The Need for Vitamins and Minerals During Pregnancy

Requirements for most vitamins and minerals increase dur-ing pregnancy due to metabolic demands associated with placental and fetal growth, expansion of maternal tissues and Preeclampsia A pregnancy-specific

condition that usually occurs after 20 weeks of pregnancy (but may occur earlier). It is characterized by increased blood pressure and protein in the urine and is associated with decreased blood flow to maternal organs and through the placenta.

Table 4.22 EPA 1 DHA, vitamin A, and vitamin D content of 4.5 g (1 tsp) cod liver oil and of fish oil¹²⁸

EPA 1 DHA Vitamin A Vitamin D

Cod liver oil, 1 tsp 810 mg 4500 IU 450 IU

Fish oil, salmon, 1 tsp 1410 mg 0 15 IU

Fish oil, sardines, 1 tsp 940 mg 0 0

Fish oil, herring, 1 tsp 470 mg 0 0

33411_04_Ch04_p087-133.indd 114

33411_04_Ch04_p087-133.indd 114 4/30/10 3:40:31 PM4/30/10 3:40:31 PM

It is now well accepted that inadequate availability of folate between 21 and 27 days after conception (when the embryo is only 2–3 mm in length) can interrupt normal cell differentiation and cause NTDs.¹⁴⁰ Neural-tube de-fects are among the most common types of congenital ab-normalities identified in infants, with approximately 4000 pregnancies affected each year in the United States.¹⁴¹ NTDs are among the most preventable types of congenital abnormalities that exist.¹⁴² Approximately 70% of cases of NTDs can be prevented by consumption of adequate folate before and during very early pregnancy.¹³⁹

Folate Status of Women in the United States Folate status is assessed by serum and red cell folate levels. Of the two measures, red cell folate levels are the preferred indicator because they represent long-term folate intake, whereas serum folate levels reflect only recent intake.

Levels of red cell folate of over 300 ng/mL (or 680 nmol/L) are associated with very low risk of NTDs.¹⁴³ These levels of red cell folate can generally be achieved by folic acid in-takes that average 400 mcg daily.¹⁴⁴ As shown in Illustra-tion 4.17, red cell folate levels are higher among women who consume folic-acid- fortified cereals or supplements compared to women consuming folate from food only.

Folate status in women of childbearing age in the United States has improved since the advent of folic-acid fortifica-tion of refined grain products in 1998. Average levels of red cell folate in U.S. women have increased from 181 to 235 ng/mL since fortification began.¹⁴² Low levels of intake of folic-acid-fortified grain products and breakfast cereals still leave some women with too little folate, however.¹⁴⁵ Low red cell folate levels are disproportionately found in black (congenital defects) in the newborn, and reduced birth

weight. Folic acid supplements (500–600 mcg per day) in the second and third trimesters of pregnancy decrease ho-mocysteine levels and improve pregnancy outcomes.

A common genetic defect has been identified in the en-zyme 5,10-methylene tetrahydrofolate reductase (MTHFR).

This variant of the normal enzyme reduces the level of activity of MTHFR by about half. Variant forms of MTHFR and defects in methionine synthase are thought to be present in approximately 30% of the population.¹³⁷

Folate and Congenital Abnormalities

“There is a widespread belief that congenital malformations are always the result of defective genes. Perhaps a nutritional deficiency resulting in a defective gene leads to the same congenital abnormality.”

R. D. Mussey, 1949⁸⁸

Researchers have known since the 1950s that low and high intakes of certain vitamins and minerals cause congeni-tal abnormalities in laboratory animals. They have also known that neural-tube defects, brain and heart defects, and cleft palate can be caused by feeding pregnant rats folate- deficient diets.¹³⁸ Firmly held beliefs that only severe malnutrition affects fetal growth and that genetic errors are the sole cause of congenital abnormalities delayed recogni-tion of the importance of folate to human pregnancy.¹³⁹

Neural-tube defects (NTDs) are malformations of the spinal cord and brain. There are three major types of NTDs:

Spina bifida is marked by the spinal cord failing to

●

close, leaving a gap where spinal fluid collects dur-ing pregnancy (see Illustration 4.16). Paralysis be-low the gap in the spinal cord occurs in severe cases.

