1. Introduction
Vitamin D is produced by our skin upon exposure to ultraviolet B (UVB) irradiation (280–320 nm) from sunlight. More than 90% of the circulating vitamin D in the body originates from the cutaneous production. Sunlight converts 7-dehydrocholesterol in the skin to vitamin D3, which will be transported to the liver and hydroxylated to 25-hydroxyvitamin D (25(OH)D). It will then be conveyed to the kidney and hydroxylated to 1,25-dihydroxyvitamin D (1,25(OH)D) [
1,
2]. Although 1,25(OH)D is the metabolically active prohormone, the 25(OH)D level is accepted as the determinant of vitamin D status [
3]. Vitamin D deficiency will result in osteomalacia and rickets. Recent studies have uncovered that a low vitamin D level is associated with medical conditions, such as osteoporosis, metabolic disease, cancer, diabetes mellitus, infection and multiple sclerosis, but causality has not been proven for most outcomes [
4].
The definitions of vitamin D deficiency and insufficiency are the subjects of much debate. Both the Institute of Medicine (IOM) of the United States and the Endocrine Society have come up with different definitions [
5,
6]. According to IOM, a serum 25(OH)D level above 50 nmol/L covers the requirement of at least 97.5% of the population [
6]. This is in agreement with the statement by the World Health Organization, that the suppression of parathyroid hormone (PTH) with vitamin D supplementation only occurs in individuals with a baseline serum 25(OH)D level less than 50 nmol/L [
7]. Thus, the current study adopted the cut-off value of 50 nmol/L for vitamin D insufficiency.
Vitamin D insufficiency is prevalent in countries with temperate climates that receive limited sunlight, especially during winter [
8]. It is also prevalent in populations in the Middle East, whereby the covering of body parts is demanded by religious and cultural reasons [
9,
10,
11]. Recent studies have demonstrated that vitamin D insufficiency is common in tropical countries, such as Vietnam, Malaysia and Indonesia [
12,
13,
14]. In Malaysia, the studies on vitamin D status are limited to pockets of populations, such as school children [
15], Malay employees of an academic institution [
13], women of child-bearing age [
12] and postmenopausal women [
16]. There is a paucity of studies on men in this multiracial setting. Previous studies in the United States have revealed a significant racial discrepancy in vitamin D status among Caucasian, African American and Hispanic men [
17,
18]. We speculated that such a difference might occur in Malaysians.
The current study aimed to determine the prevalence of vitamin D status in Chinese and Malay adult men in Malaysia and its associating determinants. The relationships among serum 25(OH)D, PTH and bone health as reflected by calcaneal quantitative ultrasound (QUS) were also investigated. We hypothesized that there were significant relationships between ethnicity, age, physical activity and vitamin D status among Malaysian men. We also speculated that there was an inverse relationship between serum 25(OH)D and PTH and a positive relationship between 25(OH)D and bone health status. The information obtained from this study would identify risk factors for suboptimal vitamin D status in Malaysian men.
3. Results
The serum 25(OH)D level was available for 401 subjects who attended the screening session. After eliminating univariate outliers, data from 383 subjects (60.8% Chinese and 39.2% Malay) were used in the current analysis. Data from 382 subjects were used in MLR analysis after the removal of one multivariate outlier. The Chinese subjects were significantly older, taller and had a lower BMI and physical activity level compared to the Malay subjects (
p < 0.05). The Malay subjects also had significantly higher serum total calcium and inorganic phosphate levels, but a significantly lower serum 25(OH)D level compared to the Chinese subjects (
p < 0.05) (
Table 1).
Table 1.
The difference in characteristics between Chinese and Malay subjects. IPAQ, International Physical Activity Questionnaire; MET, metabolic rate; n/a, not applicable; PTH, parathyroid.
Table 1.
The difference in characteristics between Chinese and Malay subjects. IPAQ, International Physical Activity Questionnaire; MET, metabolic rate; n/a, not applicable; PTH, parathyroid.
