Vitamin D status and parathyroid hormone concentrations in Chinese women and men from north-east of the People's Republic of China

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1 (2000) 54, 68±72 ß 2000 Macmillan Publishers Ltd. All rights reserved 0954±3007/00 $ Vitamin D status and parathyroid hormone concentrations in Chinese women and men from north-east of the People's Republic of China L Yan 1 *, A Prentice 1, H Zhang 2, X Wang 2, DM Stirling 1 and MM Golden 3 1 Medical Research Council Human Nutrition Research, Cambridge, UK; 2 Department of Preventive Medicine, Shenyang Medical College, Shenyang, People's Republic of China; and 3 Department of Medicine and Therapeutics, University of Aberdeen, Foresterhill, Aberdeen, UK Study objective: To evaluate vitamin D status of young and old women and men living in Shenyang, north-east People's Republic of China in early spring and to explore the relationship between vitamin D metabolites and parathyroid hormone (PTH) in this population. Design: Cross-sectional study. Setting: Shenyang, north-east China. Subjects: 194 healthy volunteers: 48 young women and 48 young men aged 25 ± 35 y, and 48 old women and 50 old men aged 65 ± 75 y. Results: Fasting blood samples were used to measure plasma 25-hydroxyvitamin D (25(OH)D), 1,25- dihydroxyvitamin D (1,25(OH) 2 D) and PTH using radioimmunoassays. The proportion of subjects who could be regarded as vitamin D de cient ( < 25 nmol=l) was 48%, 29%, 15% and 13% for old men, young men, old women and young women, respectively. There was no association between plasma 1,25(OH) 2 D and 25(OH)D concentrations. PTH concentrations were elevated in both old women and men compared with young subjects (P < 0.01). A negative association between PTH and 25(OH)D was only found in old women (P < 0.001), but not in old men, nor in young subjects. Conclusions: Vitamin D status was poor in this population in early spring, especially in men. There was no clear evidence to show that the secretion of PTH and the conversion of 1,25(OH) 2 D were affected by the low 25(OH)D concentration. Sponsorship: Partly supported by the Sandoz Foundation for Gerontological Research and the Nestle Foundation, Switzerland, and Medical Research Council, UK. Descriptors: 25-hydroxyvitamin D; 1,25-dihydroxyvitamin D; parathyroid hormone (2000) 54, 68±72 Introduction Vitamin D is a major regulator of calcium metabolism and, therefore, an important determinant of bone health through its effects on calcium absorption and the secretion of parathyroid hormone (PTH) (Chapuy & Meunier, 1997; Par tt et al, 1982). In countries where food is not forti ed with vitamin D, vitamin D status depends mainly on cutaneous synthesis on exposure to sunlight. This is in uenced by many environmental factors (Haddad, 1992; Par tt et al, 1982). The simplest and most widely used biochemical marker of vitamin D status is the level of circulating 25-hydroxyvitamin D (25(OH)D), the most abundant vitamin D metabolite. Limits of 25(OH)D of 25 and 30 nmol=l have been suggested as indicators of vitamin D de ciency in adult populations (Lips et al, 1988; Par tt et al, 1982). Others have suggested that the threshold of the plasma concentration that separates vitamin D suf ciency from insuf ciency should be determined by the secretion of PTH and the synthesis of 1,25-dihydroxyvitamin D (1,25(OH) 2 D) from 25(OH)D (Bouillon et al, 1987; Chapuy et al, 1997). Low concentrations of 25(OH)D have been reported in adults who live in the north-eastern United States and Europe (Chapuy & Meunier, 1997; Scharla, 1998; Webb et al, 1990). Inverse associations between 25(OH)D and PTH concentrations have been reported not only in elderly women and men (Chapuy et al, 1997; Dawson-Hughes et al, 1997), but also in male adolescents (Guillemant et al, 1995). However, this association has not been found in all studies (Chapuy et al, 1987). Vitamin D status in the Chinese population has been reported from Hong Kong and Taiwan (Lau et al, 1989; Tsai et al, 1996), but not from the north of China where the diet, lifestyle and climate are different. The objective of this study was to evaluate vitamin D status of young and old women and men living in Shenyang in early spring and to examine the relationships between vitamin D metabolites and PTH in this population accustomed to a low calcium intake. *Correspondence: L Yan, Medical Research Council Human Nutrition Research, Downham's Lane, Milton Road, Cambridge CB4 1XJ, UK. Liya.Yan@mcr-hnr.cam.ac.uk. Received 9 May 1999; revised 2 August 1999; accepted 6 August Subjects and methods Subjects Shenyang, the capital of Liaoning Province, is located at a latitude of ± N and altitude 47.8 m in the

