Calcium intake is weakly negatively associated with iron status j nutr 1999
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Calcium intake is weakly negatively associated with iron status j nutr 1999 Calcium intake is weakly negatively associated with iron status j nutr 1999 Document Transcript

  • Human Nutrition and MetabolismCalcium Intake Is Weakly but Consistently Negatively Associated with IronStatus in Girls and Women in Six European Countries1 L.P.L. van de Vijver,*2 A.F.M. Kardinaal,* J. Charzewska,† M. Rotily,** P. Charles,‡ M. Maggiolini,†† S. Ando,†† K. Vaananen,‡‡ B. Wajszczyk,† J. Heikkinen,# A. Deloraine§ ¨¨ ¨ and G. Schaafsma*3 *Division of Human and Animal Nutrition, TNO Nutrition and Food Research Institute, 3700 AJ Zeist, The Netherlands, †Department of Epidemiology, National Institute for Food and Nutrition, 02-903 Warsaw, Poland, **ORS PACA INSERM and CIC APHM-INSERM, 13006 Marseille, France, ‡Osteoporosis Clinic, Aarhus Amtssygehus, 8000 Aarhus, Denmark, ††Centro Sanitario, University of Calabria, 87030 Arcavacata di Rende, Italy, ‡‡Department of Anatomy, University of Oulu, 90220 Oulu, Finland, #Deaconess Institute of Oulu, 90100 Oulu, Finland, and §CAREPS, 38043 Grenoble, France ABSTRACT Several studies indicate that intake of calcium can inhibit iron absorption especially when taken simultaneously. In the CALEUR study, a cross-sectional study among girls (mean 13.5 y) and young women (mean Downloaded from by on February 21, 2009 22.0 y) in six European countries, the association between calcium intake and iron status was studied. In 1,080 girls and 524 women, detailed information on calcium intake was collected by means of a 3-d food record, and serum ferritin, serum iron, serum transferrin and transferrin saturation were measured as indicators of iron status. The mean levels of serum iron, ferritin and transferrin were 15.8 6.1 mmol/L, 34.5 19.1 g/L and 3.47 0.47 g/L, respectively, in girls and 16.9 7.5 mmol/L, 40.2 30.5 and g/L, 3.59 0.60 g/L, respectively, in women. A consistent inverse association between calcium intake and serum ferritin was found, after adjusting the linear regression model for iron intake, age, menarche, protein, tea and vitamin C intake and country, irrespective of whether calcium was ingested simultaneously with iron. The adjusted overall regression coefficients for girls and women were 0.57 0.20 and 1.36 0.46 per 100 mg/d increase in calcium intake, respectively. Only in girls, transferrin saturation as a measure for short-term iron status was inversely associated with calcium intake (adjusted overall coefficient 0.18 0.08). However, analysis per country separately showed no consistency. We conclude that dietary calcium intake is weakly inversely associated with blood iron status, irrespective of whether calcium was ingested simultaneously with iron. J. Nutr. 129: 963–968, 1999. KEY WORDS: ● dietary calcium ● iron status ● ferritin ● transferrin saturation ● cross-sectional ● girls ● women Iron deficiency is the most prevalent nutritional deficiency is important. Heme iron is absorbed more efficiently thanin the world, highest in developing countries. Also in devel- nonheme iron and is thought to be less affected by otheroped countries some populations show a high incidence of iron dietary constituents (Fairbanks 1994).deficiency (Turnlund et al. 1990). Differences in bioavailabil- Studies of Hallberg et al. (1991, 1992a, 1992b) showed thatity of dietary iron may be one of the factors explaining regional calcium inhibits iron absorption when taken during the samedifferences (Galan et al. 1991). Less than 10% of daily iron meal. This inhibitory effect was observed for both heme andintake has to be absorbed to compensate for daily losses (up to nonheme iron. The inhibitory effect of calcium seems to be20% for menstruating and pregnant women). Therefore, fac- limited to the meals in which both calcium and iron aretors determining absorption may be more relevant to iron consumed (Cook et al. 1991, Dawson et al. 1986, Deehr et al.status than the absolute intake of iron (Passmore and East- 1990, Galan et al. 1991, Gleerup et al. 1993, Hallberg et al.wood 1986). Also, the form in which iron is present in the diet 1992a). Further, a dose-dependent relationship was found in which calcium intake in excess of 300 mg/d did not lead to further inhibition of iron absorption (Hallberg et al. 1991). 