Ethnicity | Chinese (n = 233) | Malays (n = 150) | p-value |
---|
Variable | Mean | SD | Mean | SD |
---|
Age (years) | 46.6 | 13.5 | 39.4 | 17.0 | <0.001 |
Height (cm) | 168.5 | 6.4 | 166.4 | 6.6 | 0.002 |
Body Fat Percentage (%) | 21.9 | 5.7 | 22.9 | 7.0 | 0.130 |
Waist Circumference (cm) | 87.5 | 8.7 | 89.0 | 12.3 | 0.206 |
Calcaneal Speed of Sound (m/s) | 1516.5 | 26.7 | 1527.0 | 28.2 | 0.108 a |
Serum Total Calcium (mmol/L) | 2.2 | 0.1 | 2.3 | 0.1 | <0.001 |
Serum Inorganic Phosphate (mmol/L) | 1.1 | 0.1 | 1.2 | 0.1 | <0.001 |
Variable | Median | IQR | Median | IQR | p-value |
Body Weight (kg) | 68.1 | 14.7 | 67.9 | 21.0 | 0.298 |
Body Mass Index (kg/m2) | 24.0 | 5.1 | 24.8 | 7.7 | 0.013 |
Serum 25(OH)D Level (nmol/L) | 60.2 | 13.9 | 54.6 | 16.0 | <0.001 |
Serum PTH Level | 43.4 | 22.0 | 42.8 | 20.2 | 0.609 b |
IPAQ Score (MET-min/week) | 1099.0 | 2055.0 | 1,854.0 | 3,711.3 | <0.001 b |
Vitamin D Status | n | % (95% CI) | n | % (95% CI) | p-value |
Deficiency | 0/2 | 0 | 2/2 | 100 | n/a |
Insufficiency | 36/85 | 42.4 (31.9–52.9) | 49/85 | 57.7 (47.2–68.2) | 0.159 |
Normal | 197/296 | 66.6 (61.2–72.0) | 99/296 | 33.5 (28.1–38.9) | <0.001 c |
According to the definition by IOM, this study found that 22.7% of the overall subjects had vitamin D insufficiency and 0.5% suffered from deficiency. The two vitamin D-deficient subjects were Malay men. The proportion of subjects having vitamin D insufficiency was 15.5% for Chinese men and 32.7% for Malay men when segregated by ethnicity (
Figure 1). Based on vitamin D status, there were significantly more Chinese subjects categorized as normal compared to Malay subjects (
p < 0.05) (
Table 1).
Subjects having a suboptimal serum 25(OH)D level (<50 nmol/L) were significantly younger, had a higher body weight, BMI, body fat percentage, waist circumference and serum inorganic phosphate level compared to subjects with an optimal 25(OH)D level (≥50 nmol) (
p < 0.05) (
Table 2).
Figure 1.
Vitamin D status of the subjects.
Figure 1.
Vitamin D status of the subjects.
Table 2.
The difference in characteristics between subjects with an optimal (≥50 nmol) and suboptimal (<50 nmol) 25(OH)D level. PTH, parathyroid; IPAQ, International Physical Activity Questionnaire.
Table 2.
The difference in characteristics between subjects with an optimal (≥50 nmol) and suboptimal (<50 nmol) 25(OH)D level. PTH, parathyroid; IPAQ, International Physical Activity Questionnaire.
Vitamin D status | Subjects with 25(OH)D Level < 50 nmol (n = 87) | Subjects with 25(OH)D Level ≥ 50 nmol (n = 296) | p-value |
---|
Variable | Mean | SD | Mean | SD |
---|
Age (years) | 40.0 | 16.2 | 44.9 | 15.0 | 0.009 |
Height (cm) | 167.2 | 6.2 | 167.8 | 6.6 | 0.519 |
Percentage of Body Fat (%) | 23.8 | 6.4 | 21.8 | 6.1 | 0.011 |
Waist Circumference (cm) | 90.2 | 10.6 | 87.5 | 10.1 | 0.027 |
Calcaneal Speed of Sound (m/s) | 1522.0 | 29.2 | 1520.2 | 27.3 | 0.292 a |
Serum Total Calcium (mmol/L) | 2.3 | 0.1 | 2.3 | 0.1 | 0.120 |
Serum Inorganic Phosphate (mmol/L) | 1.1 | 0.1 | 1.1 | 0.1 | 0.037 |
Variable | Median | IQR | Median | IQR | p-value |
Body Weight (kg) | 69.9 | 20.0 | 67.0 | 16.4 | 0.012 |
Body Mass Index (kg/m2) | 25.6 | 6.6 | 24.0 | 5.3 | 0.002 |
Serum 25(OH)D Level (nmol/L) | 44.3 | 6.8 | 61.6 | 11.8 | <0.001 |
Serum PTH Level | 43.3 | 26.5 | 43.0 | 20.7 | 0.279 b |
IPAQ Score (MET-min/week) | 1580.0 | 2284.0 | 1386.0 | 2730.0 | 0.465 b |
The serum vitamin D level was significantly lower in subjects aged 20–29 years compared to subjects aged 40–49, 50–59 and more than 60 years (
p < 0.05). Correspondingly, more subjects were categorized as having vitamin D deficiency and insufficiency in the younger age groups (
Table 3).
Using stepwise MLR, BMI (β = −0.136;
p = 0.007) was chosen as the best predictor of serum 25(OH)D level among the obesity indicators measured (waist circumference and body fat percentage were excluded), along with age (β = 0.150;
p = 0.003), ethnicity β = 0.205, (Chinese
versus Malays (reference);
p < 0.001) and physical activity (β = 0.131, high
versus low (reference) physical activity;
p = 0.034). Being Chinese, older in age, having a lower BMI and a high physical activity status were significantly associated with a higher serum 25(OH)D level (
p < 0.05) (
Table 4).
Table 3.
