Key words
bone mineral density - osteoporosis - vitamin D deficiency - vitamin D receptor polymorphism
- lactase polymorphism
Introduction
Vitamin D status is highly different in various countries of Europe, the Middle East
and Asia and a well known problem especially during the winter time [1]
[2]. A north-south gradient was observed for 25-OH vitamin D3 with higher levels in Scandinavia and lower levels in Italy, Spain, and some Eastern
European countries. This observation points to other determinants than sunshine, e. g.,
nutrition, food fortification and supplement use. Very low levels of 25-OH vitamin
D3 have been reported in the Middle East, e. g., Turkey, Lebanon, Jordan and Iran [1]
[3]. Therefore, in Europe immigrants from these countries inherit a high risk of vitamin
D deficiency and subsequent severe clinical musculoskeletal problems [4]
[5]. Severe vitamin D deficiency causes rickets (defect of mineralization affecting
the growing skeleton) or osteomalacia (abnormal mineralization of the mature skeleton)
and bone loss (osteopenia/osteoporosis). Less severe vitamin D deficiency causes an
increase of serum parathyroid hormone (PTH) leading to bone resorption, osteoporosis
and fractures (secondary hyperparathyroidism) [6]
[7]
[8]. The major cause of osteomalacia is vitamin D deficiency, which is most often due
to reduced cutaneous production of vitamin D in housebound elderly people, immigrants
to Northern countries and women who obey strict dress codes which prohibit exposure
of uncovered skin [4]
[9]
[10].
The etiology of osteoporosis is multi-factorial and caused by both environmental and
genetic factors. As a possible genetic component of osteoporosis, an association between
bone mineral density (BMD), calcium absorption and polymorphisms of the vitamin D
receptor gene (VDR) has been reported [11]
[12]. Lactase deficiency is a common autosomal recessive condition resulting in decreased
intestinal lactase degradation. A 13910 T/C dimorphism (LCT) near the lactase phlorizin
hydrolase gene, reported to be strongly associated with adult lactase non-persistence,
may also have an impact on calcium supply, and subsequently on bone metabolism and
bone mineral density [3]
[13]
[14]
[15].
In a recent study, we found a high prevalence of vitamin D deficiency, secondary hyperparathyroidism
and generalized bone pain in Turkish immigrants living in Germany [4]. However, precise data about the described parameters that influence bone metabolism
and bone mineral density are scarce in this population. To elucidate potential causative
factors for alteration of bone metabolism we performed a cross-sectional study that
investigated Turkish immigrants living in Germany, who were compared to a group of
healthy Germans. To our knowledge, this is also the first report, in which the prevalence
of vitamin D deficiency, secondary hyperparathyroidism, loss of bone mineral density,
markers of bone metabolism and genetically defined lactose maldigestion, and polymorphism
of the vitamin D receptor gene in Middle East immigrants to a central European country
was examined.
Patients and Methods
Study population
To exclude short-term effects on bone metabolism (e. g., by migration stress), the
study population comprised 183 Turkish immigrants living in Germany for more than
5 years (98 males, 85 females, mean age 40.96 years) and 46 age and sex matched healthy
Germans. All women were premenopausal with a physiological sexual hormone status.
In addition, participants with inflammatory bowel disease, malabsorption, liver disease,
hypophosphatemia, tubular dysfunction and anticonvulsive therapy were excluded from
the study. To minimize short-term effects of nutrition and sunlight exposure, the
he study period was limited to March and April 2008.
