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Western Human Nutrition Research Center, U.S. Department of Agriculture/ARS, University of California, Davis, California 95616
2To whom correspondence should be addressed. E-mail: jking{at}whnrc.usda.gov
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONBioavailability of calcium and...Maternal calcium status and...Selenium bioavailability during...Future studiesREFERENCES |
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2-fold during pregnancy but dropped to
values for nonpregnant women during lactation. The calcium needs for
lactation were met by renal conservation and bone resorption. In women
chronically consuming a low calcium diet, fractional calcium absorption
increased to >80% during reproduction. Zinc absorption tended to
increase during pregnancy and lactation; renal conservation was not
evident at any time during the reproductive cycle. Selenium absorption
was high, 80% of intake, in both pregnant and nonpregnant women.
Pregnant women conserved selenium by decreasing urinary selenium
excretion. Studies defining the impact of maternal status and the
dietary mineral source and amount on mineral bioavailability are needed
to determine the potential benefits of mineral supplementation during
reproduction.
KEY WORDS: • calcium • zinc • selenium • pregnancy • lactation • bioavailability
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONBioavailability of calcium and...Maternal calcium status and...Selenium bioavailability during...Future studiesREFERENCES |
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). The proportion of the nutrient absorbed
from the gastrointestinal tract is a major determinant of
bioavailability, but that is not the only factor that influences
bioavailability; tissue utilization of the absorbed nutrient or renal
conservation can also be factors. Two types of minerals are discussed
in this review: those regulated primarily by gastrointestinal
absorption (calcium and zinc) and those regulated by renal conservation
(selenium).
Bioavailability is influenced by endogenous and exogenous factors. Body
status, or need, is a primary endogenous factor. Need increases during
growth, pregnancy and lactation, and therefore the bioavailability of
nutrients changes during those periods. Other endogenous factors
include the previous intake of the nutrient, the efficiency of
digestion, gut transit time and the presence of gastrointestinal
disorders or disease (Jackson 1997
). The purpose of this
workshop was to evaluate the bioavailability of vitamin/mineral
supplements. Exogenous factors likely to influence the bioavailability
of supplements include whether they were consumed with food or water
and, if they were consumed with food, the characteristics of the food
eaten.
The bioavailability of supplemental iron during pregnancy and lactation
has been studied and is reported in another article in this supplement.
The bioavailability of other prenatal supplements has not been
evaluated. This review therefore focuses on the physiological
adjustments in nutrient utilization during reproduction in women. Using
stable isotopes as tracers, we measured calcium, zinc and selenium
homeostasis in women during reproduction. The results of those studies
are reviewed here (Fung et al. 1997
, Ritchie et al. 1998
, Swanson et al. 1983
).
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Bioavailability of calcium and zinc during pregnancy and lactation |
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TOPABSTRACTINTRODUCTIONBioavailability of calcium and...Maternal calcium status and...Selenium bioavailability during...Future studiesREFERENCES |
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In addition to tissue mobilization, changes in intake or nutrient
bioavailability are two options for meeting calcium and zinc
requirements for reproduction. A change in nutrient bioavailability may
reflect an increase in true absorption, a reduction in endogenous
gastrointestinal excretion or renal conservation. To ascertain the
effects of reproduction on calcium and zinc bioavailability, we
undertook a longitudinal study of 14 women. Exclusion criteria included
low calcium intake (<800 mg/d), vegetarian diet, age of <22 or >42
y, self-reported body mass index (in kg/m2)
of <17 or >27, daily intake of more than three cups of coffee or
equivalent in caffeine-containing beverage, cigarette smoking, drug
or alcohol abuse and chronic health problems (Fung et al. 1997
, Ritchie et al. 1998
). All of the women
were white and were of upper to middle income stratum. The women were
recruited before conception and studied at six time points: before
pregnancy, 8–10 wk, 23–26 wk, 34–36 wk, early lactation, 7–9 wk and
5 mo after menses. The data from gestation and early lactation are
reported here.