2 north-east of the People's Republic of China. The urban population of Shenyang was 3,195,046 in There are four distinct seasons in Shenyang, mild in the spring and autumn, a hot summer and a long and cold winter from late November to early March (715 to 730 C). There are typically about 15 h of daylight in summer and 8 h in winter. During winter, people stay indoors most of the time. The economy of Shenyang is based on heavy industry and there are a number of large factories, each employing 10,000 ± 30,000 people. Therefore, a large proportion of the city population are factory workers. A hundred and ninety-four healthy volunteers, 48 young women and 48 young men aged 25 ± 35 y, and 48 old women and 50 old men, aged 65 ± 75 y, were recruited to take part in this study. The young subjects were factory workers. The old subjects were retired factory workers from the same representative factory area in the city. Recruitment was made through factory workshops and residences. No subject had any disorder know to alter calcium or bone metabolism, such as cancer, parathyroidism and renal disease or diabetes. No subject had taken any vitamin D supplement for at least 2 months before or during the study. None of the old women were or had ever taken hormone replacement therapy. The investigation was part of a larger project of factors that may in uence bone mineral status in Shenyang. It was approved by the Joint Ethical Committee of the University of Aberdeen and Grampian Health Board and the Academic Board of Shenyang Medical College. All participants provided informed written consent. Sample collection and analytical procedure A fasting blood sample was collected from each subject between 8 and 10 a.m. The collection of blood samples from female subjects was conducted in April to May and that of male subjects was in March to May. Approximately 4 ml blood, from the antecubital vein, was collected into an monovette containing potassium ethylenediamine tetraacetate as anticoagulant from each subject. The blood was kept cold throughout the collection. Samples were spun in a refrigerated centrifuge, stored at 7 70 C and transported on dry ice to MRC Human Nutrition Research (formerly Dunn Nutritional Laboratory), Cambridge, UK for analysis. Calcium intakes of subjects were assessed by 5 day weighed food records using the Chinese food composition table (Institute of Nutrition and Food Hygiene, 1992; Yan, 1999; Yu, 1989). Plasma 25(OH)D was measured by radioimmunoassay (Incstar, Stillwater, MN, USA; Abbotts et al, 1995). The intra- and interassay coef cients of variation (CVs) were 10% and 9%, respectively. The accuracy and precision of 25(OH)D assay was also monitored using Lyphocheck (Bio-Rad, Anaheim, CA) and other in-house reference material. The laboratory is a member of Vitamin D External Quality Assessment Scheme (London, UK). Plasma 1,25-(OH) 2 D was determined using Gamma-B 1,25-dihydroxy vitamin D kit (IDS, Boldon, Tyne and Wear, UK). This kit is a complete assay system for the puri cation of 1,25(OH) 2 D in plasma by immunoextraction using a highly speci c solid-phase monoclonal anti-1,25(oh) 2 D, followed by quantitation by 125 I radioimmunoassay. The intra- and interassay CVs were 5% and 4%, respectively. For nancial reasons, 1,25(OH) 2 D was measured in a subset of 20 subjects randomly selected from the rst 30 subjects in each group. Plasma Intact PTH 1 ± 84 was measured by an immunoradiometric assay (Incstart, Stillwater, MN, USA). The intra- and interassay CVs were 7% and 6%, respectively. Statistical analysis was performed by ANOVA, analysis of covariance and regression analysis using Linear Model software in Data Desk 5.0 (Data Descriptions, Ithaca, NY, 1995). Scheffe post hoc tests following ANOVA were used to examine the signi cance of differences between groups. Where necessary, data were transformed to natural logarithms to correct for skewed distributions. The w 2 test was used to examine the proportional difference in prevalence of 25(OH)D de ciency between sexes. P < 0.05 was used as the level of signi cance for all tests. Results The plasma 25(OH)D concentrations of the four study groups are presented in Table 1, together with general information and other biochemical measurements. In general, 25(OH)D concentrations were low in each of the four groups, according to the currently suggested de nition of vitamin D status (Mckenna & Freaney, 1998). The mean 25(OH)D concentration (mean s.d.) was nmol=l for all subjects together. Men had a lower mean value compared with women, especially in the older age group (Table 1). After adjusting for month of blood sampling, the difference between young women and men disappeared, but that between old men and men remained (P ˆ 0.001). There were no age differences in 25(OH)D concentration in either sex. A level of plasma 25(OH)D of 25nmol=l has conventionally been used as a cut-off for de ning the lower limit of adequacy of vitamin D status (Department of Health, 69 Table 1 Characteristics and plasma vitamin D metabolites and PTH of subjects a Women Men Young Old Young Old (n ˆ 48) (n ˆ 48) (n ˆ 48) (n ˆ 50) Age (y) * Weight (kg) ** **,*** Height (cm) *** ** **,*** Ca intake (mg) * ** 25(OH)D (nmol=l) * ** 1,25(OH) 2 D (pmol=l) (n ˆ 20) (n ˆ 20) (n ˆ 20) (n ˆ 20) PTH (pg=ml) *** *** a Values are the mean s.d. Signi cant difference examined by Scheffe post hoc tests in ANOVA: *P < 0.05; **P < 0.01 signi cantly different from female counterparts; ***P < 0.01 signi cantly different between young and old in same sex.