Laboratory parameters
Phlebotomy was performed in the morning after an overnight fasting period. Blood samples
for plasma separation were collected in EDTA-containing tubes and centrifuged immediately
(<1/2 h after phlebotomy). Serum tubes were centrifuged after clotting and total serum
calcium concentration (sCa2 + , reference range 2.15–2.55 mM/l), phosphate (P, reference range 0.87–1.45 mM/l),
alkaline phosphatase (AP, reference range 35–129 U/L), 25-OH vitamin D3 (25-OH D3, reference range 30–60 ng/ml; Incstar Corporation, Stillwater, MN, USA), parathyroid
hormone (PTH, reference range 8.3–68 pg/ml; PTH intact ELISA, DRG Instruments GmbH,
Marburg, Germany), osteocalcin (OC, reference range males: 12–51.1 ng/ml, females
6.5–42.3 ng/ml; LIAISON Osteocalcin, DiaSorin Inc, Stillwater, MN, USA), β-crosslaps
(CL, reference range males: 30–50 years <0.584 ng/ml, 50–70 years <0.704 ng/ml; premenopausal
females: <0.573 ng/ml; β-crosslaps/serum, Roche Diagnostics, Indianapolis, IN, USA),
and tartrate-resistant acid phosphatase isoform 5b (TRAP5b, reference range males:
4.0±1.4 U/l, females 2.9±1.4 U/l; Metra TRAP5b EIA Kit, Quidel Corporation, San Diego,
CA, USA), were batch analyzed in serum samples.
Genotyping
Genomic DNA was extracted from samples of peripheral venous blood. Single nucleotide
polymorphism (SNP) genotyping was performed using a microplate fluorometer (Fluoroskan
Ascent, Thermo Fisher Scientific, Waltham, MA, USA). Identification of C/T exchanges
was carried out by an allele-specific polymerase chain reaction (PCR) (Amplifluor
SNPs HT Genotyping System FAM-SR; Serologicals Corporation, Norcross, GA, USA) as
described previously [16]. The polymorphic site was amplified by using the sequence 5′-GTTCCTTTGAGGCCAGGGA-3′
as specific primer for LCT-T and the sequence 5′-TTCCTTTGAGGCCAGGGG-3′ as specific
primer for LCT-C. PCR and VDR FokI- and BsmI-genotyping were performed as described
previously [17]
[18]. Alleles were genotyped for the presence (f, b) or the absence (F, B) of the FokI
or BsmI restriction sites, respectively.
Osteodensitometry
BMD (g/cm2) was measured at the lumbar spine (first to fourth vertebrae, antero-posterior view)
and the total right femur by dual-energy X-ray absorptiometry (DXA; Prodigy Lunar,
Milwaukee, Wisconsin, USA). The coefficient of variation of repeated measurements
in vivo was 0.9% for the lumbar spine and 1.6% for the femur. T-score (number of standard
deviations (SD) from the normal mean obtained from young healthy adults) and Z-score
(number of SD from the mean of age- and sex-matched controls) were also calculated.
A low BMD was defined according to the world health organization (WHO) guidelines:
Osteopenia as a T-score between − 1 and − 2.5 SD and osteoporosis as a T score < − 2.5
SD. The respective reference values have been published previously for women and men.
For the purpose of this study, the reference values of the manufacturer were used.
Ethical approval
The study was approved by the local Ethics Committee of the Faculty of Medicine, Justus-Liebig-University,
Giessen, Germany, and all study participants gave written informed consent prior to
inclusion into the study.
Statistical analysis
All data are presented as mean±SD. Data were tested for normality by the Kolmogorov-Smirnov
test. For comparison of the groups, numeric values (e. g., age) were analyzed by Student’s
t-test. Data that showed unequal variance or abnormal distribution were analyzed by
the Mann-Whitney rank sum test. Proportions of the groups were compared by Chi-square
analysis and Fisher’s exact test. A p value of <0.05 was regarded as significant for
all analyses. Data were evaluated using SPSS Release 12.0.1 (SPSS Inc., Chicago, IL,
USA).
Results
Vitamin D metabolism and other laboratory parameters
Basic demographic and osteologic data of the patient groups are shown in [Table 1]. Both groups did not differ in age, lumbar and femoral BMD (including T- and Z-scores)
as well as serum levels of AP and CL. Significantly higher mean values were found
in the Turkish group for body mass index (BMI), and PTH levels. In contrast, the serum
levels of calcium, phosphate, 25-OH D3, OC and TRAP5b were significantly reduced.