1 Supported by the European Commission DG XII (grant BMH1-CT94-1523), The risk of iron deficiency is elevated in rapidly growingthe Dairy Foundation for Nutrition and Health, CERIN, Laboratoire Innothera, the persons such as teenagers and in women during their repro-Dutch Ministery of Public Health, Oda og Hans Svenningsens Fond, Mejeri-foreningen, Danish Dairy Board, the Polish State Committee of Scientific Re- ductive years. Thus far, studies have focused mainly on thesearch (grant 4P05D02910), Tetra Laval Service GmbH-Warsaw Branch, Dairy influence of supplementary calcium intake on iron absorptionFarmers of Canada, the Swedish Dairy Board and the Centre de Recherche et (Hallberg et al. 1991, 1992a, 1992b). To study the influence ofd’Information Nutritionelles, France. 2 To whom correspondence should be addressed. nonsupplement calcium intake on iron in a group of persons 3 Current address: DMV international, 5460 BA Veghel, The Netherlands. most at risk of iron deficiency, we investigated the association0022-3166/99 $3.00 © 1999 American Society for Nutritional Sciences.Manuscript received 10 June 1998. Initial review completed 24 September 1998. Revision accepted 20 January 1999. 963
  • 964 VAN DE VIJVER ET AL.between calcium intake and iron status in the CALEUR study, mean level of 3.3 g/L and for analysis of serum iron the variationa large cross-sectional study among girls and women in six coefficient was 1.8% at a mean value of 18 mol/L in the qualityEuropean countries. (Kardinaal et al. 1999) control sample. Blood samples were available for 1,083 girls and 525 women. In a selection of samples with extremely high or low ferritin levels (ferritin MATERIALS AND METHODS 3.0 or 10.0 g/L), duplicate measurement was performed. When the second measurement was comparable to the first measurement, The CALEUR study was conducted in Denmark, Finland, France, the first value was used. Samples of four subjects (three girls and oneItaly, The Netherlands and Poland, initially to study the association woman) were excluded from statistical analyses because of instabilitybetween calcium intake and peak bone mass (Kardinaal et al. 1999). of the sample, leaving data for 1,080 girls and 524 women for analysis.In each of the participating countries, girls aged 11–15 y and young Statistical analyses. Mean and SD of serum iron, serum ferritin,adult women aged 20 –23 y with high and low calcium intake were serum transferrin and transferrin saturation and of dietary intakeselected from random population samples of about 750 girls and 375 levels were calculated. Pearson correlations between potential con-women. All subjects were of Caucasian origin. In Finland (Oulu), founding factors and calcium intake and parameters for iron statusDenmark (Aarhus), Poland (Warsaw) and The Netherlands (Zeist), were calculated. As potential confounders, age, height, weight, men-random samples from the local population registries were obtained ses, smoking, tea and coffee consumption, alcohol consumption,and subjects invited to participate; response rates varied from 25.4 to energy intake, protein intake and vitamin C intake were considered.51.5% for girls and from 28.6 to 62% for women. In France, girls and Variables associated with calcium intake and serum ferritin levels orwomen were recruited via general practitioners and gynecologists in transferrin saturation were included as a covariable in the statisticaltwo geographical areas, Rhone-Alps and Marseille; in Italy, girls were models with serum ferritin or transferrin saturation as independentrecruited from all eight secondary schools in the town of Rende, and variables. Categorical variables were put into the model as dummywomen from the University of Calabria (response rates were 100%). variables. Though the