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Dietary vitamin D intake is not associated with 25-hydroxyvitamin D3 or parathyroid hormone in elderly subjects, whereas the calcium-to-phosphate ratio affects parathyroid hormone
N U TR IT ION RE S E ARCH 3 3 ( 2 0 13 ) 66 1 –6 6 7
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Dietary vitamin D intake is not associated with 25-hydroxyvitamin D3 or parathyroid hormone in elderly subjects, whereas the calcium-to-phosphate ratio affects parathyroid hormone☆ Alexandra Jungert, Monika Neuhäuser-Berthold⁎ Institute of Nutritional Science, Justus-Liebig-University, Giessen, Germany
ARTI CLE I NFO
A BS TRACT
Article history:
This cross-sectional study investigates whether serum 25-hydroxyvitamin D3 [25(OH)D3]
Received 30 October 2012
and intact parathyroid hormone (iPTH) are affected by vitamin D, calcium, or phosphate
Revised 11 May 2013
intake in 140 independently living elderly subjects from Germany (99 women and 41 men;
Accepted 23 May 2013
age, 66-96 years). We hypothesized that habitual dietary intakes of vitamin D, calcium, and
Keywords:
these associations. Serum 25(OH)D3 and iPTH were measured by an electrochemi-
25-Hydroxyvitamin D3
luminescence immunoassay. Dietary intake was determined using a 3-day estimated
Parathyroid hormone
dietary record. The median dietary intake levels of vitamin D, calcium, and phosphate were
phosphate are not associated with 25(OH)D3 or iPTH and that body mass index confounds
Vitamin D intake
3 μg/d, 999 mg/d, and 1250 mg/d, respectively. Multiple regression analyses confirmed that
Calcium intake
dietary vitamin D and calcium did not affect 25(OH)D3 or iPTH; however, supplemental
Phosphate intake
intakes of vitamin D and calcium were associated with 25(OH)D3 after adjustment for
Elderly German subjects
age, sex, body composition, sun exposure, physical activity, and smoking. In addition, phosphate intake and the calcium-to-phosphate ratio were associated with iPTH after multiple adjustments. In a subgroup analysis, calcium and vitamin D supplements, as well as phosphate intake, were associated with 25(OH)D3 and/or iPTH in normal-weight subjects only. Our results indicate that habitual dietary vitamin D and calcium intakes have no independent effects on 25(OH)D3 or iPTH in elderly subjects without vitamin D deficiency, whereas phosphate intake and the calcium-to-phosphate ratio affect iPTH. However, vitamin D and calcium supplements may increase 25(OH)D3 and decrease iPTH, even during the summer, but the impact of supplements may depend on body mass index. © 2013 Published by Elsevier Inc.
Abbreviations: 25(OH)D, 25-hydroxyvitamin D; 25(OH)D3, 25-hydroxyvitamin D3; BMI, body mass index; Ca/P ratio, calcium-tophosphate ratio; GISELA, longitudinal study on nutrition and health status in senior citizens of Giessen; iPTH, intact parathyroid hormone; log, logarithmically transformed; PAL, physical activity level; rS, Spearman correlation coefficient; TBF, total body fat. ☆ The authors do not have any competing interests to declare. This investigation received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. ⁎ Corresponding author. Institute of Nutritional Science, Justus-Liebig-University, Goethestrasse 55, 35390 Giessen, Germany. Tel.: +49 (0) 641 99 39067; fax: +49 (0)641 99 39069. E-mail address: [email protected] (M. Neuhäuser-Berthold). 0271-5317/$ – see front matter © 2013 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.nutres.2013.05.011
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1.
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Introduction
In addition to the role of vitamin D in osteoporosis, there is evidence that vitamin D is also involved in the pathogenesis of other widespread diseases such as hypertension and certain types of cancer [1,2]. Elderly people have an increased risk of developing vitamin D deficiency because of age-related declines in cutaneously produced vitamin D, sun exposure, and dietary intake [3]. In Germany, the dietary vitamin D intake of elderly subjects is usually well below the reference value [4,5]. Thus, sun exposure or the use of supplements may be required to achieve adequate vitamin D status. In addition to vitamin D intake, the amounts of calcium and phosphate in the diet may interfere with vitamin D metabolism. Elevated intact parathyroid hormone (iPTH) levels, which can result from inadequate calcium intake or high phosphate intake, induce the formation of 1,25-dihydroxyvitamin D3, which may promote the turnover of circulating 25-hydroxyvitamin D3 [25(OH)D3]. The existing evidence is not sufficient to determine whether dietary calcium and phosphate and widespread low vitamin D intake affect 25(OH)D3 and/or iPTH in free-living elderly individuals and whether the use of supplements impacts the vitamin D and/or iPTH status of these individuals. Previous studies have provided inconsistent results [6-16]; phosphate intake and the dietary calcium-to-phosphate ratio (Ca/P ratio) were not frequently taken into consideration as influencing factors. To date, several studies focused on nutrient intake without considering biochemical parameters [4,17] such as 25(OH)D3 or iPTH levels, whereas others reported nonfasting measurements [8] and/or did not simultaneously examine intakes of vitamin D, calcium, and phosphate [6-16]. In addition, earlier reports often exclusively focused on 25(OH)D3 or iPTH without considering the interaction between both [6,16]. Overall, studies with elderly people who are in general good health and do not have vitamin D deficiency and severely elevated iPTH values are scarce. Currently, there is a great debate on higher recommended dietary intakes of vitamin D for elderly people. At present, it is unclear to what extent all elderly subjects, independent of their health, vitamin D status, or body mass index (BMI), would benefit from such a strategy. Other dietary factors, for example, calcium and phosphate, may have a stronger impact on 25(OH)D3 or iPTH. We hypothesized that (1) in free-living elderly subjects, habitual dietary intakes of vitamin D, calcium, and phosphate show no independent associations with 25(OH)D3 or iPTH and (2) BMI confounds these associations. The objectives of the present study were to (1) evaluate vitamin D, calcium, and phosphate intakes; (2) investigate whether intakes of vitamin D, calcium, and phosphate or the Ca/P ratio contribute to 25(OH)D3 and/or iPTH, independent of potential confounders such as age, sex, body composition, sun exposure, physical activity, and smoking behavior, in an aging population; (3) examine the impact of a mutual adjustment for 25(OH)D3 and iPTH in this context, respectively; and (4) assess the influence of BMI on the associations between 25(OH)D3 and iPTH and vitamin D, calcium, and phosphate intakes by comparing normal-weight with overweight/obese individuals. The present study provides a comprehensive examination of the
relationships between 25(OH)D3 and iPTH concentrations and vitamin D, calcium, and phosphate intakes in free-living elderly subjects, a population that was rarely the focus of previous investigations.
2.
Methods and materials
2.1.
Study population
The present investigation reports cross-sectional data from the longitudinal study on nutrition and health status in senior citizens of Giessen, Germany (50°35´North), (GISELA study) obtained in 2008. The GISELA study is an ongoing prospective cohort study initiated in 1994. To be enrolled in the study, participants had to be at least 60 years of age and physically mobile. Investigations took place in the Institute of Nutritional Science in Giessen from July to October. Written informed consent was obtained from each participant. The study protocol was approved by the ethical committee of the Faculty of Medicine at the Justus-Liebig-University, Giessen. Subjects with incomplete data were excluded, as were subjects who took diuretics or had edema or chronic kidney disease. Five subjects were identified as outliers based on their 25(OH)D3 and iPTH measurements and/or the residuals of the regression analyses and thus not included in the analyses. After applying these exclusion criteria to the 275 individuals who took part in the follow-up in 2008, the present study assessed data from 99 women and 41 men who were all whites.
2.2.
Biochemical analyses
Fasting blood samples were collected, and serum aliquots were stored at −70°C until further analysis. Both 25(OH)D3 and iPTH were measured by an electrochemiluminescence immunoassay (ECLIA; Roche Diagnostics, Mannheim, Germany) [18,19]. Concentrations of 25(OH)D3 less than 25 nmol/L were defined as vitamin D deficiency [3], and 2 cutoff values were applied to define adequate 25(OH)D3 levels: ≥50 and ≥75 nmol/L [1].