Table 1 Demographic data and results of bone-specific characteristics of Turkish immigrants
and German controls. Values shown are mean±standard deviation; reference ranges are
given in brackets. Level of significance (Mann-Whitney-U-test) for difference between
the 2 populations: p<0.05.
|
Parameter
|
Turkish immigrants (n=183)
|
German controls (n=46)
|
p-value
|
|
age (years)
|
40.96±10.39
|
39.67±13.18
|
0.339
|
|
BMI (kg/m2)
|
28.16±4.53
|
25.29±4.54
|
<0.0005
|
|
BMD L1–4 (g/cm2)
|
1.16±0.17
|
1.17±0.14
|
0.581
|
|
T-score L1–4
|
− 0.34±1.45
|
− 0.15±1.14
|
0.363
|
|
Z-score L1–4
|
− 0.37±1.38
|
− 0.27±1.16
|
0.486
|
|
BMD right femur (g/cm2)
|
1.04±0.12
|
1.03±0.14
|
0.626
|
|
T-score right femur
|
− 0.04±0.99
|
0.05±1.07
|
0.837
|
|
Z-score right femur
|
0.07±0.89
|
0.01±0.96
|
0.728
|
|
25-OH D3 (30–60 ng/ml)
|
8.85±9.90
|
21.86±24.25
|
<0.0005
|
|
PTH (8.3n – 68 pg/ml)
|
109.44±42.54
|
58.48±12.75
|
<0.005
|
|
AP (35–129 U/L)
|
64.81±19.78
|
59.74±16.65
|
0.098
|
|
Phosphate (0.87–1.45 mmol/l)
|
1.10±0.18
|
1.20±0.1
|
<0.0005
|
|
sCa2+ (2.15–2.55 mmol/l)
|
2.33±0.14
|
2.39±0.14
|
0.005
|
|
CL (males, 30–50 years: <0.548 ng/ml; males, 50–70 years: <0.704 ng/ml; females: <0.573 ng/ml)
|
0.35±0.18
|
0.41±0.21
|
0.071
|
|
OC (males: 12–51.1 ng/ml; females: 6.5–42.3 ng/ml)
|
14.25±4.86
|
16.91±7.89
|
0.034
|
|
TRAP5b (males: 4.0±1.4 U/l; females: 2.9±1.4 U/l)
|
2.13±1.07
|
2.75±1.15
|
0.001
|
Specifically, the vast majority of Turkish immigrants presented with a vitamin D deficiency.
A severe vitamin D deficiency (25-OH D3 <10 ng/ml) was found in 83.1%, vs. 0% of the Germans, a moderate vitamin D deficiency (25-OH D3 10–19.9 ng/ml) in 9.8% vs. 0% of the Germans, and a mild vitamin D insufficiency (25-OH D3 20–29.9 ng/ml) in 4.9% vs. 45.7% of the Germans ([Table 2]). By contrast, sufficient levels of vitamin D were only seen in 2.2% of Turkish
immigrants vs. 54.3% of the Germans ([Table 2]). No gender-specific differences were observed for all parameters. Furthermore,
Turkish immigrants had decreased levels of osteocalcin in 11.5%, elevated levels of
PTH in 82%, of TRAP5b in 7.7%, of CL in 12.6%, and of AP in 6.6% ([Table 2]). An additional regression analysis of the parameters did not show a significant
dependence.
Table 2 Total number (percent) of Turkish immigrants exhibiting pathological changes of biochemical
markers of bone metabolism in comparison with German controls. An elevated value for
TRAP5b is defined as exceeding the shown mean±standard deviation.
