Characteristic changes in calcium metabolism in preeclamptic women in Japan

Abstract

Aim: Calcium intake is relatively low in Japanese women. The present study was to investigate longitudinal calcium metabolism during pregnancy and its characteristic changes in women with pregnancy induced hypertension (PIH) in Japan.

Methods: A total of 435 pregnant Japanese women participated in this study. Fifty nine developed PIH, including 24 with preeclampsia and 35 with gestational hypertension. The urinary calcium to creatinine (Ca/Cr) ratio and serum calcium concentrations were measured at the 1st, 2nd, and 3rd trimesters and postpartum. In preeclampsia, they were classified on focusing onset time, severity and intrauterine fetal growth.

Results: The urinary Ca/Cr ratio at the 3rd trimester was reduced, while serum calcium concentrations were reduced at the 2nd trimester in three groups. In preeclamptic women, urinary Ca/Cr ratios at 3rd trimester and after onset of disease were significantly reduced, while the serum calcium concentration at the 2nd trimester was markedly reduced. Changes in urinary calcium and serum calcium were similar in all groups of preeclampsia.

Conclusions: The characteristic changes in calcium metabolism seen in preeclampsia might be present in Japanese women, who were known to have low calcium intake.

Introduction

Calcium intake is relatively low in Japanese women, although the Ministry of Health and Welfare of Japan recommends an oral intake of calcium >600 mg/day in non-pregnant women because the requirement dramatically increases with fetal storage during pregnancy. It was reported that calcium homeostasis in pregnant women with low calcium intake (i.e., <500 mg/day) might be different from that in women with high calcium intake.^1,2) Thus, investigations of calcium homeostasis during pregnancy in Japanese women are warranted.

Epidemiological and clinical studies have shown an inverse relationship between the development of pregnancy induced hypertension (PIH), especially preeclampsia and calcium intake, possibly shortage of calcium.^3,4,5,6) A previous review reported that oral supplementation of calcium during pregnancy significantly reduced the risk of PIH (gestational hypertension and preeclampsia). This effect was more prominent in studies in which participants had low baseline calcium intake compared to that of adequate calcium intake.^7,8) This finding was not confirmed in a large trial in Western developed countries.⁹⁾ It was more effective in developing countries.¹⁰⁾

In this study, longitudinal changes in calcium metabolism during pregnancy were investigated in Japanese women.

Materials and methods

A total of 435 pregnant women participated in Nagoya City University Hospital. This study was approved by the ethics committee of the institution, and all subjects gave informed consent.

The urinary concentrations of calcium and creatinine, and serum levels of calcium, albumin and uric acid were longitudinally measured at the 1st, 2nd and 3rd trimesters and postpartum. In 24 women who developed preeclampsia, these parameters were also assessed after onset of the disease. Urine was collected in the morning. Blood samples of about 10 ml were drawn into syringes from the antebrachial vein. The concentrations of urinary calcium and creatinine were measured by use of Hitachi 7450 (Hitachi Co., Ltd., Japan) and values for calcium, albumin and uric acid by use of Hitachi 7150 (Hitachi Co., Ltd., Japan). Urinary calcium excretion was estimated as the calcium to creatinine (Ca/Cr) ratio. The concentration of serum calcium was adjusted with the following equation; corrected serum calcium (mg/dl)=serum calcium (mg/dl)+4−serum albumin (g/dl), if serum was less than 4 mg/dl.¹¹⁾