2.3.
Anthropometric data and body composition
Body mass index was determined as previously described [20,21]. Subjects were separated into 2 groups; subjects with a BMI less than 25 kg/m2 were classified as normal-weight, and subjects with a BMI of 25 kg/m2 or greater were defined as overweight/obese. The percentage total body fat (%TBF) was recorded by a single-frequency (50 kHz) bioelectrical impedance analyzer (Akern-RJL BIA 101/S; Data Input, Frankfurt, Germany), according to the manufacturer's instructions and the predictive formula from Roubenoff et al [22].
2.4.
Lifestyle factors
Nutritional intake was determined using a 3-day estimated dietary record, which was developed and validated for the GISELA study [23]. This dietary record consisted of 146 food items subdivided into 16 food groups. For every food item,
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both typical household measures (for example, slice, cup, and spoon) and the appropriate weights were given. The participants were instructed to record their entire food intake on 3 consecutive days directly after consumption, starting on a Sunday. The energy, vitamin D, calcium, and phosphate contents of the subjects' diets were calculated according to the German Food Code and Nutrient Data Base [24] version II.3 via the nutrient calculation program DGE-PC professional (German Nutrition Society, Frankfurt, Germany). The nutritional intake assessment did not include nutrient intake by supplements. Time spent outdoors as an indicator for sun exposure, physical activity patterns, smoking behavior, and data regarding age, diseases, medications, and supplement intake were collected via self-administered questionnaires. Participants who reported using vitamin D or calcium supplements sometimes or regularly were classified as “supplement users”. Like supplement use, smoking behavior was coded as a dummy variable (never-smokers vs current and exsmokers). The physical activity level (PAL) of each participant was assessed as previously described [20].
2.5.
Statistical analyses
The characteristics of the study participants are expressed as the median and the 5th to 95th percentiles because of nonnormally distributed data, which was tested by the Shapiro-Wilk test, the Kolmogorov-Smirnov test, and by visual inspection of histograms. Descriptive characteristics were compared between groups via the Mann-Whitney U test for continuous variables. The χ2 test or Fisher exact test was used for categorical variables. Spearman correlation coefficients (rS) were applied to evaluate correlations between 25(OH)D3, iPTH, age, BMI, and %TBF and relevant nutritional parameters. We examined the associations of vitamin D, calcium, and phosphate intakes with 25(OH)D3 and iPTH by multiple regression analyses and created 4 models with different levels of adjustment. Model 1 represented the age-
and sex-adjusted associations, whereas model 2 additionally adjusted for PAL, sun exposure, and smoking. Model 3 further considered the %TBF. In model 4, 25(OH)D3 or iPTH was included as a confounding variable. Serum iPTH was logarithmically transformed (log) to obtain normally distributed residuals. Because there were no major changes in the results when 25(OH)D3 was log transformed, we report the nontransformed data. Finally, we divided the study population into 2 groups based on BMI to examine whether normal-weight subjects differed from overweight/obese subjects regarding the associations between vitamin D, calcium, and phosphate intakes and 25(OH)D3 and log iPTH by multiple regression analyses. Statistical analyses were conducted with SPSS 20.0 for Windows (SPSS Inc, Chicago, IL, USA). The significance level was set at P < .05. All tests were 2-tailed.
3.
Results
The characteristics of the subjects are presented in Table 1. Both sexes failed to meet the recommended daily intake of 20 μg vitamin D [5]; only 1 woman had an intake greater than 20 μg/d. A vitamin D intake less than 10 μg/d and less than 5 μg/d was observed in 93.9 % and 62.6 % of the women and 90.2 % and 56.1 % of the men, respectively. The median calcium intake for both sexes was close to the recommendation of 1000 mg/d [25], and only 19.2 % of the women and 14.6 % of the men had a calcium intake less than 800 mg/d. In contrast, except for 2 women, phosphate intakes of all subjects were above the reference value of 700 mg/d [25]. Consequently, the corresponding Ca/P ratios were clearly below the recommendation of 1.4, and only 2 women had ratios above this recommendation. None of the subjects had 25(OH)D3 levels less than 25 nmol/ L; however, 25(OH)D3 levels less than 50 nmol/L were noted in 22.1 % of the subjects, and 80.0 % of the study population had 25(OH)D3 levels less than 75 nmol/L. Using both cutoff values
Table 1 – Descriptive characteristics of the study population (n = 140) a Women (n = 99) Age (y) BMI (kg/m2) TBF (%) 25(OH)D3 (nmol/L) iPTH (pmol/L) Vitamin D intake (μg/d) Vitamin D intake (μg/MJ) Calcium intake (mg/d) Calcium intake (mg/MJ) Phosphate intake (mg/d) Phosphate intake (mg/MJ) Ca/P ratio Time spent outdoors (min/d) PAL Current or ex-smokers Vitamin D supplement users Calcium supplement users a b
75.0 26.4 42.2 60.6 4.5 2.5 0.3 1000 128 1198 166 0.8 120 1.7 23 16 37
(68.0-86.0) (21.2-33.8) (32.5-50.1) (39.7-93.0) (2.4-8.6) (0.3-10.1) (0.1-1.6) (526-1782) (84-239) (777-2103) (126-215) (0.5-1.2) (40-360) (1.5-2.0) (23.2%) (16.2%) (37.4%)
Men (n = 41)
Pb
76.0 (70.0-84.8) 26.4 (22.9-32.3) 29.0 (21.5-37.9) 67.5 (39.7-88.9) 4.1 (2.1-8.3) 3.5 (1.0-11.3) 0.4 (0.1-1.2) 994 (602-1635) 114 (76-179) 1434 (1053-2385) 163 (124-198) 0.7 (0.5-1.0) 150 (23-396) 1.7 (1.4-1.9) 29 (70.7%) 2 (4.9%) 5 (12.2%)
.395 .591 <.0001 .156 .290 .125 .383 .911 .017 .003 .610 .003 .062 .517 <.0001 .096 .004
Median and 5th to 95th percentile for continuous variables, absolute and relative frequencies for categorical variables. Mann-Whitney U test for continuous variables and χ2 test or alternatively Fisher exact test for categorical variables.