Using a weighed, 24-h food intake record, the women reported consuming 1054 ± 262 mg calcium/d and 10 ± 0.04 mg zinc/d before pregnancy. Because their intakes of calcium and zinc were good, they were told to maintain their dietary pattern throughout gestation. Changes occurred, however. The women increased their intake of dairy products, and consequently dietary calcium and zinc increased 28 and 32%, respectively, by the third trimester. Calcium and zinc intakes returned to near prepregnancy values during lactation.
The true, fractional absorption of calcium and zinc was estimated at
each time point using stable isotopes of the two minerals and the dual
isotopic tracer method (Eastell et al. 1989
,
Friel et al. 1992
). The women consumed 19 mg
44Ca and 3.0 mg 67Zn with a
standard breakfast meal (an English muffin and peanut butter) at
0800 h after an overnight fast. Twenty-five minutes after the
start of breakfast, 5.9 mg 42Ca and 0.8 mg
70Zn were infused into the antecubital vein. The
stable isotopic ratios of calcium were measured in the first 24-h urine
collection after isotope administration by thermal ionization mass
spectrometry with a magnetic sector mass spectrometer (model 261;
Finnigan MAT, San Jose, CA). The zinc isotopic ratios were measured in
the first morning urinary voids on days 4, 5 and 6 after isotope
administration and analyzed by inductively coupled plasma mass
spectrometry (model Sciex ELAN 500; Perkin Elmer, Norwalk, CT).
The true absorption of calcium increased significantly by 57 and 72%
at the second and third trimester, respectively (P < 0.001), and it returned to near the prepregnancy level at early
lactation (Ritchie et al. 1998
). The true zinc
absorption increased 29 and 33% at the second and third trimester,
respectively, but this increase was not statistically significant. At
early lactation, however, zinc absorption increased 73% and was
significantly (P < 0.05) greater than that measured
before conception. Based on an average intake of 1350 mg calcium in the
third trimester, the women absorbed 380 mg more calcium than during
the prepregnancy period. The increase in dietary zinc and true zinc
absorption permitted the women to absorb an average of an additional
1.0 mg zinc/d in the third trimester and an additional 1.3 mg/d during
lactation.
There was no evidence that either calcium or zinc was conserved by the kidney during pregnancy. Compared with before pregnancy, urinary calcium increased significantly by 46% and urinary zinc increased by 79% in the third trimester. The changes in urinary zinc were quite variable, however, and this large average increase did not reach significance. During lactation, the kidney reabsorption of calcium increased and the daily urinary output declined by 56% (P < 0.001) compared with prepregnancy values. A similar decline in urinary zinc excretion did not occur; zinc excretion during lactation was similar to that in midpregnancy.
Approximately 99% of the whole body calcium and 30% of the whole
body zinc are found in bone. Although bone is not a true "store" of
calcium and zinc, it appears that some of the mineral is a
"functional reserve" that can be mobilized in time of need. The
hormonal mechanisms for mobilizing calcium are known; similar
mechanisms for zinc have not been identified. Possibly, the release of
zinc from bone is passive in conjunction with normal bone
turnover rates.
We measured the changes in trabecular bone mineral density before
conception, within 7 d of delivery and at early lactation using
quantitative computed tomography (model 9800; General Electric,
Milwaukee, WI). Pregnancy did not cause a significant change in
trabecular bone mineral density of the lumbar spine. A 9% spinal
trabecular bone loss was measured after only 2 mo of lactation,
however. This bone loss during the early postpartum period is
consistent with the findings of previous studies (Cross et al. 1995
, Hayslip et al. 1989
, Sowers et al. 1993
). We estimate that this loss of bone in early lactation
released daily 120 mg calcium and 0.9 mg zinc into the
extracellular/plasma pool for uptake by the mammary gland and use in
milk synthesis.
In sum, the bioavailability, or fraction of ingested calcium and zinc that is retained, increases during pregnancy and lactation in response to an increased body need. The adjustments for calcium differ from those for zinc during both pregnancy and lactation. In the study women, the additional need for calcium during pregnancy was met by an increased intake and a significant increase in gastrointestinal absorption. Zinc intakes also increased, but the rise in gastrointestinal absorption did not reach significance. There was no evidence of bone mineral mobilization or renal conservation. During early lactation, trabecular bone was mobilized, releasing both calcium and zinc. Renal calcium conservation helped meet the calcium needs for lactation, whereas the high demand for zinc was met by an increase in fractional absorption. These diverse adjustments in calcium and zinc during pregnancy and lactation suggest that their hormonal controls at these time points differ. However, the hormonal controls specific for pregnancy or lactation have not been identified for either mineral.