3 70 Table 2 Proportion (%) of subjects with vitamin D de ciency at different levels Women Men Young Old Young Old Total (n ˆ 48) (n ˆ 48) (n ˆ 48) (n ˆ 50) (n ˆ 194) 25(OH)D < 30 nmol=l (12 ng=ml) 25(OH)D < 25 nmol=l (10 ng=ml) ; Par tt et al, 1982), although others have suggested the slightly higher level of 30 nmol=l (Lips et al, 1988). The prevalence of low 25(OH)D status using the two cut-offs is given in Table 2. Using 25 nmol=l as the lower limit, the proportion of subjects who could be regarded as vitamin D de cient was substantial, especially in old men in whom the proportion almost reached 50%. In addition, 28% old men had a 25(OH)D concentration below 20 nmol=l (8 ng=ml). The prevalence of 25(OH)D concentration below 25 nmol=l was higher in men than women, both among the young (w 2 ˆ 4.0, P ˆ 0.04) and old (w 2 ˆ 12.7, P ˆ ). Mean 1,25(OH) 2 D concentrations in the four groups of subjects are presented in Table 1. The concentration was higher in women than in men, but the difference was not statistically signi cant. 1,25(OH) 2 D concentration was not related to either age or the month of blood sampling. There was a positive relationship between 1,25(OH) 2 D and 25(OH)D in female subjects taken together using multiple regression analysis. This relationship was not found in men nor in the total population (with adjustment for sex), nor when each of the four groups was analysed separately. Mean PTH concentrations were higher in both old women and old men compared with their young counterparts (Table 1). The levels were 42.4% higher in older women and 26.9% in older men. There was a weak trend for PTH to be negatively related to 25(OH)D when the four subject groups were anlaysed together by regression analysis (P ˆ 0.053). When each group was examined separately, there was a negative relationship between 25(OH)D and PTH in old women (P ˆ ), but not in old men or either of the younger groups (Figures 1 and 2). In addition, Figure 1 25(OH)D and PTH in old subjects Figure 2 25(OH)D and PTH in young subjects a weak negative relationship was also found between the two variables in old subjects together (P ˆ 0.045), but not in young subjects (P ˆ 0.98). Therefore, there was no indication that the high prevalence of low 25(OH)D among men was associated with elevated PTH. Discussion A striking feature of the data was the high proportion of individuals with low 25(OH)D. The proportion with a poor vitamin D status was particularly high among men. The low 25(OH)D concentration in this population was probably due to the lack of both substantial dietary sources of vitamin D and sunlight exposure during a long, cold and dark winter. Dairy produce and fatty sh, which are the main natural sources, were rarely consumed by these subjects (Yan, 1999). The body stores of vitamin D are largely dependent on the cutaneous synthesis of vitamin D 3. However, in Shenyang during the winter, people tend to go outdoors infrequently and wear heavy clothing, thereby decreasing the surface area exposed to sunlight. In addition, the zenith angle of the sunlight increases in the winter, and the amount of solar ultraviolet radiation that reaches the earth's surface is low (Holick, 1995). Furthermore, the polluted atmosphere in Shenyang, caused by heavy industry and domestic res for heating in winter, would also increase the sunlight ltering. Each of these factors may contribute to the low plasma 25(OH)D concentrations in the Shenyang population. In contrast with previous studies (Heaney, 1993; Tsai & Wahner, 1987), no age difference in 25(OH)D concentration was found in the present study. This may be because the upper age limit of subjects was only 75 y and the older subjects were ambulant and could take part in outdoor activities. In addition, the effect of the long winter on suppressing vitamin D synthesis could affect both young and old people in Shenyang and result in very low 25(OH)D concentrations in both young and old people, so that no differences could be detected with age. However, a difference in 25(OH)D status was detected between men and women, with a high proportion of men with low 25(OH)D. The reason for this is unclear but might be related to different lifestyles of women and men. In Shenyang, women tend to be involved in more outdoor activities, such as shopping in the open market, looking after children and talking with neighbours in the yard,