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to define the optimal vitamin D status, the dietary intakes of vitamin D, calcium, and phosphate in subjects with adequate and insufficient vitamin D status were almost equal (all P > .05); however, the analysis showed borderline significance for a higher vitamin D intake in subjects with 25(OH)D3 levels of 75 nmol/L or greater compared with subjects with levels less than 75 nmol/L, independent of whether μg/d (median, 2.5 vs 5.2 μg/d; P = .078) or μg/MJ (median, 0.3 vs 0.6 μg/MJ; P = .077) was used. Spearman correlation analyses of dietary parameters with 25(OH)D3 showed no significant associations (all P > .05). However, the association between 25(OH)D3 and vitamin D supplement use was close to statistical significance (rS = 0.157; P = .064). Log iPTH was inversely correlated with the Ca/P ratio (rS = −0.221; P = .009) and indicated a trend toward an association with calcium supplement use (rS = −0.164; P = .053). Age was positively linked to the daily intakes of vitamin D (rS = 0.244; P = .004), calcium (rS = 0.168; P = .047), and phosphate (rS = 0.181; P = .033). Except for vitamin D (rS = 0.208; P = .014), there were no such associations when nutrients were assessed in relation to the energy content of the diet (all P > .05). Furthermore, age was positively associated with the percentage of subjects who used calcium supplements (rS = 0.257; P = .002). Body mass index was not significantly correlated with intakes of vitamin D, calcium, or phosphate (all P > .05), whereas the %TBF was negatively correlated with phosphate intake in mg/d (rS = −0.264; P = .002) and showed trends toward associations with the Ca/P ratio (rS = 0.157; P = .063) and vitamin D intake in µg/d (rS = −0.152; P = .073). We found an inverse association between 25(OH)D3 and log iPTH, which persisted after controlling for age, sex, body composition, sun exposure, PAL, smoking, use of calcium and vitamin D supplements, and dietary intakes of vitamin D, calcium, and phosphate within the multiple regression analysis (β = −0.224; P = .015). Therefore, mutual adjustments were performed in the subsequent multiple regression analyses. In the multiple regression analyses, no significant associations between dietary vitamin D or calcium intakes and 25(OH)D3 or log iPTH were found after adjustment for age, sex, sun exposure, PAL, smoking, and %TBF (all P > .05). In contrast, dietary phosphate intake in mg/d was positively associated
with log iPTH after the above-mentioned adjustments (β = 0.193; P = .034), but this association disappeared after including 25(OH)D3 in the regression model (β = 0.162; P = .068). The Ca/P ratio was inversely associated with log iPTH before (β = −0.317; P < .001) and after (β = −0.282; P = .001) additional controlling for 25(OH)D3 and showed a trend toward a positive association with 25(OH)D3 after adjustment for age, sex, lifestyle factors, and %TBF (β = 0.144; P = .087). Vitamin D and calcium supplements were associated with 25(OH)D3 after multiple adjustments (Table 2). Although the association between calcium supplements and 25(OH)D3 vanished after log iPTH was added to the model, the association between vitamin D supplements and 25(OH)D3 remained significant. After dividing the cohort into 2 groups based on BMI, dietary intakes of vitamin D, calcium, and phosphate, the Ca/P ratio and the percentage of supplement users did not differ between the 2 BMI groups (all P > .05). However, the vitamin D status (median, 66.4 vs 58.7 nmol/L; P = .006) but not the iPTH status (median, 4.4 vs 4.5 pmol/L; P = .540) of normal-weight subjects was higher than that in overweight/obese subjects. In normal-weight subjects, we observed an inverse association between calcium supplementation and log iPTH after controlling for sex, age, and sun exposure (Table 3), which persisted after further adjustment for 25(OH)D3 (β = −0.305; P = .048). In addition, calcium supplementation was only associated with 25(OH)D3 in normal-weight subjects, but there was no association after further controlling for log iPTH (β = 0.227; P = .116). After adjustment for sex, age, and sun exposure, the use of vitamin D supplements was linked to 25(OH)D3 and log iPTH in normal-weight subjects, but both associations vanished after mutual adjustment for log iPTH (β = 0.198; P = .143) and 25(OH)D3 (β = −0.206; P = .165), respectively. In contrast, dietary vitamin D and calcium intakes were not associated with 25(OH)D3 or log iPTH in either BMI category, but phosphate intake in mg/d was associated with log iPTH in normal-weight subjects, even after adjustment for 25(OH)D3 (β = 0.353; P = .018). Likewise, the associations between the Ca/P ratio and log iPTH persisted after additional controlling for 25(OH)D3 in both normal-weight (β = −0.309; P = .041) and overweight/obese (β = −0.268; P = .012) subjects. These results
Table 2 – Impact of vitamin D and calcium supplements on 25(OH)D3 and log-transformed iPTH in 140 elderly women and men of the GISELA study analyzed by multiple linear regression analyses Vitamin D supplement Model 1 Model 2 Model 3 Model 4 Adjusted R2
25(OH)D3 (nmol/L) B
β
P
B
β
P
7.941 8.907 8.213 7.042
0.181 0.203 0.187 0.160
.035 .013 .021 .043
−0.048 −0.049 −0.046 −0.026
−0.122 −0.124 −0.116 −0.065
.158 .157 .187 .455
0.195 for model 4
Log iPTH (pmol/L)
Calcium supplement Model 1 Model 2 Model 3 Model 4 Adjusted R2
0.042
25(OH)D3 (nmol/L)
Log iPTH (pmol/L)
B
β
P
B
β
P
5.146 5.971 5.978 4.704
0.160 0.186 0.186 0.146
.079 .035 .033 .088
−0.049 −0.051 −0.051 −0.037
−0.169 −0.176 −0.176 −0.127
.064 .062 .061 .172
0.188
0.052
for model 4
Multiple regression analyses with 25(OH)D3 and log iPTH as dependent variables. The results of the regression analyses are expressed in terms of B (the nonstandardized coefficient), β (the standardized coefficient), and the adjusted coefficient of determination (R2) for model 4. Model 1: association adjusted for age (in years) and sex (female/male). Model 2: model 1 further adjusted for PAL, sun exposure (in min/d), and smoking (yes/no). Model 3: model 2 additionally adjusted for TBF (%). Model 4: model 3 additionally adjusted for log iPTH (in pmol/L) and 25(OH)D3 (in nmol/L), respectively.
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Table 3 – Impact of nutritional parameters on 25(OH)D3 and log-transformed iPTH in 140 elderly women and men of the GISELA study in relation to BMI category analyzed by multiple linear regression analyses BMI <25 kg/m2 (n = 51) 25(OH)D3 (nmol/L) 25(OH)D3 (nmol/L) Vitamin D intake (μg/d) Vitamin D intake (μg/MJ) Vitamin D supplement (no/yes) Calcium intake (mg/d) Calcium intake (mg/MJ) Calcium supplement (no/yes) Phosphate intake (mg/d) Phosphate intake (mg/MJ) Ca/P ratio
– −0.016 −0.052 0.280 ⁎ 0.019 0.051 0.327 ⁎ −0.080 −0.201 0.147
Log iPTH (pmol/L) −0.395 ⁎ 0.203 0.101 −0.299 ⁎ 0.046 −0.237 −0.400 ⁎⁎ 0.382 ⁎ 0.136 −0.360 ⁎
BMI ≥25 kg/m2 (n = 89) 25(OH)D3 (nmol/L) – 0.018 −0.015 0.190 −0.019 0.043 0.114 −0.110 −0.094 0.148
Log iPTH (pmol/L) −0.253 ⁎ −0.098 −0.087 −0.071 −0.074 −0.127 −0.081 0.091 0.135 −0.299 ⁎⁎
Multiple regression analyses with 25(OH)D3 and log iPTH as dependent variables. The results of the regression analyses are expressed in β (the standardized coefficient) and adjusted for sex (female/male), age (in years), and sun exposure (in min/d). ⁎ P < .05. ⁎⁎ P < .01.
did not change substantially when we replaced sun exposure with PAL or smoking (data not shown).
4.