Anencephaly is the absence of the brain or spinal

●

cord.

Encephalocele is characterized by the protrusion of

●

the brain through the skull.

Illustration 4.16 A newborn child with spina bifida.

source: Biophoto Associates/Photo Researchers, Inc.

Illustration 4.17 Mean red cell folate level in preconceptional women by level of intake of various sources of folate.

1359 (600)

1133 (500)

906 (400)

680 (300)

453 (200)

227

(100) 0 250 500 750 1000 1250

Red Cell Folate, Mean + SE, nmol/L (ng/mL)

Food Intake, mcg/d Food

Cereal

Supplements, no cereal Supplements, cereal

source: Brown JE et al., JAMA, Vol. 277, No. 6, p. 551 (Feb 19, 1997).

33411_04_Ch04_p087-133.indd 115

33411_04_Ch04_p087-133.indd 115 4/30/10 3:40:31 PM4/30/10 3:40:31 PM

NTDs develop before women may realize they are preg-nant, adequate folate should be consumed several months prior to, as well as throughout, pregnancy.

Women who have previously delivered an infant affected by an NTD are being urged to take 4000 mcg (4.0 mg) of folic acid in a supplement to reduce the risk of recurrence.¹⁴⁷ This dose, however, may be much higher than needed based on results of clinical trials.^148,149 The upper limit for intake of folic acid from fortified foods and supplements is set at 1000 mcg per day. There is no upper limit for folate consumed in its naturally occurring form in foods. The 1000 mcg level represents an amount of folic acid that may mask the neurological signs of vitamin B₁₂ deficiency. If left untreated, vitamin B₁₂ defi-ciency leads to irreversible neurological damage.¹⁴¹

Choline

Choline is a B-complex vitamin that humans can produce, but not in high enough amounts to meet needs when die-tary intake of choline is very limited. The need for choline increases during pregnancy due to its roles as a compo-nent of phosopholipids in cell membranes and a precursor of intracellular messengers.¹⁵⁰ Choline can be converted to betaine, which, like folate, serves as a source of methyl groups used to regulate gene function, neural-tube and brain development, and the conversion of homocysteine to methionine.¹⁵¹ Large amounts of choline are trans-ported via the mother’s blood to the embryo and fetus during pregnancy.¹⁵⁰

The RDA for choline in pregnancy is 450 mg daily.

Average intake of choline by women between the ages of 20 and 39 years in the United States, however, is 274 mg per day.¹¹⁴ Some experts are concerned that pregnant women may not be getting enough choline for optimal fetal brain growth and intellectual development.¹⁵² Although this is a much discussed and exciting area of research, it is not yet clear whether low dietary choline availability in women during pregnancy represents a risk factor for fetal brain and intellectual development.¹⁵³

Choline status tends to be adequate in women who regularly consume eggs and meat, the two major sources of this vitamin.¹⁵⁰

Vitamin A

Vitamin A is a key nutrient in pregnancy because it plays important roles in reactions involved in cell differentiation.

Deficiency of this vitamin is rare in pregnant women in industrialized countries, but it is a major problem in many developing nations. Vitamin A deficiency that occurs early in pregnancy can produce malformations of the fetal lungs, urinary tract, and heart.¹⁵⁴

Of more concern than vitamin A deficiency in the United States are problems associated with excessive in-takes of vitamin A in the form of retinol or retinoic acid (but not beta-carotene). Intakes of these forms of vitamin A women in the United States compared to white or Mexican

American women.¹⁴⁶

Dietary Sources of Folate Many vegetables and fruits are good sources of folate (see Table 1.9 in Chapter 1), but only a few foods contain the highly bioavailable form of folate. Table 4.23 lists some foods that natu-rally contain the highly bioavailable, monoglutamate form of folate and foods that provide folic acid through fortification.