PIH was diagnosed according to the classification and criteria of the Japan Society for the Study of Hypertension in Pregnancy.¹²⁾ The main features include preeclampsia (hypertension with proteinuria) and gestational hypertension (hypertension without proteinuria) after 20 weeks of gestation, but resolving by 12 weeks postpartum. Mild PIH was defined as systolic blood pressure higher than 140 mmHg but not exceeding 160 mmHg, and/or diastolic blood pressure higher than 90 mmHg but not exceeding 110 mmHg, as well as daily proteinuria higher than 300 mg but not exceeding 2 g. Severe PIH was defined as systolic blood pressure exceeding 160 mmHg and/or diastolic blood pressure exceeding 110 mmHg, as well as daily proteinuria exceeding 2 g. PIH with an onset earlier than 32 weeks of gestation was defined as early onset (EO), and PIH with an onset after 32 weeks of gestation was defined as late onset (LO). Women with a history of smoking or diabetes mellitus, renal ailments, collagen disease, anti-phospholipid syndrome or infections were excluded since these are known to impact on endothelial functions. Furthermore, none of the patients were taking any type of regular medication, including antihypertensive drugs or MgSO₄, before or during the study. Small for gestational age (SGA) was diagnosed with reference to the standard curve for newborn-infant body weights in Japan.¹³⁾

Statistical analysis

Mean±SD values were calculated from data and analyzed by two-way repeated-measures ANOVA (followed by Scheffé’s F-test for post hoc analysis) and a Student’s unpaired t-test with an F-test. Analyses were performed using Statview for Windows (SAS Institute Inc., USA). The level of significance was set at P<0.05.

Results

Of 435 pregnant Japanese women, 24 developed preeclampsia and 35 developed gestational hypertension (Table 1). In normotensive pregnant women, the serum calcium concentrations significantly decreased in the 2nd and 3rd trimesters compared to the 1st trimester. The serum calcium concentration was similar in women who developed gestational hypertension and preeclampsia. In women who developed preeclampsia, the concentration of serum calcium was markedly reduced in the 2nd trimester compared with normotensive pregnant women and those with gestational hypertension (P<0.05 by ANOVA and Student’s t-test). However, both in the 3rd trimester and after onset of disease, serum concentrations of calcium were similar in all three groups (Table 1, Figure 1).

Table 1. Urinary calcium/creatinine, serum calcium and uric acid during pregnancy

	Normotensive pregnant	Gestational hypertension	Preeclampsia
Number	376	35	24
Age (years)	29.6±4.4	29.6±4.4	31.9±4.9
PP/MP	176/200	23/12	19/5
BMI	20.1±2.1	21.6±2.7	20.6±2.7
Blood pressure (mmHg)
1st sBP	112±12	115±10	113±12
dBP	68±9	71±8	68±7
2nd sBP	110±10	124±11	117±17
dBP	62±8	67±11	69±13
3rd sBP	113±11	124±10 **	131±16 **
dBP	67±8	75±9 **	82±12 **
Post sBP	114±11	125±15 **	117±13
dBP	69±10	74±9 **	69±13
Birth time (weeks)	39.3±1.2	39.3±1.1	37.6±3.0 *
Birth weight (g)	3,136±375	3,115±450	2,506±824 **
Rate of SGA (%)	1.9	11.4 *	37.5 **
Sampling time
1st (weeks)	8.7±2.6	8.7±2.5	10.0±2.3
2nd (weeks)	21.8±1.8	21.8±1.8	21.7±1.7
3rd (weeks)	34.1±0.5	34.1±0.4	33.7±2.4
Post (days)	30.6±4.7	30.4±5.8	29.0±5.5
UCa/Cr
1st	0.1979±0.1139	0.1968±0.1111	0.2017±0.1243
2nd	0.1657±0.1043 ††	0.1535±0.0779	0.1657±0.1043 ††
3rd	0.1339±0.1011 ††	0.1343±0.1264 ††	0.0705±0.0563 *††
Post	0.0863±0.0769 ††	0.0858±0.0799 ††	0.0740±0.0350 ††
SCa (mg/dl)
1st	9.68±0.57	9.70±0.35	9.57±0.33
2nd	9.46±0.33 ††	9.39±0.38 ††	9.17±0.24 **††§
3rd	9.49±0.34 ††	9.52±0.38	9.42±0.41
Post	9.32±0.7 ††	9.36±0.33 ††	9.37±0.33 ††
SUA (mg/dl)
1st	2.92±0.66	3.37±0.66	3.15±0.76
2nd	3.39±0.69 ††	3.94±0.78 †	3.68±0.76 †
3rd	4.41±0.91 ††	4.68±1.37 ††	5.46±1.39 *††
Post	5.01±1.03 ††	5.27±0.86 ††	5.13±0.95 ††
Onset Time (weeks)	–	34.8±4.4	34.4±8.4
EO/LO	–	9/26	3/21
Severity (%)	–	6	46
sBP (mmHg)	–	146±11 §	159±18 §
dBP (mmHg)	–	93±8 §	100±11 §
Proteinuria (g/day)	–	–	2.6±2.2
Sampling time (weeks)	–	–	34.4±8.4
UCa/Cr	–	–	0.0240±0.0316 §
SCa (mg/dl)	–	–	9.40±0.46
SUA (mg/dl)	–	–	6.73±1.30 §