|
Parameters
|
Turkish immigrants (n=183)
|
German controls (n=46)
|
|
25-OH D3
|
|
|
|
vitamin D sufficiency (≥30 ng/ml)
|
4 (2.2%)
|
25 (54.3%)
|
|
hypovitaminosis:
|
|
mild vitamin D insufficiency (20–29.9 ng/ml)
|
9 (4.9%)
|
21 (45.7%)
|
|
moderate vitamin D deficiency (10–19.9 ng/ml)
|
18 (9.8%)
|
0
|
|
severe vitamin D deficiency (<10 ng/ml)
|
152 (83.1%)
|
0
|
|
reduced OC (males: <12 ng/ml; females: 6.5 ng/ml)
|
21 (11.5%)
|
3 (6.5%)
|
|
elevated TRAP5b (males: >4.0±1.4 U/l; females: >2.9±1.4 U/l)
|
14 (7.7%)
|
4 (8.7%)
|
|
elevated CL (males, 30–50 years >0.548 ng/ml; males, 50–70 years: >0.704 ng/ml; females:
>0.573 ng/ml)
|
23 (12.6%)
|
8 (17.4%)
|
|
elevated PTH (>68 pg/ml)
|
150 (82%)
|
37 (80.4%)
|
|
elevated AP (>129 U/L)
|
12 (6.6%)
|
0
|
Of note, although vitamin D deficiency was highly prevalent in the Turkish group,
the typical laboratory constellation of secondary hyperparathyroidism (increased PTH,
decreased sCa2+ ) was observed only in 12 cases with a predominance of females (10 females, 2 males).
This prevalence of secondary hyperparathyroidism (6.6%) did not differ from the German
group (6.5%). Furthermore, none of these Turkish individuals had osteoporosis, and
only one of them presented with osteopenia. Phosphate levels were decreased in 7 cases,
but only one of them had a true vitamin D hypovitaminosis.
BMD and bone metabolism parameters
An alteration of BMD was seen in Turkish immigrants in 40.2%. Osteoporosis was observed
in 15 cases (8.2%), osteopenia in 59 cases (32.2%; [Table 3a]), whereas the German controls showed no osteoporosis and osteopenia was observed
in 17 cases (37%; [Table 3b]). There was no gender-specific preference in either group. Turkish immigrants with
osteoporosis were significantly older than immigrants with osteopenia (50.5±11.4 years
vs. 39.8±8.7 years, p<0.002). The incidence of severe vitamin D deficiency (25-OH D3 <10 mg/ml) was identical in the groups with osteoporosis and osteopenia, respectively
([Table 3a]), whereas the Germans with osteopenia did not exhibit severe vitamin D deficiency
([Table 3b]). Osteoporosis in Turkish immigrants was more frequently associated with reduced
OC and elevated AP levels than osteopenia ([Table 3a]).
Table 3 Comparison of the prevalence of pathological changes in biochemical markers of bone
metabolism between osteopenia and osteoporosis in Turkish immigrants (A) and German controls (B). An elevated value for TRAP5b is defined as exceeding the shown mean±standard deviation.
|
A
|
|
Parameter
|
No. (%) of immigrants with
|
|
Osteopenia (n=59/183)
|
Osteoporosis (n=15/183)
|
|
severe 25-OH D3 deficiency (<10 mg/ml)
|
47 (79.7%)
|
13 (86.7%)
|
|
reduced OC (males: <12 ng/ml; females: <6.5 ng/ml)
|
3 (5.1%)
|
4 (26.7%)
|
|
elevated TRAP5b (males: >4.0±1.4 U/l; females: >2.9±1.4 U/l)
|
10 (17.0%)
|
4 (26.7%)
|
|
elevated CL (males, 30–50 years: >0.548 ng/ml; males, 50–70 years: >0.704 ng/ml; females:
>0.573 ng/ml)
|
10 (17.0%)
|
2 (13.3%)
|
|
elevated AP (>129 U/L)
|
2 (3.4%)
|
8 (53.3%)
|
|
elevated PTH (>68 pg/ml)
|
50 (84.7%)
|
10 (66.7%)
|
|
B
|
|
Parameter
|
No. (%) of controls with
|
|
Osteopenia (n=17/46)
|
Osteoporosis (n=0/46)
|
|
severe 25-OH D3 deficiency (<10 mg/ml)
|
0
|
0
|
|
reduced OC (males: <12 ng/ml; females: <6.5 ng/ml)
|
2 (11.8%)
|
0
|
|
elevated TRAP5b (males: >4.0±1.4 U/l; females: >2.9±1.4 U/l)
|
4 (23.5%)
|
0
|
|
elevated CL (males, 30–50 years: >0.548 ng/ml; males, 50–70 years: >0.704 ng/ml; females:
>0.573 ng/ml)
|
4 (23.5%)
|
0
|
|
elevated AP (>129 U/L)
|
0
|
0
|
|
elevated PTH (>68 pg/ml)
|
13 (76.5%)
|
0
|
Genetic pattern
Turkish immigrants showed a significant predominance (54.1%, p=0.009; [Table 4]) of the FokI genotype FF. Apart from that, there were no significant differences
in the distribution of the BsmI- and Fok I polymorphisms of the VDR-gene between Turkish
immigrants and Germans.
