MINERALS AND TRACE ELEMENTS IN PREGNANCY
Iron
Iron is essential to the production of hemoglobin. Its dietary sources include animal protein, dried beans, fortified grains, and any food cooked in cast iron cookware. Despite its numerous sources, women have difficulty maintaining iron balance using only a healthy diet. A well-balanced diet with 10,460 kJ/d (2500 kcal) contains approximately 15 mg of elemental iron; however, the absorption of iron is very inefficient and only approximately 10% of this is absorbed. With each normal menses, 12-15 mg of elemental iron is lost. Estimates indicate that a woman's diet must include 1.5-2 mg/d of elemental iron to compensate for menstrual losses alone. In pregnancy, 500 mg of additional iron is needed to expand maternal red cell mass. Another 500 mg is needed to supply fetal and placental tissues. On average, an additional 3 mg/d of elemental iron must be absorbed from dietary sources.
The RDA for elemental iron reflects these increased requirements. The RDA for women who are not pregnant is 15 mg, 1.5 mg of which is absorbed. In pregnancy, the RDA is 30 mg, 3 mg of which is absorbed. During lactation, the RDA returns to 15 mg. Well-balanced diets do not provide the pregnancy RDA for elemental iron; therefore, iron supplementation is recommended in normal pregnancy.
Various iron preparations are commercially available, and each delivers a slightly different amount of elemental iron. These preparations include ferrous sulfate, ferrous fumarate, ferrous gluconate, and polysaccharide iron complex. Pure elemental iron is available in 50-mg caplets of carbonyl iron. Providers should be aware of the elemental iron contained in any one specific preparation, and they should understand that only 10% of this is absorbed from the maternal gut. Absorption is enhanced by concurrent ingestion of foods containing vitamin C. Usually, one dose of any preparation containing at least 30 mg of elemental iron meets the RDA. Larger doses are required only to treat maternal iron deficiency anemia and only serve to constipate patients without anemia. Physicians should remember that iron competes with zinc at absorption sites. The clinical implications of this are discussed in the Zinc section.
Iron deficiency anemia is one of the most common pregnancy complications. Screening for iron deficiency anemia is recommended at the first prenatal visit and, thereafter, as indicated. Iron deficiency anemia is suggested if the complete blood cell count suggests a microcytic, hypochromic anemia. Confirmatory test results include a reduced serum iron level, increased total iron-binding capacity, decreased transferrin saturation, and reduced serum ferritin levels. If a provider is unable to perform the complete battery of confirmatory tests, the serum iron and ferritin levels usually suggest the correct diagnosis. Treatment is increased oral iron supplementation.
Many studies have shown that high hemoglobin values are associated with adverse pregnancy outcomes; however, iron supplementation cannot, in itself, raise hemoglobin to these levels (Yip, 2000). Any adverse outcomes are more likely secondary to underlying conditions responsible for high hemoglobin values.
Calcium
Calcium is a major component of bone; therefore, large quantities of calcium are required in pregnancy for construction of fetal tissues, especially in the third trimester. Pregnant women younger than 25 years also still require calcium for maternal bone mass. Hormonal adaptations and increased intestinal absorption protect maternal bone while meeting fetal calcium requirements. A well-balanced diet provides adequate calcium to meet all of these needs, and supplementation is not recommended. The RDA for nonpregnant, pregnant, and lactating women is 1000 mg (Allen, 1998).
Calcium is found in dairy products and leafy green vegetables such as collard, kale, turnip, and mustard greens. Vitamin D is required for calcium absorption.
Phosphorus
Along with calcium, phosphorus is required for bone formation. Maternal serum inorganic phosphorus levels remain constant during pregnancy because of maternal adaptations. The RDA for nonpregnant, pregnant, and lactating women is 1000 mg. Well-balanced diets easily provide the RDA for nonpregnant, pregnant, and lactating women; supplementation is not recommended. In fact, phosphorus is not usually in vitamin supplements.
Zinc
Zinc is involved in nucleic acid and protein metabolism; therefore, zinc is important in early gestation. The RDA is 12 mg. The RDA for pregnant women is 15 mg, which increases to 19 mg during lactation. Well-balanced diets provide the RDA for women who are pregnant and lactating, and supplementation is not recommended. Both iron and copper compete with zinc at absorption sites; therefore, zinc supplementation is recommended when elemental iron supplementation exceeds 60 mg/d. Likewise, whenever zinc supplements are used, copper should also be supplemented. Different prenatal vitamin formulations contain different amounts of copper and zinc. Usually, copper or zinc supplementation can be accomplished by careful selection of a prenatal vitamin formulation.
Sodium
Sodium is present in large quantities in the average American diet. It has received much attention. In pregnancy, sodium should neither be restricted nor used excessively. Well-balanced diets “salted to taste” satisfy sodium requirements and obviate any need for supplementation. Pregnant women should remember that most processed and pre-prepared foods are high in sodium.