covariables were not significantly associatedOf those selected, the participation rate varied between 64 and 84% with all the parameters of iron status, we choose to use a fixed set offor girls and between 52 and 91% for women: data were collected for covariables in the statistical models. With analysis of covariance,1,116 girls and 526 women. The study was approved by local medical- mean levels of the serum iron parameters were calculated per quartile Downloaded from by on February 21, 2009ethical committees, and all participants (and their parents, if re- of calcium intake, adjusted for the covariables. Linear regressionquired) gave their informed consent. Subjects were excluded when analysis was used to calculate the effect of calcium intake on serumindicating a chronic disease in general, diseases related to bone or levels overall and per country, unadjusted and adjusted for covari-calcium metabolism in particular, use of corticosteroids, participating ables. The influence of the simultaneous intake of iron and calciumin sports more than 7 h/wk, current or prior pregnancy, menstruation during a meal was measured by dividing calcium intake into calciumat irregular intervals (for the women only), vegetarianism or macro- ingested simultaneously with iron and the remaining calcium. Simul-biotism or a prescribed diet other than an energy-restricted diet. taneous intake of iron and calcium was defined as a moment of the Dietary intake. To estimate calcium intake in a comparable way day when at least 20% of the total daily iron and at least 20% of thein the six countries, a 3-d food record method was used. The subjects total daily calcium was consumed. For statistical analysis the BMDPwere asked to record everything they consumed during a consecutive statistical package was used (BMDP version 7.0; BMDP StatisticalWednesday, Thursday and Friday, the week before their visit to the Software Inc., Los Angeles, CA). P values 0.05 were consideredinstitute. Time of day, food, quantity and recipes of composite dishes significant.were recorded. The parent responsible for meal preparation was askedto assist in completing the food records. At the visit to the institute,the food records were checked by a dietitian for completeness; house- RESULTShold measures were verified by comparison with standard measures.Daily consumption of food products in grams were converted to The study sample included girls with a mean age of 13.5nutrient intake using local food composition tables. Mean intakes of 1.5 y (37.8% were premenarcheal) and women with a meancalcium, iron and energy were calculated as the average over 3 d. age of 22.0 1.1 y. Use of oral contraceptives was reported by Height and weight were measured with the subjects wearing light 1.0% of the girls and 48.1% of the women. Smoking wasclothing and no shoes. Subjects completed a self-administered ques-tionnaire on menstrual function and use of oral contraceptives, smok- reported by 7.0% of the girls and 29.4% of the habits, alcohol use, time spent outdoors, height, weight and The ranges for (serum iron, serum ferritin, serum transferrineducation of parents and physical activity. Physical activity was and the transferrin saturation) the different countries weredetermined for the previous month and covered activities at school, comparable (Table 1). For the pooled data, serum iron rangedat work and in leisure time (sports and household activities). For the from 2.9 to 46.8 mmol/L for girls and from 2.1 to 45.1 mmol/Lgirls, the questionnaire comprised 58 items, and for the women 88. for women. Ranges for serum ferritin and serum transferrinThe questionnaire was checked in an interview setting. were 2.3–161.1 g/L and 2.0 –5.3 g/L, respectively, for girls A 10-mL blood sample was drawn from nonfasting subjects in a and 1.1–191.6 g/L and 2.1–5.5 g/L, respectively, for women.10-mL tube with clot activator and cooled to 4°C. Serum was Transferrin saturation ranged from 3.4 to 60.3% for girls andprepared within 2 h by centrifugation at 3000 g for 10 min. Serum from 3.0 to 63.3% for women, with the lowest levels