Discussion
The results of our study confirm that in noninstitutionalized elderly subjects, habitual dietary intakes of vitamin D, calcium, and phosphate exhibit no independent associations with 25(OH)D3 or iPTH, although the Ca/P ratio was negatively related to iPTH, even after multiple adjustments. Therefore, our first hypothesis must be partially rejected as well as the second hypothesis that BMI functions as a confounder in this context. The second hypothesis was only confirmed for the association between phosphate intake and iPTH and for the associations of vitamin D and calcium supplements with 25(OH)D3 and iPTH, which were significant in normal-weight subjects but not in overweight/obese subjects. A high percentage of our subjects had a vitamin D intake far below the reference intake, which is in agreement with other national [4,6,17] and international [9,12,14] studies. Subjects with such a low vitamin D intake are regarded as having a high risk for vitamin D deficiency [16,17] and, as a result, secondary hyperparathyroidism [27]. However, none of our subjects had a serious vitamin D deficiency, which is contrary to other studies of elderly individuals [6,26]. GISELA subjects with 25(OH)D3 levels of 75 nmol/L or greater tended to have higher dietary vitamin D intakes than did subjects with lower levels, indicating that dietary vitamin D might contribute to a higher vitamin D status. However, only 20 % of the population had 25(OH)D3 levels of 75 nmol/L or greater, and in multiple regression analyses, dietary vitamin D amounts were unrelated to 25(OH)D3 and iPTH. This is in agreement with some [10,12,14] but not all [6-9,16] previous studies. One reason for these missing associations may be the low vitamin D intake levels, but there is evidence that dietary vitamin D intake has a minor impact on vitamin D status in cases where subjects have sufficient sun exposure [28,29]. Considering the amount of self-reported outdoor activities of
the GISELA subjects, dietary vitamin D intake may become a negligible factor. This assumption is in agreement with Bates et al [11], who reported an association between vitamin D intake and 25-hydroxyvitamin D [25(OH)D] in every season except for summer. In addition to vitamin D, calcium intake may impact 25(OH) D3 or iPTH. In cases of chronic low calcium intake, higher 25(OH)D levels may be required to maintain iPTH in the reference range [13]. A high level of calcium intake, in turn, has been linked to reduced 25(OH)D turnover caused by the suppression of iPTH secretion [27]. However, we did not find a direct association between dietary calcium intake and either 25(OH)D3 or iPTH. The relationship between calcium and iPTH might be more evident in individuals with high iPTH levels, inadequate calcium intake, and/or vitamin D deficiency. In a cross-sectional study, iPTH was only associated with calcium intake in the presence of 25(OH)D levels less than 25 nmol/L, which suggests that calcium intake becomes an important determinant in vitamin D deficiency [13]. In the present study, only a few subjects had iPTH values above the reference range, dietary calcium intake was close to adequate, and 25(OH)D3 levels were primarily greater than 50 nmol/L. Along with calcium, phosphate is essential for maintaining bone health, but an excess of phosphate, especially in combination with low intakes of calcium and/or vitamin D, can lead to secondary hyperparathyroidism [30]. In our study, dietary phosphate intake was far above the reference value, which is in accordance with previous studies [30-32]. Because of the high phosphate intake, the Ca/P ratio failed to reach the suggested ratio of 1.4. In the regression analyses, phosphate intake and the Ca/P ratio were associated with iPTH, especially in normal-weight subjects. Why these associations were more pronounced in normal-weight subjects is up for discussion, but the differences appear not to be caused by varying intake levels. It is possible that the normal-weight and overweight/ obese subjects consume different phosphate sources, and organic and inorganic phosphate may induce divergent effects on iPTH [32]. However, because the Ca/P ratio in both subgroups was inversely associated with iPTH, this indicates
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that a high calcium intake may counteract phosphate intake's impact on iPTH. Therefore, consumption of foods with a high Ca/P ratio, such as dairy products, in conjunction with a restriction of foods containing high amounts of phosphate, such as meat products, convenience foods, and soft drinks, should be encouraged to reduce iPTH levels [32]. In our study, vitamin D and calcium supplements were associated with 25(OH)D3 after adjustment for sex, age, sun exposure, PAL, smoking, and %TBF. Several studies have confirmed significant effects of calcium/vitamin D supplements on 25(OH)D levels in elderly subjects [1,14,16], but controlling for iPTH was not performed. Our results indicate that the effect of calcium supplements on 25(OH)D3 may be caused by alterations in circulating iPTH, whereas the effect of vitamin D supplements on 25(OH)D3 appears to be independent of iPTH. However, in our study, the proportion of supplement users was small, and we did not evaluate the dosage or duration of vitamin D supplementation. Therefore, these results must be interpreted with caution. Although we found no independent associations between vitamin D and calcium supplements and iPTH levels, other studies have reported inverse associations [13,27,33]. In a previous followup of the GISELA study, median supplemental vitamin D intake was 10 μg/d in women and 5 μg/d in men, whereas median supplemental calcium intake was 500 mg/d in women and 187 mg/d in men [34]. These amounts may have been too low to affect iPTH, especially when iPTH is mainly in the reference range. Furthermore, the effect of calcium supplements on iPTH may be less pronounced in elderly subjects because of a progressive age-related decrease in calcium absorption [33]. Our results indicate that the impact of vitamin D and calcium supplements on 25(OH)D3 and iPTH depends on BMI, which is in accordance with a study conducted in patients with severe vitamin D deficiency [35]. Furthermore, in a recent randomized, placebo-controlled trial, an inverse association between BMI and the 25(OH)D response to 15 μg cholecalciferol was noted in 48 older Irish adults, but the iPTH response to 15 μg cholecalciferol was not affected by body size or adiposity [36]. In our study, the use of supplements was predominantly associated with 25(OH)D3 and iPTH in normal-weight subjects. Thus, individuals with higher %TBF may require a higher vitamin D intake than normal-weight subjects to maintain or achieve adequate 25(OH)D3 concentrations because of an increased sequestration of vitamin D in the TBF [10,21,35]. The relatively strong association between 25(OH)D3 and log iPTH in our normal-weight subjects might explain why most associations weakened or disappeared after mutual adjustment. The association between 25(OH)D3 and log iPTH tended to be stronger in subjects with a BMI less than 25 kg/m2 than in subjects with a BMI of 25 kg/m2 or greater. Apparently, a higher quantity of TBF weakens this association. Future studies should consider the interaction between 25(OH)D3 and iPTH when studying the impact of nutrients on 25(OH)D3 and iPTH status. There are some limitations of our study. First, the crosssectional design allows no inference on the causality of the observed relationships. Second, the observed relationships should be interpreted with caution given the relatively small sample size and the low number of supplement users. Third, the lifestyle variables measured were based on self-reported data and indirect assessments of sun exposure and physical
activity. Fourth, we did not collect information on the period of supplementation or supplement composition and dosage. All participants (except for 1 man) who took vitamin D supplements also used calcium supplements. Thus, a combined effect of both nutrients cannot be ruled out. In conclusion, a notably high percentage of our subjects did not reach half of the recommended intake level for vitamin D, whereas calcium intake from food matched the recommendation and phosphate intake markedly exceeded the dietary guidelines. Neither dietary vitamin D nor calcium intakes affect 25(OH)D3 or iPTH during the summer, but vitamin D and calcium supplements appear to have a positive effect on the vitamin D status of elderly subjects, especially in those who are not overweight or obese. More research is needed on the different results between normal-weight and overweight/ obese subjects. Vitamin D supplements may be advantageous to achieve desirable 25(OH)D3 levels and thus should be recommended for subjects who are at high risk for vitamin D deficiency. In addition, high dietary phosphate intakes are associated with higher iPTH levels, especially in normalweight subjects, whereas high Ca/P ratios may have beneficial effects on bone health in elderly subjects by decreasing iPTH independent of BMI. Therefore, public health interventions should devote more attention to improving the Ca/P ratio, particularly by decreasing dietary phosphate intake.