Adequacy of folic acid intake before and during pregnancy can be estimated by adding up the amount of folic acid in foods typically consumed in the daily diet using the data in the table. Whole-grain products includ-ing breads and pastas, brown rice, oatmeal, shredded wheat, and organic grain products may or may not be fortified with folic acid. You have to check food labels to find that out.

Recommended Intake of Folate Due to variation in folate bioavailability, the DRI for folate takes into con-sideration a measure called dietary folate equivalents, or DFE. One DFE equals any of the following:

1 mcg food folate

●

0.6 mcg folic acid consumed in fortified foods

●

or a supplement taken with food

0.5 mcg of folic acid taken as a supplement

●

on an empty stomach

Folic acid taken in a supplement without food provides twice the dietary folate equivalents as does an equivalent amount of folate from food.

It is recommended that women consume 600 mcg DFE of folate per day during pregnancy and include 400 mcg folic acid from fortified foods or supplements.¹³⁵ The remaining 200 mcg DFE should be obtained from vegetables and fruits. These nutrient-dense foods provide an average of 40 mcg of folate per serving.¹⁴¹ Because

Table 4.23 Food sources of folate

Amount Folic Acid (mcg) A. Foods

Orange 1 40

Orange juice 6 oz 82

Pineapple juice 6 oz 44

Papaya juice 6 oz 40

Dried beans ¹⁄2c 50

B. Fortified Foods

Highly fortified 1 c or 1 oz 400 breakfast cereal^a

Breakfast cereal 1 c or 1 oz 100 Bread, roll 1 slice or 1 oz 40

Pasta ¹⁄2c 30

Rice ¹⁄2c 30

aIncludes Product 19, Smart Start, Special K, and Total.

33411_04_Ch04_p087-133.indd 116

33411_04_Ch04_p087-133.indd 116 4/30/10 3:40:32 PM4/30/10 3:40:32 PM

to women with poor vitamin D status tend to be smaller than average, more likely to have low blood calcium levels (hypocalcaemia) at birth, and more likely to have poorly calcified bones and abnormal enamel. They are also more likely to develop dental caries in childhood.^157–160

Vitamin D also supports normal functioning of the immune system and can inhibit inflammation. There are indications that vitamin D deficiency during pregnancy may be related to adverse outcomes based on these roles.

Miscarriage, preeclampsia, preterm birth, maternal infec-tion, and the development of type 1 diabetes and asthma in children are under investigation for their relationship to maternal vitamin D insufficiency.¹⁶¹

Prevalence of Vitamin D Inadequacy Vitamin D inadequacy during pregnancy is common. A study of 206 pregnant women in Toronto found vitamin D deficiency in 35%.¹⁵⁸ Of 400 women tested in Pittsburgh, vitamin D deficiency or insufficiency was identified in 83% of black and 47% of white pregnant women.¹⁶¹ These high rates of inadequate vitamin D status during pregnancy were mir-rored in the vitamin D status of infants. During the first month of life, 93% of black and 66% of white infants were vitamin D insufficient.¹⁶¹ Intakes of 10 mcg (400 IU) vitamin D daily are related to low serum levels of 25 hydroxyvitamin D (vitamin D₃, the nutrition biomarker of vitamin D status).¹⁵⁷

of over 10,000 IU per day, and the use of medications such as Accutane and Retin-A for acne and wrinkle treatment, increase the risk of fetal abnormalities. Effects are partic-ularly striking in infants born to women using Accutane or Retin-A early in pregnancy (see Illustration 4.18). Fetal exposure to the high doses of retinoic acid in these drugs tends to result in “retinoic acid syndrome.” Features of this syndrome include small ears or no ears, abnormal or miss-ing ear canals, brain malformation, and heart defects.¹⁵⁵

Due to the potential toxicity of retinol, it is recom-mended that women take no more than 5,000 IU of vi-tamin A as retinol from supplements during pregnancy.²⁴ Most supplements made today contain beta-carotene rather than retinol. High intakes of beta-carotene have not been related to birth defects.⁵⁵ Although women were is-sued strong warnings not to take Retin-A or Accutane if pregnancy was possible, ill-timed use continue to occur, as did the retinoic acid syndrome. Accutane is no longer on the market, but other retinoic acid-based medications for acne remain available.¹⁶⁶