Table 4 Distribution of VDR-polymorphisms in Turkish immigrants and German controls.
|
VDR BsmI and FokI genotypes
|
Turkish immigrants (n=183)
|
German controls (n=46)
|
|
BB
|
49 (26.8%)
|
10 (21.7%)
|
|
Bb
|
62 (33.9%)
|
12 (26.1%)
|
|
bb
|
72 (39.3%)
|
24 (52.2%)
|
|
FF
|
99 (54.1%)
|
16 (34.8%)
|
|
Ff
|
69 (37.7%)
|
22 (47.8%)
|
|
ff
|
15 (8.2%)
|
8 (17.4%)
|
Besides, Ff-genotyped Turkish females exhibited significantly decreased BMD and T-Scores
of the lumbar spine (p<0.046) and the right total femur compared with FF-genotyped
females ([Table 5]). Due to the small group size of ff-genotyped females, the value of a statistical
analysis is limited. However, since the mean BMD and T-score values in the ff group
were higher than those in the FF group, significance can be inferred. Of note, significant
associations with any laboratory parameters could not be demonstrated, and male Turkish
immigrants did not show any correlation between BMD and FokI-genotypes.
Table 5 Relation between BMD and VDR FokI genotypes in pre-menopausal Turkish women. Values
shown are mean±standard deviation; levels of significance (Fishers-Exact-Test; p<0.05)
are provided only for the comparison of the FF and Ff genotypes due to the small group
size of the ff genotype.
|
FokI genotype
|
p-value
|
|
FF (n=53)
|
Ff (n=38)
|
ff (n=7)
|
|
|
lumbar BMD
|
1.19±0.17
|
1.11±0.16
|
1.23±0.14
|
0.046
|
|
lumbar T-score
|
0.03±1.43
|
− 0.65±1.36
|
0.34±1.09
|
0.045
|
|
right total femur BMD
|
1.05±0.13
|
0.98±0.13
|
1.06±0.11
|
0.033
|
|
right total femur T-score
|
0.31±0.99
|
− 0.19±1.04
|
0.47±0.89
|
0.045
|
A genetically determined lactose malabsorption (adult-type hypolactasia; genotype
LCT CC) was detected in 154 Turkish immigrants (84.2%) and in 14 Germans (30.4%) without
gender-specific preference in both groups ([Table 6]). Among the immigrants, this genotype was the most frequent one, whereas the TT-genotype
was the least frequent one. In contrast, among the Germans, all 3 genotypes were distributed
equally. An alteration of bone mineral density was seen in 64 (41.6%) of the immigrants
with an LCT CC-genotype: osteopenia in 51 cases (33.2%) and osteoporosis in 13 cases
(8.4%), also without gender-specific preference. In contrast, 4 of 14 Germans (28.6%)
with an LCT CC-genotype presented an osteopenia, none an osteoporosis. Immigrants
with genetically determined lactose malabsorption (LCT CC-genotype) had a significantly
higher BMI and significantly decreased levels of TRAP-5b compared with the LCT genotype
CT (BMI: p<0.018, TRAP-5b: p<0.047) and the German controls (BMI: p<0.005, TRAP-5b:
p<0.05), without gender-specific preference. There was no significant association
of the LCT genotype with age, 25-OH D3, PTH, AP, phosphate, calcium, crosslaps, osteocalcin, BMD or BsmI-/FokI-polymorphisms
in adult-type hypolactasia.