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[25] Deutsche Gesellschaft für Ernährung, Österreichische Gesellschaft für Ernährung, Schweizerische Gesellschaft für Ernährungsforschung, Schweizerische Vereinigung für Ernährung. Referenzwerte für die Nährstoffzufuhr. Frankfurt/Main, Germany: Umschau Braus; 2000. [26] Van der Wielen RP, Löwik MRH, van den Berg H, de Groot LC, Haller J, Moreiras O, et al. Serum vitamin D concentrations among elderly people in Europe. Lancet 1995;346:207–10. [27] Lips P. Vitamin D deficiency and secondary hyperparathyroidism in the elderly: consequences for bone loss and fractures and therapeutic implications. Endocr Rev 2001;22: 477–501. [28] Macdonald HM, Mavroeidi A, Fraser WD, Darling AL, Black AJ, Aucott L, et al. Sunlight and dietary contributions to the seasonal vitamin D status of cohorts of healthy postmenopausal women living at northerly latitudes: a major cause for concern? Osteoporos Int 2011;22:2461–72. [29] Ashwell M, Stone EM, Stolte H, Cashman KD, Macdonald H, Lanham-New S, et al. UK Food Standards Agency Workshop Report: an investigation of the relative contributions of diet and sunlight to vitamin D status. Br J Nutr 2010;104:603–11. [30] Kemi VE, Kärkkäinen MU, Rita HJ, Laaksonen MM, Outila TA, Lamberg-Allardt CJ. Low calcium:phosphorus ratio in habitual diets affects serum parathyroid hormone concentration and calcium metabolism in healthy women with adequate calcium intake. Br J Nutr 2010;103:561–8. [31] Karp HJ, Vaihia KP, Kärkkäinen MUM, Niemistö MJ, Lamberg-Allardt CJE. Acute effects of different phosphorus sources on calcium and bone metabolism in young women: a whole foods approach. Calcif Tissue Int 2007;80:251–8. [32] Kemi VE, Rita HJ, Kärkkäinen MU, Viljakainen HT, Laaksonen MM, Outila TA, et al. Habitual high phosphorus intakes and foods with phosphate additives negatively affect serum parathyroid hormone concentration: a cross-sectional study on healthy premenopausal women. Public Health Nutr 2009;12:1885–92. [33] Guillemant J, Oberlin F, Bourgeois P, Guillemant S. Agerelated effect of a single oral dose of calcium on parathyroid function: relationship with vitamin D status. Am J Clin Nutr 1994;60:403–7. [34] Schwarzpaul S, Strassburg A, Lührmann PM, NeuhäuserBerthold M. Intake of vitamin and mineral supplements in an elderly German population. Ann Nutr Metab 2006;50: 155–62. [35] Lee P, Greenfield JR, Seibel MJ, Eisman JA, Center JR. Adequacy of vitamin D replacement in severe deficiency is dependent on body mass index. Am J Med 2009;122:1056–60. [36] Forsythe LK, Livingstone MB, Barnes MS, Horigan G, McSorley EM, Bonham MP, et al. Effect of adiposity on vitamin D status and the 25-hydroxycholecalciferol response to supplementation in healthy young and older Irish adults. Br J Nutr 2012;107:126–34.
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Dietary vitamin D intake is not associated with 25-hydroxyvitamin D3 or parathyroid hormone in elderly subjects, whereas the calcium-to-phosphate ratio affects parathyroid hormone☆ Alexandra Jungert, Monika Neuhäuser-Berthold⁎ Institute of Nutritional Science, Justus-Liebig-University, Giessen, Germany
ARTI CLE I NFO
A BS TRACT
Article history:
This cross-sectional study investigates whether serum 25-hydroxyvitamin D3 [25(OH)D3]
Received 30 October 2012
and intact parathyroid hormone (iPTH) are affected by vitamin D, calcium, or phosphate
Revised 11 May 2013
intake in 140 independently living elderly subjects from Germany (99 women and 41 men;
Accepted 23 May 2013
age, 66-96 years). We hypothesized that habitual dietary intakes of vitamin D, calcium, and
Keywords:
these associations. Serum 25(OH)D3 and iPTH were measured by an electrochemi-
25-Hydroxyvitamin D3
luminescence immunoassay. Dietary intake was determined using a 3-day estimated
Parathyroid hormone
dietary record. The median dietary intake levels of vitamin D, calcium, and phosphate were
phosphate are not associated with 25(OH)D3 or iPTH and that body mass index confounds
Vitamin D intake
3 μg/d, 999 mg/d, and 1250 mg/d, respectively. Multiple regression analyses confirmed that
Calcium intake
dietary vitamin D and calcium did not affect 25(OH)D3 or iPTH; however, supplemental
Phosphate intake
intakes of vitamin D and calcium were associated with 25(OH)D3 after adjustment for
Elderly German subjects
age, sex, body composition, sun exposure, physical activity, and smoking. In addition, phosphate intake and the calcium-to-phosphate ratio were associated with iPTH after multiple adjustments. In a subgroup analysis, calcium and vitamin D supplements, as well as phosphate intake, were associated with 25(OH)D3 and/or iPTH in normal-weight subjects only. Our results indicate that habitual dietary vitamin D and calcium intakes have no independent effects on 25(OH)D3 or iPTH in elderly subjects without vitamin D deficiency, whereas phosphate intake and the calcium-to-phosphate ratio affect iPTH. However, vitamin D and calcium supplements may increase 25(OH)D3 and decrease iPTH, even during the summer, but the impact of supplements may depend on body mass index. © 2013 Published by Elsevier Inc.
Abbreviations: 25(OH)D, 25-hydroxyvitamin D; 25(OH)D3, 25-hydroxyvitamin D3; BMI, body mass index; Ca/P ratio, calcium-tophosphate ratio; GISELA, longitudinal study on nutrition and health status in senior citizens of Giessen; iPTH, intact parathyroid hormone; log, logarithmically transformed; PAL, physical activity level; rS, Spearman correlation coefficient; TBF, total body fat. ☆ The authors do not have any competing interests to declare. This investigation received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. ⁎ Corresponding author. Institute of Nutritional Science, Justus-Liebig-University, Goethestrasse 55, 35390 Giessen, Germany. Tel.: +49 (0) 641 99 39067; fax: +49 (0)641 99 39069. E-mail address: [email protected] (M. Neuhäuser-Berthold). 0271-5317/$ – see front matter © 2013 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.nutres.2013.05.011
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1.
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Introduction
In addition to the role of vitamin D in osteoporosis, there is evidence that vitamin D is also involved in the pathogenesis of other widespread diseases such as hypertension and certain types of cancer [1,2]. Elderly people have an increased risk of developing vitamin D deficiency because of age-related declines in cutaneously produced vitamin D, sun exposure, and dietary intake [3]. In Germany, the dietary vitamin D intake of elderly subjects is usually well below the reference value [4,5]. Thus, sun exposure or the use of supplements may be required to achieve adequate vitamin D status. In addition to vitamin D intake, the amounts of calcium and phosphate in the diet may interfere with vitamin D metabolism. Elevated intact parathyroid hormone (iPTH) levels, which can result from inadequate calcium intake or high phosphate intake, induce the formation of 1,25-dihydroxyvitamin D3, which may promote the turnover of circulating 25-hydroxyvitamin D3 [25(OH)D3]. The existing evidence is not sufficient to determine whether dietary calcium and phosphate and widespread low vitamin D intake affect 25(OH)D3 and/or iPTH in free-living elderly individuals and whether the use of supplements impacts the vitamin D and/or iPTH status of these individuals. Previous studies have provided inconsistent results [6-16]; phosphate intake and the dietary calcium-to-phosphate ratio (Ca/P ratio) were not frequently taken into consideration as influencing factors. To date, several studies focused on nutrient intake without considering biochemical parameters [4,17] such as 25(OH)D3 or iPTH levels, whereas others reported nonfasting measurements [8] and/or did not simultaneously examine intakes of vitamin D, calcium, and phosphate [6-16]. In addition, earlier reports often exclusively focused on 25(OH)D3 or iPTH without considering the interaction between both [6,16]. Overall, studies with elderly people who are in general good health and do not have vitamin D deficiency and severely elevated iPTH values are scarce. Currently, there is a great debate on higher recommended dietary intakes of vitamin D for elderly people. At present, it is unclear to what extent all elderly subjects, independent of their health, vitamin D status, or body mass index (BMI), would benefit from such a strategy. Other dietary factors, for example, calcium and phosphate, may have a stronger impact on 25(OH)D3 or iPTH. We hypothesized that (1) in free-living elderly subjects, habitual dietary intakes of vitamin D, calcium, and phosphate show no independent associations with 25(OH)D3 or iPTH and (2) BMI confounds these associations. The objectives of the present study were to (1) evaluate vitamin D, calcium, and phosphate intakes; (2) investigate whether intakes of vitamin D, calcium, and phosphate or the Ca/P ratio contribute to 25(OH)D3 and/or iPTH, independent of potential confounders such as age, sex, body composition, sun exposure, physical activity, and smoking behavior, in an aging population; (3) examine the impact of a mutual adjustment for 25(OH)D3 and iPTH in this context, respectively; and (4) assess the influence of BMI on the associations between 25(OH)D3 and iPTH and vitamin D, calcium, and phosphate intakes by comparing normal-weight with overweight/obese individuals. The present study provides a comprehensive examination of the
relationships between 25(OH)D3 and iPTH concentrations and vitamin D, calcium, and phosphate intakes in free-living elderly subjects, a population that was rarely the focus of previous investigations.