Data are shown as mean±SD. * P<0.05, ** P<0.01 vs. normotensive pregnant;

† P<0.05, †† P<0.01 vs. at the 1st trimester, § P<0.01 vs. normotensive pregnant at the 3rd trimester.

PP, primiparous; MP, multiparous; BMI, body mass index; 1st, first trimester; 2nd, second trimester; 3rd, third trimester; Post, postpartum; sBP, systolic blood pressure; dBP, diastolic blood pressure; SGA, small for gestational age; EO, early onset preeclampsia; LO, late onset preeclampsia; UCa/Cr, Urinary calcium/creatinine; SCa, serum calcium; SUA, serum uric acid.

Figure 1.

Longitudinal changes in urinary calcium excretion and serum calcium and uric acid concentrations in preeclamptic women.

Data are shown for serum calcium and urinary calcium to creatinine ratios in normotensive pregnant women (○), patients with gestational hypertension (△) and patients with preeclampsia (●). Values are presented as mean±SD. * P<0.05, ** P<0.01 vs. normotensive pregnant women, † P<0.01 vs. 1st trimester, § P<0.05 vs. 2nd trimester.

In normotensive pregnant women, the urinary Ca/Cr ratio gradually decreased during pregnancy. The urinary Ca/Cr ratio was similar in women who developed both gestational hypertension and preeclampsia. In women who developed preeclampsia, the urinary Ca/Cr ratio was significantly reduced in the 3rd trimester compared with those in both normotensive pregnant women and women who developed gestational hypertension in the 3rd trimester, and decreased further after onset of disease (P<0.05 by Student’s t-test) (Table 1, Figure 1).

The concentration of serum uric acid gradually increased as pregnancy progressed in all three groups, but this was most prominent in women who developed preeclampsia in the 3rd trimester and after onset of disease (Table 1).

In 24 women who developed preeclampsia, neither the urinary Ca/Cr ratio nor the serum concentration of calcium differed with onset time or severity of disease. In LO preeclampsia, similar values were obtained for severe and mild types, and with or without SGA (Table 2).

Table 2. Urinary calcium/creatinine, serum calcium and uric acid in established preeclampsia by onset time, severity and growth restriction