Vitamin D

Vitamin D supports fetal growth, the addition of calcium to bone, and tooth and enamel formation. Lack of a sufficient supply of vitamin D during pregnancy compromises fetal as well as childhood bone development.¹⁵⁷ Infants born

Illustration 4.18 An 8-month-old infant exposed to high levels of retinoic acid in utero. Note the high forehead, flat nasal bridge, and malformed ear.

source: Used by permission of Harcourt Health Sciences, Inc. Lott I T et al., “Fetal hydrocephalus and ear abnormalities associated with maternal use of isotretinoin,” J Pediatr 1984;105:597–600.

33411_04_Ch04_p087-133.indd 117

33411_04_Ch04_p087-133.indd 117 4/30/10 3:40:32 PM4/30/10 3:40:32 PM

Calcium

Calcium is primarily needed in pregnancy for fetal skeletal mineralization and maintenance of maternal bone health.

Approximately 30 grams of calcium (a little over an ounce) is transferred from the mother to the fetus during pregnancy. Fetal demand for calcium peaks in the third tri-mester when fetal bones are mineralizing at a high rate.¹⁶³ Calcium metabolism changes meaningfully during pregnancy. Absorption of calcium from food increases, excretion of calcium in urine decreases, and bone mineral turnover takes place at a higher rate.¹⁶⁴ The additional re-quirement for calcium in the last quarter of pregnancy is approximately 300 mg per day and may be obtained by in-creased absorption and by release of calcium from bone.¹⁶⁵ (Calcium is not taken from the teeth, however.¹⁶⁶) Calcium lost from bones appears to be replaced after pregnancy in women with adequate intakes of calcium and vitamin D.¹⁶⁵ Inadequate calcium intake has been related to increased blood pressure during pregnancy, decreased subsequent bone remineralization, increased blood pressure of infants, and decreased breast-milk concentration of calcium.¹⁶³

Calcium and the Release of Lead from Bones Lead in maternal blood can cross the placenta and be taken up by the fetus.¹⁶⁸ Elevated blood lead levels in pregnancy are a cause for concern because they are related to miscarriage, preterm birth, low-birth-weight infants, impaired central nervous system development, and subsequent developmental delays in children.¹⁶⁷ Poor, urban, and immigrant popula-tions are at greater risk for exposure to lead-based paint and environmental contamination than are other groups in the United States.¹⁶⁹

Pregnant women who do not consume enough calcium show greater increases in blood lead levels than women who consume 1000 mg (the DRI for calcium) or more per day. Bone tissues contain about 95% of the body’s lead content, and the lead is released into the bloodstream when bones demineralize. Bone tissues de mineralize to a greater extent in pregnant women who fail to consume adequate calcium.¹⁶⁷

Calcium needs during pregnancy can be met by drink-ing 3 cups of milk or calcium-fortified soymilk, or 2 cups of calcium-fortified orange juice plus a cup of milk, or by choosing a sufficient number of other good sources of calcium daily. (Table 1.14 in Chapter 1 lists food sources of calcium.)

Fluoride

Teeth begin to develop in utero, so why isn’t it recom-mended that pregnant women consume sufficient fluoride so that the fetus builds cavity-resistant teeth? A limited amount of fluoride is transferred from the mother’s blood to the developing enamel of the fetus. Major gains in the fluoride composition of enamel, however, occur in the Obese women appear to be at increased risk for

inadequate vitamin D status due to low levels of release of stored vitamin D from fat cells. As many as 61% of obese women have been identified as having low serum levels of vitamin D compared to approximately 36% in women who are not obese.¹⁶¹ Vegan women are at risk for poor vitamin D status because vitamin D is naturally present only in ani-mal products. Risk factors for inadequate vitamin D status

Dalam dokumen Through the Life Cycle (Halaman 142-146)