in Italywas stored at 20°C. Serum ferritin was measured as the main markerfor iron status. Serum ferritin levels reflect stored iron. Serum iron and the highest in Denmark. The lowest levels of serum ironand the main iron-binding protein, transferrin, reflect the iron in and serum ferritin were seen in Finland, both for girls andtransit and were used to calculate the transferrin saturation {(trans- women. Low-serum iron and ferritin levels were also seen inferrin saturation (%) iron (g/L)/[transferrin (g/L) 1.41)]} (Wick Italian women. Latent iron deficiency (ferritin 12 g/L) was1996). Transferrin saturation can be used as a short-term marker of found in 4.3% of the girls and 7.4% of the women (Fig. 1).iron concentration (Wick 1996). Serum iron, transferrin and ferritin Calcium intakes of both girls and women were lowest inwere measured with the Hitachi 911 (Boehringer Mannheim, Mann- Italy and highest in Finland (Table 2). Intakes in The Neth-heim, Germany). Ferritin and transferrin were analyzed immunotur- erlands, Finland and Denmark were all relatively high. Thebidimetrically with a ferritin test kit (No. 1661400; Boehringer lowest iron intakes were in Italy, whereas girls and women inMannheim) and a transferrin test kit (No. 1360752; Boehringer Poland had the highest iron intakes.Mannheim). Serum iron was analyzed after separation of Fe3 fromtransferrin. After reduction, Fe2 forms colored complexes with To assess the association between calcium intake and serumFerroZine™ (Hach Chemical Co., Ames, IA). The coefficient of iron levels, the potential confounders age, height, weight,variation for the analysis of serum ferritin was 8.1 and 3.9% at mean menses, smoking, tea and coffee consumption, alcohol con-values of 30 and 60 g/L in the quality control samples, respectively. sumption, energy intake, protein intake and vitamin C intakeThe variation coefficient for analysis of transferrin was 2.5% at a were first considered. Inverse associations between serum fer-
  • CALCIUM INTAKE AND IRON STATUS 965 TABLE 1Serum iron, ferritin and transferrin concentrations and transferrin saturation of girls and women from several European countries1 n Ferritin Iron Transferrin Transferrin saturation g/L mol/L g/L %Girls The Netherlands 165 35.2 16.3 16.5 5.3 3.45 0.45 19.4 7.1 Finland 190 28.3 16.1 13.7 5.7 3.38 0.43 16.6 7.9 Denmark 165 38.2 21.3 17.6 6.3 3.31 0.40 21.5 8.2 Italy 197 33.4 21.4 15.2 6.1 3.78 0.51 16.4 7.1 France 167 36.4 19.6 15.4 5.7 3.36 0.43 18.6 7.5 Poland 196 36.1 17.9 16.6 6.4 3.51 0.43 19.1 7.8 Overall 1080 34.5 19.1 15.8 6.1 3.47 0.47 18.5 7.8Women The Netherlands 96 47.0 38.1 17.9 7.7 3.93 0.59 18.5 8.2 Finland 82 29.7 25.3 14.4 8.7 3.39 0.62 17.6 11.4 Denmark 77 50.2 36.4 19.4 7.8 3.67 0.66 21.5 9.2 Italy 99 33.1 19.2 14.4 5.8 3.47 0.51 16.9 7.1 France 74 39.6 30.2 17.3 7.7 3.69 0.53 19.1 8.5 Poland 96 42.2 26.9 18.0 6.4 3.43 0.49 21.1 7.5 Overall 524 40.2 30.5 16.9 7.5 3.59 0.60 19.1 8.8 1 Values are means SD. Downloaded from by on February 21, 2009ritin and age and between serum transferrin and vitamin C women, respectively, P 0.05). After adjustment was madeintake were observed. Age, weight, height, protein intake and for the set of covariables, the adjusted linear regression coef-energy intake were positively associated with serum trans- ficient for the pooled data was 0.57 0.20 for girls. Thisferrin. Calcium intake was positively associated with age, means that for every 100-mg/d increase of calcium intake theheight, energy intake, protein intake and vitamin C intake. ferritin level is reduced by 0.57 g/L. For an average ferritin In Table 3 the mean levels of serum iron, ferritin, trans- level of 34.5 g/L, this means a reduction in ferritin level offerrin and transferrin saturation per quartile of calcium intake 1.6%. For women a linear regression coefficient of 1.37are presented. The means were adjusted for a fixed set of 0.46 was found, i.e., a reduction in ferritin concentration ofcovariables, age, country, tea, protein, vitamin C and iron 3.3% for every 100-mg/d increase in calcium intake at anconsumption and