	Number	Onset time (weeks)	Severity (%)	SGA (%)	1st trimester		2nd trimester		3rd trimester		after onset
					UCa/Cr	SCa (mg/dl)	UCa/Cr	SCa (mg/dl)	UCa/Cr	SCa (mg/dl)	UCa/Cr	SCa (mg/dl)
EO	3	26.6±5.1	100	100	0.6649±0.7540	9.30±0.45	0.1244±0.0700	8.97±0.23	0.0233±0.0008	–	0.0052±0.0060	9.20±0.25
LO	21	36.5±1.8	38	29	0.2190±0.1290	9.52±0.33	0.1689±0.0810	9.07±0.27	0.0730±0.0532	9.48±0.42	0.0277±0.0340	9.40±0.70
Mild	13	37.2±1.8	0	23	0.2135±0.1391	9.48±0.38	0.1348±0.0702	9.05±0.23	0.0750±0.0579	9.54±0.39	0.0287±0.0404	9.50±0.50
Severe	11	33.8±5.2	100	55	0.3476±0.4063	9.52±0.30	0.1972±0.0800	8.98±0.39	0.0603±0.0468	9.30±0.43	0.0177±0.0138	9.30±0.36
Mild LO	13	37.2±1.8	0	23	0.1992±0.1353	9.48±0.38	0.1433±0.0764	9.06±0.21	0.0834±0.0590	9.55±0.38	0.0287±0.0404	9.50±0.50
Severe LO	8	36.6±2.0	100	38	0.2695±0.1039	9.63±0.22	0.2328±0.0540	8.95±0.52	0.0467±0.0205	9.23±0.52	0.0213±0.0121	9.20±0.41
LO+SGA (−)	15	37.2±1.9	33	0	0.1792±0.1123	9.47±0.30	0.1540±0.0770	9.13±0.23	0.0737±0.0598	9.36±0.28	0.0195±0.0240	9.40±0.32
LO+SGA (+)	6	36.5±1.7	50	100	0.3194±0.1190	9.65±0.41	0.2062±0.0860	8.78±0.38	0.0711±0.0357	9.80±0.61	0.0399±0.0456	9.60±0.72

Data are shown as mean±SD. EO, early onset preeclampsia; LO, late onset preeclampsia; Mild, mild preeclampsia; Severe, severe preeclampsia; SGA, small for gestational age; UCa/Cr, urinary calcium/creatinine; SCa, serum calcium.

Discussion

During pregnancy, 200−300 mg of calcium is transferred via the placenta to the fetus on a daily basis,¹⁾ and the provision requires considerable a physiologic adaptation of calcium homeostasis by 1) increased intestinal calcium absorption, 2) decreased urinary calcium excretion, and 3) increased maternal bone calcium turnover. These data were based on women with relatively high daily calcium intake of >1000 mg from pre-pregnancy to postpartum.^1,2) However, calcium homeostasis in pregnant women with low calcium intake (i.e., <500 mg/day) might be different from that in women with higher calcium intake.¹⁴⁾ Japanese women are known to have low calcium intake.

The present study showed that urinary calcium excretion slowly decreased with the progression of pregnancy in normotensive pregnant women, while the serum concentration of calcium decreased at the 2nd trimester. This result suggested that calcium demand might induce changes in homeostasis in both urinary and serum calcium concentrations. These changes might be consistent with results from the low calcium intake group.¹⁴⁾

In the present study, characteristic changes in calcium metabolism were seen in women with preeclampsia. However, neither marked decrease in urinary calcium excretion nor alteration in concentrations of serum calcium were apparent in women who developed gestational hypertension, consistent with previous studies.^3,4,11) However, the decrease in urinary calcium excretion seen in preeclampsia was much more marked, independent of time of onset or severity of the disease, or in the presence or absence of SGA.

The decrease in serum calcium concentration at the 2nd trimester might be unclear and independent on the increase of plasma volume, since plasma volume increases from the early gestation and reached to a plateau in the 2nd trimester.

Our results could support the correlation between the development and shortage of calcium in Japanese women. Recently, it was suggested that PIH, especially preeclampsia, might activate and/or impair vascular endothelial function, characterized by changed actions of endothelium-derived relaxing factors, such as nitric oxide,^15,16,17) prostacyclin¹⁸⁾ and endothelium-derived hyperpolarizing factor.^16,18) It was unclear the shortage of calcium change the endothelial function from present study. It needs further work to clarify with it.

In conclusion, the characteristic changes in calcium metabolism seen in preeclampsia might be present in Japanese women, who were known to have low calcium intake.

Acknowledgements

This work was partly supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (23592408 and 23791848).

Conflict of interest

None.

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