2.
Methods and materials
2.1.
Study population
The present investigation reports cross-sectional data from the longitudinal study on nutrition and health status in senior citizens of Giessen, Germany (50°35´North), (GISELA study) obtained in 2008. The GISELA study is an ongoing prospective cohort study initiated in 1994. To be enrolled in the study, participants had to be at least 60 years of age and physically mobile. Investigations took place in the Institute of Nutritional Science in Giessen from July to October. Written informed consent was obtained from each participant. The study protocol was approved by the ethical committee of the Faculty of Medicine at the Justus-Liebig-University, Giessen. Subjects with incomplete data were excluded, as were subjects who took diuretics or had edema or chronic kidney disease. Five subjects were identified as outliers based on their 25(OH)D3 and iPTH measurements and/or the residuals of the regression analyses and thus not included in the analyses. After applying these exclusion criteria to the 275 individuals who took part in the follow-up in 2008, the present study assessed data from 99 women and 41 men who were all whites.
2.2.
Biochemical analyses
Fasting blood samples were collected, and serum aliquots were stored at −70°C until further analysis. Both 25(OH)D3 and iPTH were measured by an electrochemiluminescence immunoassay (ECLIA; Roche Diagnostics, Mannheim, Germany) [18,19]. Concentrations of 25(OH)D3 less than 25 nmol/L were defined as vitamin D deficiency [3], and 2 cutoff values were applied to define adequate 25(OH)D3 levels: ≥50 and ≥75 nmol/L [1].
2.3.
Anthropometric data and body composition
Body mass index was determined as previously described [20,21]. Subjects were separated into 2 groups; subjects with a BMI less than 25 kg/m2 were classified as normal-weight, and subjects with a BMI of 25 kg/m2 or greater were defined as overweight/obese. The percentage total body fat (%TBF) was recorded by a single-frequency (50 kHz) bioelectrical impedance analyzer (Akern-RJL BIA 101/S; Data Input, Frankfurt, Germany), according to the manufacturer's instructions and the predictive formula from Roubenoff et al [22].
2.4.
Lifestyle factors
Nutritional intake was determined using a 3-day estimated dietary record, which was developed and validated for the GISELA study [23]. This dietary record consisted of 146 food items subdivided into 16 food groups. For every food item,
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both typical household measures (for example, slice, cup, and spoon) and the appropriate weights were given. The participants were instructed to record their entire food intake on 3 consecutive days directly after consumption, starting on a Sunday. The energy, vitamin D, calcium, and phosphate contents of the subjects' diets were calculated according to the German Food Code and Nutrient Data Base [24] version II.3 via the nutrient calculation program DGE-PC professional (German Nutrition Society, Frankfurt, Germany). The nutritional intake assessment did not include nutrient intake by supplements. Time spent outdoors as an indicator for sun exposure, physical activity patterns, smoking behavior, and data regarding age, diseases, medications, and supplement intake were collected via self-administered questionnaires. Participants who reported using vitamin D or calcium supplements sometimes or regularly were classified as “supplement users”. Like supplement use, smoking behavior was coded as a dummy variable (never-smokers vs current and exsmokers). The physical activity level (PAL) of each participant was assessed as previously described [20].
2.5.
Statistical analyses
The characteristics of the study participants are expressed as the median and the 5th to 95th percentiles because of nonnormally distributed data, which was tested by the Shapiro-Wilk test, the Kolmogorov-Smirnov test, and by visual inspection of histograms. Descriptive characteristics were compared between groups via the Mann-Whitney U test for continuous variables. The χ2 test or Fisher exact test was used for categorical variables. Spearman correlation coefficients (rS) were applied to evaluate correlations between 25(OH)D3, iPTH, age, BMI, and %TBF and relevant nutritional parameters. We examined the associations of vitamin D, calcium, and phosphate intakes with 25(OH)D3 and iPTH by multiple regression analyses and created 4 models with different levels of adjustment. Model 1 represented the age-
and sex-adjusted associations, whereas model 2 additionally adjusted for PAL, sun exposure, and smoking. Model 3 further considered the %TBF. In model 4, 25(OH)D3 or iPTH was included as a confounding variable. Serum iPTH was logarithmically transformed (log) to obtain normally distributed residuals. Because there were no major changes in the results when 25(OH)D3 was log transformed, we report the nontransformed data. Finally, we divided the study population into 2 groups based on BMI to examine whether normal-weight subjects differed from overweight/obese subjects regarding the associations between vitamin D, calcium, and phosphate intakes and 25(OH)D3 and log iPTH by multiple regression analyses. Statistical analyses were conducted with SPSS 20.0 for Windows (SPSS Inc, Chicago, IL, USA). The significance level was set at P < .05. All tests were 2-tailed.
3.
Results
The characteristics of the subjects are presented in Table 1. Both sexes failed to meet the recommended daily intake of 20 μg vitamin D [5]; only 1 woman had an intake greater than 20 μg/d. A vitamin D intake less than 10 μg/d and less than 5 μg/d was observed in 93.9 % and 62.6 % of the women and 90.2 % and 56.1 % of the men, respectively. The median calcium intake for both sexes was close to the recommendation of 1000 mg/d [25], and only 19.2 % of the women and 14.6 % of the men had a calcium intake less than 800 mg/d. In contrast, except for 2 women, phosphate intakes of all subjects were above the reference value of 700 mg/d [25]. Consequently, the corresponding Ca/P ratios were clearly below the recommendation of 1.4, and only 2 women had ratios above this recommendation. None of the subjects had 25(OH)D3 levels less than 25 nmol/ L; however, 25(OH)D3 levels less than 50 nmol/L were noted in 22.1 % of the subjects, and 80.0 % of the study population had 25(OH)D3 levels less than 75 nmol/L. Using both cutoff values
Table 1 – Descriptive characteristics of the study population (n = 140) a Women (n = 99) Age (y) BMI (kg/m2) TBF (%) 25(OH)D3 (nmol/L) iPTH (pmol/L) Vitamin D intake (μg/d) Vitamin D intake (μg/MJ) Calcium intake (mg/d) Calcium intake (mg/MJ) Phosphate intake (mg/d) Phosphate intake (mg/MJ) Ca/P ratio Time spent outdoors (min/d) PAL Current or ex-smokers Vitamin D supplement users Calcium supplement users a b
75.0 26.4 42.2 60.6 4.5 2.5 0.3 1000 128 1198 166 0.8 120 1.7 23 16 37
(68.0-86.0) (21.2-33.8) (32.5-50.1) (39.7-93.0) (2.4-8.6) (0.3-10.1) (0.1-1.6) (526-1782) (84-239) (777-2103) (126-215) (0.5-1.2) (40-360) (1.5-2.0) (23.2%) (16.2%) (37.4%)
Men (n = 41)
Pb
76.0 (70.0-84.8) 26.4 (22.9-32.3) 29.0 (21.5-37.9) 67.5 (39.7-88.9) 4.1 (2.1-8.3) 3.5 (1.0-11.3) 0.4 (0.1-1.2) 994 (602-1635) 114 (76-179) 1434 (1053-2385) 163 (124-198) 0.7 (0.5-1.0) 150 (23-396) 1.7 (1.4-1.9) 29 (70.7%) 2 (4.9%) 5 (12.2%)
.395 .591 <.0001 .156 .290 .125 .383 .911 .017 .003 .610 .003 .062 .517 <.0001 .096 .004
Median and 5th to 95th percentile for continuous variables, absolute and relative frequencies for categorical variables. Mann-Whitney U test for continuous variables and χ2 test or alternatively Fisher exact test for categorical variables.