Table 6 Distribution of LCT-genotypes in Turkish immigrants and German controls and association
with BMI and TRAP-5b levels. Values shown for body mass index (BMI) and tartrate-resistant
acid phosphatase isoform 5b (TRAP5b) are mean±standard deviation; * p<0.047, ** p<0.018,
vs. genotype CT; # p<0.05, ## p<0.005 vs. German controls. N.D.: not done; LCT: 13910 T/C dimorphism near the lactase phlorizin
hydrolase gene.
|
LCT genotypes
|
Turkish immigrants (n=183)
|
German controls (n=46)
|
|
n (%)
|
BMI (kg/m2)
|
TRAP5b (U/l)
|
n (%)
|
BMI (kg/m2)
|
TRAP5b (U/l)
|
|
TT
|
2 (1.1%)
|
N.D.
|
N.D.
|
13 (28.3%)
|
25.46±4.37
|
2.77±1.09
|
|
CT
|
27 (14.8%)
|
26.9±5.8
|
2.8±0.9
|
19 (41.3%)
|
26.15±5.46
|
2.77±1.35
|
|
CC (adult type hypolactasia)
|
154 (84.2%)
|
28.4±4.3 ** ##
|
2.1±1.1 *#
|
14 (30.4%)
|
23.96±3.06
|
2.72±0.98
|
Discussion and Conclusions
Discussion and Conclusions
Vitamin D and its metabolites are well-established regulators of bone mineral homeostasis.
Their dominant role is the prevention and treatment of rickets and osteomalacia, bone
diseases characterized by inadequate bone formation, and mineralization. In vitamin
D deficiency, the major pathological finding observed in bone biopsy is failure of
bone to mineralize. Unless not based on a group of genetic syndromes, phosphate wasting
or deficiency, or aluminium intoxication, all of which can cause vitamin D resistance,
rickets and osteomalacia can be readily prevented and/or cured by vitamin D replacement.
Thus the most common cause of rickets and osteomalacia is vitamin D deficiency, which
is an extremely frequent problem in the Middle East, e. g., Turkey, Lebanon, Jordan
and Iran [1]
[2].
The histologic appearance of adult osteomalacia was first described in 1885 but clinical
appreciation of the disease took place much later. A large population of adult osteomalacia
patients in China provided the basis for a detailed study in the 1930s. Maxwell estimated
that there may have been 100 000 cases at that time [19]. In 1948, Albright and Reifenstein reported the presence of osteomalacia in the
United States due to lack of vitamin D supplementation [20]. The first report of overt osteomalacia in Turkish immigrants and their offspring
in Germany was described in 1978 [21] but there is also evidence for vitamin D deficiency, secondary hyperparathyroidism
and the association with reduced BMD in individuals with Pakistani and Norwegian background
living in Oslo, Norway [8]. In addition, Chapuy et al. [22] were the first to propose that a 25-OH D3 level of less than 80 nmol/l is associated with higher PTH values. In osteomalacia,
the main clinical features are bone pain, fractures and muscle weakness.
In a recent study, which to our knowledge is the first detailed analysis and characterization
of the prevalence of vitamin D deficiency, secondary hyperparathyroidism and loss
of BMD in Turkish immigrants living in Germany, we found a high prevalence of vitamin
D deficiency, secondary hyperparathyroidism and generalized bone pain, particularly
in veiled women. Based on these data, we concluded that there is an independent ethnic
risk factor for low 25-OH D3 levels in Turkish nationals compared to Germans. In fact, darker skin pigmentation
and cultural differences in clothing in Turkish immigrants, resulting in a reduced
ability to synthesize vitamin D in their skin during sunlight exposure, were identified
as risk factors [4].
However, vitamin D metabolism may be influenced by other factors such as age, seasonal
sunlight exposure (lower levels between October and April) [23] and also specific genetic alterations. As expected, we found significantly decreased
levels of 25-OH D3 in a large proportion of Turkish immigrants (97.8%; severe 25-OH D3 deficiency in 83.1%), although it needs to be considered that the observation period
was March/April, i. e., at the end of the winter season with low sunlight exposure.
In contrast, 47.3% of the German controls had abnormal 25-OH D3 levels, but none of them with moderate or severe deficieny. None of the study participants
had malabsorption, liver disease, hypophosphatemia, renal tubular dysfunction or received
anticonvulsant therapy, which could have induced a vitamin D deficiency.