for menses (in girls only). Adjusted means average ferritin level of 40.2 g/L. Calculation of the associ-differed significantly between quartiles of calcium intake for ation for each country separately revealed a consistent inversetransferrin levels in girls and serum iron and transferrin satu- association between calcium intake and serum ferritin, evenration in women. though in none of the countries this association reached Linear regression modeling was used to assess the contribu- significance. When transferrin saturation as a measure oftion of various independent variables of influence to serum short-term iron concentration was put in the model as theiron status. Calcium intake was not associated with serum dependent variable, we found a significant adjusted inverselevels of iron or transferrin. For the main indicator of iron regression coefficient for the girls of 0.18 0.08 per 100-status, serum ferritin, the linear regression coefficients are mg/d increase of calcium. The regression coefficient calculatedpresented for the separate countries and for all countries for the separate countries showed only a significant inversepooled (Table 4). Both for girls and for women a significant association in Denmark. For women, no association was ob-inverse association was found between calcium intake and served between calcium intake and transferrin saturation.serum ferritin level (r 0.09 and 0.07 for girls and Several studies suggest that only the simultaneous intake of calcium and iron leads to an inhibitory effect (Gleerup et al. 1993). Our linear regression model, adjusted for iron intake, age, menarche, country, protein intake and vitamin C intake, did not reveal a difference in inhibition between calcium taken simultaneously with iron and the remaining calcium. The two linear models in which the calcium variables were separated neither revealed a significant association with cal- cium taken with iron nor with the remaining calcium. The contribution of the two calcium sources independent of the other source was assessed by putting both calcium variables in one regression model. In the adjusted model with serum fer- ritin as the dependent variable, the linear regression coeffi- cients for calcium-consumed simultaneously with iron and for the remaining calcium were 0.56 0.25 and 0.58 0.22, respectively, in girls and 1.34 0.56 and 1.37 0.50, respectively, in women (P 0.05 for all values). The dose dependence of the association between calcium FIG. 1. Percentage of women and girls in each country with latent intake and serum ferritin levels was checked over strata ofiron deficiency (ferritin 12 g/L). calcium intake levels. A division was made between intake
  • 966 VAN DE VIJVER ET AL. TABLE 2 Daily intake of calcium, iron and energy among girls and women in the pooled data and per country1 Calcium Iron Energy intake Girls Women Girls Women Girls Women mg MJThe Netherlands 1083 379 1139 458 10.9 4.4 10.5 3.5 9.5 2.1 9.4 2.0Finland 1227 512 1265 545 11.3 9.6 10.3 5.7 7.8 2.0 7.6 2.0Denmark 1133 499 1263 596 10.7 11.1 10.3 6.8 8.0 2.1 8.1 1.9Italy 680 302 609 279 8.2 2.6 8.0 2.9 7.8 1.9 8.2 1.9France 950 381 887 335 10.9 4.3 10.4 2.7 8.6 2.2 8.8 1.9Poland 937 400 829 364 13.0 6.5 12.4 5.6 8.8 2.3 9.8 2.3Overall 992 449 988 504 10.8 7.0 10.3 5.0 8.4 2.2 8.7 2.2 1 Values are means SD.levels of 300, 300 – 600, 600 –900, 900 –1200, and 1200 prudence is called for when using these data as a measure ofmg/d. No indication of a threshold effect was seen. None of intake for a specific country. Also, blood was collected accord-the linear regression coefficients in the strata showed a signif- ing to the same protocol, and all blood samples were analyzed Downloaded from by on February 21, 2009icant association between calcium intake and serum ferritin centrally in one laboratory.levels. In all strata an inverse association was observed except Dietary assessment took place on three consecutive days.for a positive linear regression coefficient in women with a From the Dutch National Food Consumption Survey, it wascalcium intake lower than 300 mg. No trend