4 whilst men tend to prefer more indoor activities, for example, playing Mah-jong and cards. Although 25(OH)D concentrations were low, 1,25(OH) 2 D concentrations of this population were not low compared with other populations, such as the British in Cambridge (Prentice et al, 1998). Low intakes of calcium or phosphate have been reported to increase the production of 1,25(OH) 2 D in some studies (Bouillon et al, 1995; Fraser, 1980; Kumar, 1986). Therefore, the results in Shenyang may be due to an adaptation of 1,25(OH) 2 D production to low calcium intakes. 1,25(OH) 2 D concentrations were not related to age in this study. This is consistent with studies conducted by Ebeling et al (1994) and Chan et al (1992), which reported that the increase in endogeneous production of 1,25(OH) 2 D induced by a low calcium diet was age independent. PTH concentrations were higher in both old, compared to young, women and men in the present study. This is consistent with studies in both Caucasian and Asian populations (Chan et al, 1992; Chapuy et al, 1997). The mild increase in PTH secretion with ageing may represent a compensatory response to the age-related decline in the level of renal 1a-hydroxylase or its sensitivity to normal physiological stimuli (Sherman et al, 1990). Chronic calcium and vitamin D insuf ciency could also contribute to the elevation of PTH (Heaney, 1993). In spite of a marked depletion of vitamin D stores after winter, PTH concentrations of most subjects remained within the normal range. Particularly in older men, who had a mean 25(OH)D concentration near the low limit for normal adults, but only three of 50 subjects had PTH concentration above the upper limit of the adult reference range (55 pg=ml) (Chapuy et al, 1997). PTH was related to 25(OH)D in old women but not in young subjects, nor in old men who had a higher prevalence of low 25(OH)D. Studies of the relationship between vitamin D status and PTH have given con icting results. Some show that plasma 25(OH)D and PTH concentrations in old female and male subjects were inversely related (Chapuy et al, 1997; Dawson-Hughes et al, 1997), but others did not (Chapuy et al, 1987). Ooms et al (1995) have found that only 25(OH)D values below a threshold of 25 nmol=l (10 ng=ml) were related to PTH, whereas there was no relationship above this value in elderly women. Similarly, Collins et al (1998) failed to nd a correlation between serum 25(OH)D and PTH in postmenopausal women when all subjects were analysed, but did nd a signi cant negative correlation when only subjects with serum 25(OH)D below 30 nmol=l were considered. A signi cant negative correlation was found between serum PTH and 25(OH)D values in a large French population of women and men aged 35 ± 65 y (Chapuy et al, 1997). They found serum PTH held a stable plateau concentration at 36 pg=ml as long as serum 25(OH)D values were higher than 78 nmol=l (31 ng=ml), but increased when the serum 25(OH)D value fell below this threshold. When the 25(OH)D concentration became equal to or lower than 11.3 mmol=l (4.6 ng=ml), the PTH values reached the upper limit of normal values found in vitamin D replete subjects. However, in the present study, an inverse relationship between 25(OH)D and PTH was only found in old women, but not in other groups, including old men who had very low mean 25(OH)D level. In general, there was no evidence that subjects who had the lowest 25(OH)D values had the highest PTH values (Figures 1 and 2). The reason for the sex difference in the relationship between 25(OH)D and PTH in this population is not clear. However, the calcium intake of old women was signi cantly lower than old men. Since oral calcium suppresses PTH secretion (KaÈrkkaÈinen et al, 1997), the low calcium intake in old women could contribute to the increased secretion of PTH depending on their vitamin D status. Alternatively, it is possible that oestrogen could also affect the regulating system of vitamin D and PTH in women (Riggs & Melton, 1983). Guillemant et al (1995) reported that PTH in 28 male French adolescents was signi cantly higher in March ( ng=l) than in September ( ng=l). A statistically signi cant correlation between PTH and 25(OH)D concentrations was obtained but PTH values remained within the normal range all the time. In the present study, plasma 25(OH)D concentrations in young subjects were low, especially in men, but PTH concentrations were all within the normal range. There was no association between plasma PTH and 25(OH)D values (Figure 2). These results suggest that, in this population with a high proportion of low vitamin D status de ned by the current cut-off for UK population (Department of Health, 1998), a low plasma 25(OH)D does not always lead to an increase in plasma PTH, especially in young people or old men. This calls into question the usefulness of a single cut-off for indicating the adequacy of vitamin D status across all population groups. In addition, gender or calcium intake may have an effect on the relationship of 25(OH)D status and PTH secretion in elderly. In conclusion, in this northern Chinese population in early spring, vitamin D status was poor, re ected by the high proportion of plasma 25(OH)D concentrations below recognized cut-offs, especially in men. However, there was no clear evidence which shows that the secretion of PTH and the conversion of 1,25(OH)D are affected by the low 25(OH)D concentration, especially in old men and young subjects. 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