In vitamin D deficiency, but not necessarily in other causes of osteomalacia, secondary
hyperparathyroidism is frequently present. It is well known that even less severe
vitamin D deficiency can cause an increase of serum PTH leading to bone resorption,
osteoporosis and fractures. Accordingly, in the present study we found elevated PTH
levels in 82% of the Turkish immigrants and 80.4% of the German controls. However,
the mean PTH levels in the Turkish immigrants were significantly higher than in the
German controls, which is in keeping with the higher prevalence of severe 25-OH D3 deficiency among the Turks.
Of note, the typical laboratory constellation of a manifest secondary hyperparathyroidism
(elevated PTH and decreased calcium levels) was only seen in a small proportion of
either group (6.6% of the Turkish immigrants, 6.5% of the Germans) with a predominance
of women, and there was no association with a reduction in BMD. Therefore, the majority
of Turkish immigrants appear to suffer from hypovitaminosis D and incomplete secondary
hyperparathyroidism without hypocalcemia, at least for an extended period of time.
Most likely, they are able to maintain calcium homeostasis due to a mobilization of
calcium from the bone, by gastrointestinal resorption and due to normal kidney function.
Interestingly, there was no difference in the extent of 25-OH D3 reduction between Turkish immigrants with and without reduced BMD, although 40.4%
of the cases were found to have either osteopenia (32.2%) or osteoporosis (8.2%).
Bone remodeling is characterized by 2 opposing mechanisms, the formation of new bone
by osteoblasts and the resorption of aged bone by osteoclasts. Active bone metabolism
can be assessed by measuring the enzymatic activity of the osteoblasts and osteoclasts
(AP, TRAP5b, crosslaps, osteocalcin).
The AP was found to be elevated in only 6.6% of the Turkish immigrants, without a
significant correlation to BMD. Furthermore, Turkish immigrants had increased TRAP5b
levels (26.7% of the cases with osteoporosis and 17% of the cases with osteopenia).
Since TRAP5b is released into the serum by bone-resorbing osteoclasts and is an indicator
of osteoclast activity, it illustrates overall bone resorption [24]. The crosslaps as biochemical markers of bone resorption were also elevated in Turkish
immigrants (in 13.3% of the cases with osteoporosis and in 17% of the cases with osteopenia),
whereas osteocalcin as a bone formation marker was decreased in 31.9% (in 26.7% of
the cases with osteoporosis and in 5.2% of the cases with osteopenia) of the patients.
Therefore, the biochemical markers revealed an alteration of the general bone metabolism
indicating decreased bone formation and increased bone resorption. Of note, degenerative
changes as a possible cause of falsely high BMD values were excluded in immigrants
who had moderate to severe vitamin D deficiency and vitamin D insufficiency, respectively,
and normal BMD on DXA measurement. In these cases, the osteoprotective estrogen-status
in combination with the higher BMI could be the basis for this observation.
Besides environmental factors, there is substantial evidence that genetic factors
play also an important role in the multifactorial aetiology of osteoporosis [25]. Several candidate genes have been discussed as being involved in the pathogenesis
of osteoporosis. Besides a number of different loci within the vitamin D receptor
gene (VDR), such as BsmI polymorphisms at the 3′ end region of the VDR [18], FokI polymorphisms in exon 2, the translation initiation codon at the 5′ end of
the VDR gene, were defined as functional allelic variants [17] and have been shown to be significantly associated with BMD and calcium absorption
in several reports [11]
[12]
[17].
In the present study, there was no difference in the genetic distribution of the BsmI-polymorphism
of the VDR gene neither in Turkish immigrants nor in Germans. However, female Turkish
immigrants showed a significant disposition in FokI-polymorphism and Ff-genotyped
female Turkish immigrants showed a significantly decreased BMD of the right femur
and lumbar spine. There were no associations with any other laboratory parameters.