could be de- calculated that the day-to-day variation in calcium intake intected. The Netherlands was small (Van Erp-Baart 1996). A 3-d dietary record could thus give a representative estimate of DISCUSSION usual calcium intake. Day-to-day variation in iron is expected In this cross-sectional study, performed in six European to be more pronounced and consequently the 3-d record is notcountries, a weak but consistent inverse association was seen likely to accurately reflect habitual individual intake. On thebetween calcium intake and serum ferritin status, irrespective other hand, measurement of serum iron parameters variable isof whether calcium was ingested simultaneously with iron. also exposed to biological and analytical variances. The con-Further, no dose-response relationship between calcium intake sequence of these factors is that whenever an association reallyand serum ferritin was detected. exists, the association is either not found, or it seems to be The study was conducted in six countries throughout Eu- weaker than in reality.rope, which contributed to a wide variety in intakes of both In this study dietary information was collected at differ-calcium and iron. In each country, two groups of subjects were ent periods over the year for the separate countries. There-recruited, a group of girls and a group of young women. To fore, comparison of intakes between countries can give aensure comparability of the data between groups and among biased view. Blood collection took place in the same pe-countries, all procedures were similar. Because the study pop- riod as the dietary assessment was made. Because bloodulations are not necessarily representative for the countries, levels reflect intake, seasonality plays only a minor role in TABLE 3 Serum iron, ferritin and transferrin concentrations and transferrin saturation of girls and women per quartile of calcium intake1Quartile 1 2 3 4Girls2 n 268 271 268 271 Calcium intake, mg 462.0 7.5 766.0 4.2 1031.9 6.2 1686.9 24.9 Iron, mol/L 15.5 0.6 15.2 0.4 16.0 0.4 16.5 0.7 Ferritin, g/L 34.0 1.8 32.0 1.3 34.0 1.2 37.8 2.1 Transferrin,* g/L 3.55 0.04 3.54 0.03 3.45 0.03 3.35 0.05 Transferrin saturation, % 17.8 0.7 17.4 0.5 18.9 0.5 19.9 0.9Women3 n 129 130 130 131 Calcium intake, mg 482.9 10.8 798.2 6.9 1081.6 9.3 1597.9 27.8 Iron,* mol/L 15.4 1.1 17.9 0.7 15.8 0.7 18.4 1.2 Ferritin, g/L 41.2 4.5 40.8 3.0 42.9 2.7 36.4 5.1 Transferrin, g/L 3.57 0.09 3.56 0.06 3.59 0.05 3.67 0.10 Transferrin saturation,* % 17.3 1.3 20.1 0.9 18.0 0.8 21.0 1.5 1 Values are means SEM. 2 Adjusted for age, menses, country, protein, vitamin C, tea, iron. 3 Adjusted for age, country, protein vitamin C, tea, iron. * Significant difference among the means, P 0.05.
  • CALCIUM INTAKE AND IRON STATUS 967 TABLE 4 Linear regression coefficients for calcium intake (per 100 mg) as predictor of serum ferritin level Crude Adjusted1 Multivariate adjusted2 ( SD) Girls The Netherlands 0.37 0.28 0.46 0.29 0.88 0.46 Finland 0.06 0.22 0.20 0.23 0.44 0.42 Denmark 0.28 0.28 0.32 0.29 1.94 0.53** Italy 0.30 0.55 0.53 0.58 0.49 0.79 France 0.01 0.46 0.08 0.48 0.43 0.58 Poland 0.62 0.35 0.72 0.36 0.58 0.50 Overall 0.25 0.12* 0.34 0.12** 0.57 0.20** Women The Netherlands 0.46 1.03 0.32 1.09 0.49 1.94 Finland 1.77 0.52 1.83 0.51 1.37 0.87 Denmark 0.55 0.85 0.59 0.85 1.87 1.37 Italy 0.29 0.65 0.15 0.69 0.43 1.05 France 0.82 0.93 0.66 0.99 1.99 1.22 Poland 1.03 0.68 0.85 0.74 1.33 0.94 Overall 0.60 0.30* 0.62 0.30* 1.37 0.46** 1 Adjusted for iron intake. Downloaded from by on February 21, 2009 2 Adjusted for iron intake, age, menarche (in girls only), protein, tea, vitamin C and country. * P 0.05, ** P 0.01.the association between calcium intake levels and iron iron absorption is inhibited, Hallberg and colleagues (1992a)status. argued that the mechanism must involve inhibition of iron Several studies show an inhibitory effect of calcium intake extrusion from the enterocyte. Recent studies suggest thaton iron absorption (Deehr et al. 1990, Cook et al. 1991, Galan calcium competes for iron-binding sites on the intestinal iron-et al. 1991, Hallberg