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to define the optimal vitamin D status, the dietary intakes of vitamin D, calcium, and phosphate in subjects with adequate and insufficient vitamin D status were almost equal (all P > .05); however, the analysis showed borderline significance for a higher vitamin D intake in subjects with 25(OH)D3 levels of 75 nmol/L or greater compared with subjects with levels less than 75 nmol/L, independent of whether μg/d (median, 2.5 vs 5.2 μg/d; P = .078) or μg/MJ (median, 0.3 vs 0.6 μg/MJ; P = .077) was used. Spearman correlation analyses of dietary parameters with 25(OH)D3 showed no significant associations (all P > .05). However, the association between 25(OH)D3 and vitamin D supplement use was close to statistical significance (rS = 0.157; P = .064). Log iPTH was inversely correlated with the Ca/P ratio (rS = −0.221; P = .009) and indicated a trend toward an association with calcium supplement use (rS = −0.164; P = .053). Age was positively linked to the daily intakes of vitamin D (rS = 0.244; P = .004), calcium (rS = 0.168; P = .047), and phosphate (rS = 0.181; P = .033). Except for vitamin D (rS = 0.208; P = .014), there were no such associations when nutrients were assessed in relation to the energy content of the diet (all P > .05). Furthermore, age was positively associated with the percentage of subjects who used calcium supplements (rS = 0.257; P = .002). Body mass index was not significantly correlated with intakes of vitamin D, calcium, or phosphate (all P > .05), whereas the %TBF was negatively correlated with phosphate intake in mg/d (rS = −0.264; P = .002) and showed trends toward associations with the Ca/P ratio (rS = 0.157; P = .063) and vitamin D intake in µg/d (rS = −0.152; P = .073). We found an inverse association between 25(OH)D3 and log iPTH, which persisted after controlling for age, sex, body composition, sun exposure, PAL, smoking, use of calcium and vitamin D supplements, and dietary intakes of vitamin D, calcium, and phosphate within the multiple regression analysis (β = −0.224; P = .015). Therefore, mutual adjustments were performed in the subsequent multiple regression analyses. In the multiple regression analyses, no significant associations between dietary vitamin D or calcium intakes and 25(OH)D3 or log iPTH were found after adjustment for age, sex, sun exposure, PAL, smoking, and %TBF (all P > .05). In contrast, dietary phosphate intake in mg/d was positively associated
with log iPTH after the above-mentioned adjustments (β = 0.193; P = .034), but this association disappeared after including 25(OH)D3 in the regression model (β = 0.162; P = .068). The Ca/P ratio was inversely associated with log iPTH before (β = −0.317; P < .001) and after (β = −0.282; P = .001) additional controlling for 25(OH)D3 and showed a trend toward a positive association with 25(OH)D3 after adjustment for age, sex, lifestyle factors, and %TBF (β = 0.144; P = .087). Vitamin D and calcium supplements were associated with 25(OH)D3 after multiple adjustments (Table 2). Although the association between calcium supplements and 25(OH)D3 vanished after log iPTH was added to the model, the association between vitamin D supplements and 25(OH)D3 remained significant. After dividing the cohort into 2 groups based on BMI, dietary intakes of vitamin D, calcium, and phosphate, the Ca/P ratio and the percentage of supplement users did not differ between the 2 BMI groups (all P > .05). However, the vitamin D status (median, 66.4 vs 58.7 nmol/L; P = .006) but not the iPTH status (median, 4.4 vs 4.5 pmol/L; P = .540) of normal-weight subjects was higher than that in overweight/obese subjects. In normal-weight subjects, we observed an inverse association between calcium supplementation and log iPTH after controlling for sex, age, and sun exposure (Table 3), which persisted after further adjustment for 25(OH)D3 (β = −0.305; P = .048). In addition, calcium supplementation was only associated with 25(OH)D3 in normal-weight subjects, but there was no association after further controlling for log iPTH (β = 0.227; P = .116). After adjustment for sex, age, and sun exposure, the use of vitamin D supplements was linked to 25(OH)D3 and log iPTH in normal-weight subjects, but both associations vanished after mutual adjustment for log iPTH (β = 0.198; P = .143) and 25(OH)D3 (β = −0.206; P = .165), respectively. In contrast, dietary vitamin D and calcium intakes were not associated with 25(OH)D3 or log iPTH in either BMI category, but phosphate intake in mg/d was associated with log iPTH in normal-weight subjects, even after adjustment for 25(OH)D3 (β = 0.353; P = .018). Likewise, the associations between the Ca/P ratio and log iPTH persisted after additional controlling for 25(OH)D3 in both normal-weight (β = −0.309; P = .041) and overweight/obese (β = −0.268; P = .012) subjects. These results
Table 2 – Impact of vitamin D and calcium supplements on 25(OH)D3 and log-transformed iPTH in 140 elderly women and men of the GISELA study analyzed by multiple linear regression analyses Vitamin D supplement Model 1 Model 2 Model 3 Model 4 Adjusted R2
25(OH)D3 (nmol/L) B
β
P
B
β
P
7.941 8.907 8.213 7.042
0.181 0.203 0.187 0.160
.035 .013 .021 .043
−0.048 −0.049 −0.046 −0.026
−0.122 −0.124 −0.116 −0.065
.158 .157 .187 .455
0.195 for model 4
Log iPTH (pmol/L)
Calcium supplement Model 1 Model 2 Model 3 Model 4 Adjusted R2
0.042
25(OH)D3 (nmol/L)
Log iPTH (pmol/L)
B
β
P
B
β
P
5.146 5.971 5.978 4.704
0.160 0.186 0.186 0.146
.079 .035 .033 .088
−0.049 −0.051 −0.051 −0.037
−0.169 −0.176 −0.176 −0.127
.064 .062 .061 .172
0.188
0.052
for model 4
Multiple regression analyses with 25(OH)D3 and log iPTH as dependent variables. The results of the regression analyses are expressed in terms of B (the nonstandardized coefficient), β (the standardized coefficient), and the adjusted coefficient of determination (R2) for model 4. Model 1: association adjusted for age (in years) and sex (female/male). Model 2: model 1 further adjusted for PAL, sun exposure (in min/d), and smoking (yes/no). Model 3: model 2 additionally adjusted for TBF (%). Model 4: model 3 additionally adjusted for log iPTH (in pmol/L) and 25(OH)D3 (in nmol/L), respectively.
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Table 3 – Impact of nutritional parameters on 25(OH)D3 and log-transformed iPTH in 140 elderly women and men of the GISELA study in relation to BMI category analyzed by multiple linear regression analyses BMI <25 kg/m2 (n = 51) 25(OH)D3 (nmol/L) 25(OH)D3 (nmol/L) Vitamin D intake (μg/d) Vitamin D intake (μg/MJ) Vitamin D supplement (no/yes) Calcium intake (mg/d) Calcium intake (mg/MJ) Calcium supplement (no/yes) Phosphate intake (mg/d) Phosphate intake (mg/MJ) Ca/P ratio
– −0.016 −0.052 0.280 ⁎ 0.019 0.051 0.327 ⁎ −0.080 −0.201 0.147
Log iPTH (pmol/L) −0.395 ⁎ 0.203 0.101 −0.299 ⁎ 0.046 −0.237 −0.400 ⁎⁎ 0.382 ⁎ 0.136 −0.360 ⁎
BMI ≥25 kg/m2 (n = 89) 25(OH)D3 (nmol/L) – 0.018 −0.015 0.190 −0.019 0.043 0.114 −0.110 −0.094 0.148
Log iPTH (pmol/L) −0.253 ⁎ −0.098 −0.087 −0.071 −0.074 −0.127 −0.081 0.091 0.135 −0.299 ⁎⁎
Multiple regression analyses with 25(OH)D3 and log iPTH as dependent variables. The results of the regression analyses are expressed in β (the standardized coefficient) and adjusted for sex (female/male), age (in years), and sun exposure (in min/d). ⁎ P < .05. ⁎⁎ P < .01.
did not change substantially when we replaced sun exposure with PAL or smoking (data not shown).
4.