Calcium has been shown to be an important nutrient in the prevention and treatment
of osteoporosis. It is naturally supplied in the form of dairy products that contain
lactose. Lactose intolerance has also been shown to reduce dairy intake in affected
individuals, due to adult-type hypolactasia and consequent malabsorption. Adult-type
hypolactasia, as mediated by a wide-spread genetic predisposition, not only reduces
calcium intake but also calcium absorption in the presence of high amounts of lactose
and may, therefore, promote osteoporosis [3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]. The genetic background of lactose malabsorption (LM) has been described in 2002
by Finnish researchers [26] when a distinct single nucleotide polymorphism (SNP) on chromosome 2q21 was identified
(LCT-polymorphism). Individuals homozygous for the C genotype (LCT CC) have almost
undetectable levels of intestinal lactase production compared to LCT TC or LCT TT
individuals, and present with variable symptoms of lactose malabsorption. There is
also a variety of symptoms of LM after ingestion of lactose: abdominal pain, bloating,
flatulence, diarrhea, and, particularly in adolescents, vomiting.
The genetic alteration of LM (LCT CC) is common throughout the world with different
frequencies: 2% in Sweden, 20–25% in Caucasian populations in Europe and the United
States, increasing to 60–75% in South European Caucasians, and 100% in Southeast Asian
population [13]
[16]
[27]. Analysis of the LCT polymorphism has shown to be a highly reliable tool in the
diagnosis of LM. In contrast, conventional diagnostic methods for LM, such as the
hydrogen breath test, have a 20% false negative rate, due to hydrogen non-excretion.
The diagnostic sensitivity of the lactose tolerance test is about the same range [13]
[15]
[16]. The prevalence of LCT-CC in Germans is in line with previous studies [3]. However, in the present study we found a high prevalence of molecular defined lactose
malabsorption in Turkish immigrants (84.2%), but there was no significant association
of the LCT CC genotype with BMD and other laboratory parameters or with other specific
characteristics (e. g., age, BMI).
This study has several limitations. Bone mineral density can be influenced by several
environmental factors such as age, sex, social status, caloric intake and diet composition
which can represent confounding factors when analyzing BMD and hypovitaminosis D.
When selecting the German controls we did not match them in for all of these aspects
with the Turkish immigrants but only for age and sex. Cutural differences in diet
composition between Turkish immigrants and Germans can be anticipated even when investigating
immigrants who have lived in Germany for several years. The finding of a higher BMI
in the Turkish immigrants would be in keeping with this notion. In addition, we limited
our osteogenetic analysis to the analysis of polymorphisms of the VDR, the significance
of which for primary osteoporosis has been a matter of debate. Furthermore, we did
not investigate possible genetic interaction. Thus, a more detailed genetic analysis
in larger groups of Turkish and German individuals will be required to further evaluate
the genetic basis of bone metabolism and osteoporosis in Turkish immigrants.
In conclusion, vitamin D deficiency, secondary hyperparathyroidism and reduction of
the bone mineral density (osteopenia/osteoporosis) were found to be highly common
among Turkish immigrants living in Germany, without showing a gender-specific preference.
Since there is an increasing number of Turkish immigrants in Germany, osteomalacia
should always be taken into consideration if these individuals develop skeletal pain
and muscle weakness. Even minor or vague symptoms should indicate a search for an
osteometabolic cause of these sequelae. Moreover, the monitoring of vitamin D status
(i. e., 25-OH D3, calcium and PTH) and bone mineral density is warranted and once such a deficiency
is identified, it should be treated appropriately. The current screening approach
offers a fast, reliable and clinically accurate way for the identification and follow-up
of these high-risk patients. Moreover, the early identification and subsequent, carefully
monitored vitamin D treatment of high-risk patients will also result in a reduction
of secondary hyperparathyroidism – or at least related to long-term complications
such as osteomalacia, osteopenia/osteoporosis, bone and/or muscle aches and pains
and fractures. The FokI-polymorphism at the VDR gene is of clinical value in identifying
female Turkish migrants who are at risk of decreased bone mineral density (osteopenia/osteoporosis).
Furthermore, genetic predisposition for adult lactose intolerance is of high frequency
(84.1%) in Turkish immigrants but appears not to be a risk factor for an alteration
of bone mineral density.
Thus, the results of this study reveal a unique osteogenetic pattern of a large group
of migrated but ethnically homogeneous individuals in their new environment.