et al. 1992a). In these intervention binding protein mobilferrin.studies, calcium was provided via a supplement or added to the It can be argued whether the finding that higher calciummeal (Hallberg et al. 1991, 1992a, 1992b). Some studies pro- intake is associated with reduced serum ferritin levels is ofvide extra yogurt or milk during the meal (Gleerup et al. 1993, biological relevance. The normal absorption of iron from the1995). An effect of calcium on iron absorption was reported to diet is estimated to be 10% (about 1 mg/d). To maintain thebe most pronounced when calcium was provided during the iron balance, several mechanisms are involved. In case of ansame meal in which iron was consumed. Gleerup et al. (1993) iron deficiency, the uptake of iron may be increased by up toshowed that calcium given 2 or 4 h before a meal had no 20 –30% (Wick et al. 1996). Several studies showed that ironinhibitory effect on iron absorption. Some studies, however, absorption is strongly and inversely associated with serumdid not find an inhibitory effect of milk on iron absorption ferritin level (Hulten et al. 1995). Therefore, reducing calcium ´(Turnlund et al. 1990, Tidehag et al. 1995), and in a study intake does not necessarily lead to increased serum ferritinamong lactating Gambian women, no effect of calcium sup- levels. Because inadequate calcium intake may lead to otherplementation on serum ferritin level was observed (Yan et al. serious health problems such as osteoporosis (Heaney 1993), it1996). seems inappropriate to advise strongly against the consump- A far as we know, our study is the first to look at normal tion of calcium to prevent iron deficiency.dietary intake levels instead of supplementation with calcium. From our results we conclude that dietary calcium intake isContrary to most studies which evaluate iron absorption, our weakly, inversely associated with the iron status of blood ininterest was the influence of calcium intake on iron status. girls and young women, irrespective of whether calcium wasThis study, therefore, may yield a better assessment of the ingested simultaneously with iron.long-term effect of calcium on iron absorption. Our study didnot reveal an effect of time of calcium consumption on iron ACKNOWLEDGMENTSstatus. It may well be that the effect of simultaneous consump-tion of calcium and iron is readily reflected in iron absorption, We thank all those who have contributed to data collection,whereas the iron status of blood is a more long-term variable questionnaire development, data management, biochemical analysison which this effect is less clear. In our study we did not and technical and administrative support.distinguish between heme and nonheme iron. However, be-cause the inhibitory effect of calcium was observed in both LITERATURE CITEDforms of iron (Gleerup et al. 1995, Hallberg et al. 1991), we Cook, J. D., Dassenko, S. A. & Whittaker, P. (1991) Calcium supplementation:assume that the distinction is of less importance. On the other effect on iron absorption. Am. J. Clin. Nutr. 53: 106 –111.hand, it is therefore not possible to assess iron availability from Dawson-Hughes, B., Seligson, F. H. & Hughes, V. A. (1986) Effects of calciumfood as proposed by Tseng et al. (1997). carbonate and hydroxyapatite on zinc and iron retention in postmenopausal women. Am. J. Clin. Nutr. 44: 83– 88. The mechanism of inhibition of iron absorption by calcium Deehr, M. S., Dallal, G. E., Smith, K. T., Taulbee, J. D. & Dawson-Hughes, not yet clear. A complex formation with iron and phytate (1990) Effects of different calcium sources on iron absorption in postmeno-was suggested but, on the other hand, a complex formation pausal women. Am. J. Clin. Nutr. 51: 95–99. Fairbanks, V. F. (1994) Iron in medicine and nutrition. In: Modern nutrition inmay enhance rather than inhibit calcium absorption by form- health and disease (Shils, M. E., Olson, J. A. & Shike, M. eds). Lea & Febiger,ing a calcium phytate complex. As both heme and nonheme Philadelphia, A Waverly Company.
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