Discussion
The results of our study confirm that in noninstitutionalized elderly subjects, habitual dietary intakes of vitamin D, calcium, and phosphate exhibit no independent associations with 25(OH)D3 or iPTH, although the Ca/P ratio was negatively related to iPTH, even after multiple adjustments. Therefore, our first hypothesis must be partially rejected as well as the second hypothesis that BMI functions as a confounder in this context. The second hypothesis was only confirmed for the association between phosphate intake and iPTH and for the associations of vitamin D and calcium supplements with 25(OH)D3 and iPTH, which were significant in normal-weight subjects but not in overweight/obese subjects. A high percentage of our subjects had a vitamin D intake far below the reference intake, which is in agreement with other national [4,6,17] and international [9,12,14] studies. Subjects with such a low vitamin D intake are regarded as having a high risk for vitamin D deficiency [16,17] and, as a result, secondary hyperparathyroidism [27]. However, none of our subjects had a serious vitamin D deficiency, which is contrary to other studies of elderly individuals [6,26]. GISELA subjects with 25(OH)D3 levels of 75 nmol/L or greater tended to have higher dietary vitamin D intakes than did subjects with lower levels, indicating that dietary vitamin D might contribute to a higher vitamin D status. However, only 20 % of the population had 25(OH)D3 levels of 75 nmol/L or greater, and in multiple regression analyses, dietary vitamin D amounts were unrelated to 25(OH)D3 and iPTH. This is in agreement with some [10,12,14] but not all [6-9,16] previous studies. One reason for these missing associations may be the low vitamin D intake levels, but there is evidence that dietary vitamin D intake has a minor impact on vitamin D status in cases where subjects have sufficient sun exposure [28,29]. Considering the amount of self-reported outdoor activities of
the GISELA subjects, dietary vitamin D intake may become a negligible factor. This assumption is in agreement with Bates et al [11], who reported an association between vitamin D intake and 25-hydroxyvitamin D [25(OH)D] in every season except for summer. In addition to vitamin D, calcium intake may impact 25(OH) D3 or iPTH. In cases of chronic low calcium intake, higher 25(OH)D levels may be required to maintain iPTH in the reference range [13]. A high level of calcium intake, in turn, has been linked to reduced 25(OH)D turnover caused by the suppression of iPTH secretion [27]. However, we did not find a direct association between dietary calcium intake and either 25(OH)D3 or iPTH. The relationship between calcium and iPTH might be more evident in individuals with high iPTH levels, inadequate calcium intake, and/or vitamin D deficiency. In a cross-sectional study, iPTH was only associated with calcium intake in the presence of 25(OH)D levels less than 25 nmol/L, which suggests that calcium intake becomes an important determinant in vitamin D deficiency [13]. In the present study, only a few subjects had iPTH values above the reference range, dietary calcium intake was close to adequate, and 25(OH)D3 levels were primarily greater than 50 nmol/L. Along with calcium, phosphate is essential for maintaining bone health, but an excess of phosphate, especially in combination with low intakes of calcium and/or vitamin D, can lead to secondary hyperparathyroidism [30]. In our study, dietary phosphate intake was far above the reference value, which is in accordance with previous studies [30-32]. Because of the high phosphate intake, the Ca/P ratio failed to reach the suggested ratio of 1.4. In the regression analyses, phosphate intake and the Ca/P ratio were associated with iPTH, especially in normal-weight subjects. Why these associations were more pronounced in normal-weight subjects is up for discussion, but the differences appear not to be caused by varying intake levels. It is possible that the normal-weight and overweight/ obese subjects consume different phosphate sources, and organic and inorganic phosphate may induce divergent effects on iPTH [32]. However, because the Ca/P ratio in both subgroups was inversely associated with iPTH, this indicates
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that a high calcium intake may counteract phosphate intake's impact on iPTH. Therefore, consumption of foods with a high Ca/P ratio, such as dairy products, in conjunction with a restriction of foods containing high amounts of phosphate, such as meat products, convenience foods, and soft drinks, should be encouraged to reduce iPTH levels [32]. In our study, vitamin D and calcium supplements were associated with 25(OH)D3 after adjustment for sex, age, sun exposure, PAL, smoking, and %TBF. Several studies have confirmed significant effects of calcium/vitamin D supplements on 25(OH)D levels in elderly subjects [1,14,16], but controlling for iPTH was not performed. Our results indicate that the effect of calcium supplements on 25(OH)D3 may be caused by alterations in circulating iPTH, whereas the effect of vitamin D supplements on 25(OH)D3 appears to be independent of iPTH. However, in our study, the proportion of supplement users was small, and we did not evaluate the dosage or duration of vitamin D supplementation. Therefore, these results must be interpreted with caution. Although we found no independent associations between vitamin D and calcium supplements and iPTH levels, other studies have reported inverse associations [13,27,33]. In a previous followup of the GISELA study, median supplemental vitamin D intake was 10 μg/d in women and 5 μg/d in men, whereas median supplemental calcium intake was 500 mg/d in women and 187 mg/d in men [34]. These amounts may have been too low to affect iPTH, especially when iPTH is mainly in the reference range. Furthermore, the effect of calcium supplements on iPTH may be less pronounced in elderly subjects because of a progressive age-related decrease in calcium absorption [33]. Our results indicate that the impact of vitamin D and calcium supplements on 25(OH)D3 and iPTH depends on BMI, which is in accordance with a study conducted in patients with severe vitamin D deficiency [35]. Furthermore, in a recent randomized, placebo-controlled trial, an inverse association between BMI and the 25(OH)D response to 15 μg cholecalciferol was noted in 48 older Irish adults, but the iPTH response to 15 μg cholecalciferol was not affected by body size or adiposity [36]. In our study, the use of supplements was predominantly associated with 25(OH)D3 and iPTH in normal-weight subjects. Thus, individuals with higher %TBF may require a higher vitamin D intake than normal-weight subjects to maintain or achieve adequate 25(OH)D3 concentrations because of an increased sequestration of vitamin D in the TBF [10,21,35]. The relatively strong association between 25(OH)D3 and log iPTH in our normal-weight subjects might explain why most associations weakened or disappeared after mutual adjustment. The association between 25(OH)D3 and log iPTH tended to be stronger in subjects with a BMI less than 25 kg/m2 than in subjects with a BMI of 25 kg/m2 or greater. Apparently, a higher quantity of TBF weakens this association. Future studies should consider the interaction between 25(OH)D3 and iPTH when studying the impact of nutrients on 25(OH)D3 and iPTH status. There are some limitations of our study. First, the crosssectional design allows no inference on the causality of the observed relationships. Second, the observed relationships should be interpreted with caution given the relatively small sample size and the low number of supplement users. Third, the lifestyle variables measured were based on self-reported data and indirect assessments of sun exposure and physical
activity. Fourth, we did not collect information on the period of supplementation or supplement composition and dosage. All participants (except for 1 man) who took vitamin D supplements also used calcium supplements. Thus, a combined effect of both nutrients cannot be ruled out. In conclusion, a notably high percentage of our subjects did not reach half of the recommended intake level for vitamin D, whereas calcium intake from food matched the recommendation and phosphate intake markedly exceeded the dietary guidelines. Neither dietary vitamin D nor calcium intakes affect 25(OH)D3 or iPTH during the summer, but vitamin D and calcium supplements appear to have a positive effect on the vitamin D status of elderly subjects, especially in those who are not overweight or obese. More research is needed on the different results between normal-weight and overweight/ obese subjects. Vitamin D supplements may be advantageous to achieve desirable 25(OH)D3 levels and thus should be recommended for subjects who are at high risk for vitamin D deficiency. In addition, high dietary phosphate intakes are associated with higher iPTH levels, especially in normalweight subjects, whereas high Ca/P ratios may have beneficial effects on bone health in elderly subjects by decreasing iPTH independent of BMI. Therefore, public health interventions should devote more attention to improving the Ca/P ratio, particularly by decreasing dietary phosphate intake.
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