical diets consumed by most pregnant women. The recommendation is made assuming that energy needs have been met. Within reasonable limits increased energy intakes will improve protein utilization. Some clinical surveys have noted a relationship between prenatal diet and infant size at birth, infant health in early life, pregnancy complications, and fertility rate; others have failed to find these associations. Supplements of food given to pregnant women in an attempt to improve outcome are associated with significant improvement in poorly nourished women but less impact in more well nourished women. When healthy pregnant women eat to appetite, the protein and energy intakes seem to be comparable to the current NRC recommendations.