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Maternal calcium status and calcium bioavailability during reproduction |
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TOPABSTRACTINTRODUCTIONBioavailability of calcium and...Maternal calcium status and...Selenium bioavailability during...Future studiesREFERENCES |
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). Most women
around the world do not consume dairy products and have significantly
lower calcium intakes. Studies in experimental animals show that the
administration of a calcium-deficient diet during pregnancy or
lactation exacerbates the adjustments in intestinal calcium absorption
or bone demineralization (Garel 1987
). Extrapolation
from those studies to humans is not possible because of the shorter
gestational periods in animals and the very severe reductions in
dietary calcium that were studied. We therefore initiated a
longitudinal study of calcium metabolism in women living in Rio de
Janeiro, Brazil, who habitually consume 500 mg calcium/d
(Vargas-Zapata et al. 2000
). Nine women, aged 21–34 y,
participated in the study. The women had an average parity of 2.6. All
were healthy and nonsmokers and had no history of chronic disease.
Supplemental iron was given during the second half of pregnancy, but
none of the women received calcium supplements. Using the same methods
as reported previously (Ritchie et al. 1998
), dietary
calcium intake, calcium absorption and urinary calcium excretion was
measured at early pregnancy (10–12 wk), late pregnancy (34–36 wk) and
early lactation (7–8 wk).
The women did not change their food intake during pregnancy or
lactation. Calcium intake averaged 438 mg/d in early pregnancy, 514
mg/d in late pregnancy, and 451 mg/d in early lactation. At each time
point, the true calcium absorption of the Brazilian women was roughly
twice that of the U.S. women studied previously (Ritchie et al. 1998
). In late pregnancy, true fractional calcium absorption
averaged nearly 90%; absolute calcium absorption was 460 mg/d. True
absorption in early lactation was similar to that at early pregnancy,
i.e., 65%. This is the first study of intestinal calcium absorption
in women consuming low calcium intakes during pregnancy. Calcium
absorption was measured previously in lactating women from the Gambia
who report intakes of 300 mg calcium/d (Fairweather-Tait et al. 1995
). The Gambian women absorbed 1.6 times as much
calcium as did women in the United Kingdom with intakes comparable to
our U.S. women, 1200 mg/d.
It appears, therefore, that the bioavailability of calcium is enhanced in individuals with chronically low intakes and that when their need increases further due to gestation, the capacity to absorb calcium is even greater.
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Selenium bioavailability during pregnancy and lactation |
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TOPABSTRACTINTRODUCTIONBioavailability of calcium and...Maternal calcium status and...Selenium bioavailability during...Future studiesREFERENCES |
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).
In a controlled metabolic study, we compared the selenium absorption,
retention and excretion of pregnant women with that of nonpregnant
controls (Swanson et al. 1983
). A constant purified diet
that provided the recommended intakes for all nutrients during
pregnancy was consumed by all of the women. Selenium absorption was
measured from egg whites and egg yolks intrinsically labeled with
76Se on d 8 of the 21-d study.
The selenium balance and absorption data are summarized in Table 1
. Selenium absorption as estimated from the tracer and balance data did
not differ. Group means for fractional absorption ranged from 78 to
84%. Selenium apparently is not homeostatically regulated by the gut.
Renal regulation may be the means by which whole body selenium content
is controlled. In this study, the pregnant women excreted less urinary
selenium than did the nonpregnant women, and the conservation of
selenium was more pronounced in late than in early pregnancy. The
cumulative urinary excretion of 76Se supported
that observation. Thus, pregnant women seem to meet their selenium
needs for pregnancy by decreasing urinary losses.
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5 kg of lean tissue is accumulated during
pregnancy. If we assume that lean tissue contains 0.2–0.3 mg/kg
selenium, the average daily retention over 280 d of pregnancy
would be 3.5–5 µg. The net selenium retention in the second and
fourth quarters of pregnancy were 10 and 23 µg/d, respectively
(Swanson et al. 1983
). Possibly, the high retention
reflects the high intake of 150 µg/d during the 21-d metabolic
study. |
Future studies |
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TOPABSTRACTINTRODUCTIONBioavailability of calcium and...Maternal calcium status and...Selenium bioavailability during...Future studiesREFERENCES |
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) (Institute of Medicine 1990
). This formulation was
based on nutrient need during gestation. Nevertheless, most pregnant
women are prescribed prenatal supplements that contain all essential
vitamins and minerals. This Institute of Medicine report also contained
a number of research recommendations regarding nutrient supplements
during pregnancy, which still must to be addressed. In particular,
issues to be studied include the bioavailability of the chemical forms
of nutrients commonly used in supplements, the interactions between
nutrients in supplements, the effect of food on the bioavailability of
prenatal supplements and the upper tolerance level of intake for
nutrients during pregnancy and lactation.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONBioavailability of calcium and...Maternal calcium status and...Selenium bioavailability during...Future studiesREFERENCES |
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1.
Cross N. A., Hillman L. S., Allen S. H., Krause G. F., Vieira N. E. Calcium homeostasis and bone metabolism during pregnancy, lactation, and postweaning: a longitudinal study. Am. J. Clin. Nutr. 1995;61:514-523
2. Eastell R., Vieira N. E., Yergey A. L., Riggs B. L. One-day test using stable isotopes to measure true fractional calcium absorption. J. Bone Miner. Res. 1989;4:463-468[Medline]
3. Fairweather-Tait S. J. Bioavailability of selenium. Eur. J. Clin. Nutr. 1997;51:S20-S23
4.
Fairweather-Tait S., Prentice A., Heumann K. G., Jarjou L. M. A., Stirling D. M., Wharf S. G., Turnlund J. R. Effect of calcium supplements and stage of lactation on the calcium absorption efficiency of lactating women accustomed to low calcium intakes. Am. J. Clin. Nutr. 1995;62:1188-1192
5.
Friel J. K., Naake V. L., Jr, Miller L. V., Fennessey P. V., Hambidge K. M. The analysis of stable isotopes in urine to determine the fractional absorption of zinc. Am. J. Clin. Nutr. 1992;55:473-477
6.
Fung E. B., Ritchie L. D., Woodhouse L. R., Roehl R., King J. C. Zinc absorption in women during pregnancy and lactation: a longitudinal study. Am. J. Clin. Nutr. 1997;66:80-88
7.
Garel J.-M. Hormonal control of calcium metabolism during the reproductive cycle in mammals. Physiol. Rev. 1987;67:1-66
8. Hayslip C. C., Klein T. A., Wray H. L., Duncan W. E. The effects of lactation on bone mineral content in healthy postpartum women. Obstet. Gynecol. 1989;73:588-592[Medline]
9. Institute of Medicine, Subcommittee on Dietary Intake and Nutrient Supplements during Pregnancy Nutrition During Pregnancy 1990 II. Nutrient Supplements pp. 237–428, National Academy Press, Washington, D.C.
10. Jackson M. J. The assessment of bioavailability of micronutrients: introduction. Eur. J. Clin. Nutr. 1997;51:S1-S2
11. Ritchie L. D., Fung E. B., Halloran B. P., Turnlund J. R., Van Loan M. D., Cann C. E., King J. C. A longitudinal study of calcium homeostasis during human pregnancy, lactation, and after weaning. Am. J. Clin. Nutr. 1998;67:693-701[Abstract]
12.
Sowers M., Corton G., Shapiro B., Jannausch M. L., Crutchfield M., Smith M. L., Randolph J. F., Hollis B. Changes in bone density with lactation. J. Am. Med. Assoc. 1993;269:3130-3135
13.
Swanson C. A., Reamer D. C., Veillon C., King J. C., Levander O. A. Quantitative and qualitative aspects of selenium utilization in pregnant and nonpregnant women: an application of stable isotope methodology. Am. J. Clin. Nutr. 1983;38:169-180
14. Vargas Zapata C. L., Donangelo C. M., Abrams S. A., Franco M. J., King J. C. A longitudinal study of intestinal calcium absorption during pregnancy and lactation in women with low calcium intakes. Fed. Proc. 2000;14:A725
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