Serum 25(OH)D level and vitamin D status of the subjects in each age group.
Table 3.
Serum 25(OH)D level and vitamin D status of the subjects in each age group.
Age Group (Years) | N | Serum 25(OH)D Level (nmol/L) | Vitamin D Status (%) |
---|
Mean | SD | Deficiency | Insufficiency | Sufficiency |
---|
20–29 | 91 | 54.7 | 13.1 | 1.1 | 33.0 | 65.9 |
30–39 | 58 | 58.3 | 12.0 | 1.7 | 22.4 | 75.9 |
40–49 | 78 | 60.3 a | 11.1 | 0.0 | 15.4 | 84.6 |
50–59 | 93 | 60.4 a | 12.1 | 0.0 | 20.4 | 79.6 |
≥60 | 63 | 60.2 a | 11.8 | 0.0 | 17.5 | 82.5 |
Overall | 383 | 58.7 | 12.2 | 0.5 | 22.2 | 77.3 |
Table 4.
The association between the serum 25(OH)D level and its predictors. BMI, body mass index; PA, physical activity.
Table 4.
The association between the serum 25(OH)D level and its predictors. BMI, body mass index; PA, physical activity.
Variables | Standardized Regression | p-value |
---|
Ethnicity (Chinese vs. Malays (reference)) | 0.205 | <0.001 |
Age | 0.150 | 0.003 |
BMI | −0.136 | 0.007 |
Low PA vs. Moderate PA | −0.090 | 0.142 |
Low PA vs. High PA | 0.131 | 0.034 |
The associations between serum 25(OH)D and serum intact PTH, total calcium and inorganic phosphate levels were investigated using MLR. It was found that the difference between the lowest and the highest quartiles of the serum PTH level was associated with the variation of the serum 25(OH)D (β = −0.139,
p = 0.025) level. Subjects in the lowest quartile of PTH level had a significantly higher serum 25(OH)D level compared to subjects in the highest quartiles of PTH level (
p < 0.05). A plateau was not detected when the serum PTH level was plotted against the 25(OH)D level (
Figure 2). However, there was no significant association between the serum 25(OH)D level and serum total calcium and inorganic phosphate levels (
p > 0.05) (
Table 5).
Table 5.
The association between the serum 25(OH)D level and biochemical factors. PTH, parathyroid; IPAQ, International Physical Activity Questionnaire.
Table 5.
The association between the serum 25(OH)D level and biochemical factors. PTH, parathyroid; IPAQ, International Physical Activity Questionnaire.
Variable | Standardized Regression | p-value |
---|
Serum Calcium | −0.018 | 0.737 |
Serum Inorganic Phosphate | 0.009 | 0.861 |
PTH Q1 vs. Q2 | −0.063 | 0.300 |
PTH Q1 vs. Q3 | −0.068 | 0.269 |
PTH Q1 vs. Q4 | −0.139 | 0.025 |
Figure 2.
Scatter plot of the parathyroid (PTH) level versus the 25(OH)D level in the serum.
Figure 2.
Scatter plot of the parathyroid (PTH) level versus the 25(OH)D level in the serum.
The association between calcaneal SOS value and biochemical determinants of calcium homeostasis (serum 25(OH)D, serum intact PTH, serum total calcium and serum inorganic phosphate levels) was studied. None of the predictors were significantly associated with SOS values after adjustment for age, BMI and ethnicity of the subjects (
p > 0.05) (
Table 6).
Table 6.
The association between calcaneal speed of sound and biochemical variables related to calcium homeostasis. PTH, parathyroid.
Table 6.
The association between calcaneal speed of sound and biochemical variables related to calcium homeostasis. PTH, parathyroid.
Variables | Standardized Regression | p-value |
---|
PTH Q1 vs. Q2 | −0.083 | 0.152 |
PTH Q1 vs. Q3 | −0.024 | 0.687 |
PTH Q1 vs. Q4 | −0.022 | 0.706 |
25(OH)D level | 0.067 | 0.171 |
Serum Total Calcium (mmol/L) | 0.020 | 0.705 |
Serum Inorganic Phosphate (mmol/L) | −0.001 | 0.979 |
4. Discussion
The current study found that vitamin D deficiency was uncommon in Malaysian men, whereby it only occurred in 0.5% of the study population. However, a significant proportion of the study population suffered from vitamin D insufficiency (32.7% Malay men and 15.5% Chinese men). Being Chinese, older in age, having a high physical activity level and a lower BMI were associated with a higher serum 25(OH)D level in Malaysian men. Serum PTH was inversely and significantly associated with the serum 25(OH)D level in the subjects. However, none of the serum biochemical markers (PTH, total calcium, inorganic phosphate, 25(OH)D) were associated with the bone health of the subjects, as reflected by calcaneal SOS.
The definitions of vitamin D deficiency and insufficiency put forward by the Endocrine Society and IOM were significantly different. While IOM defined deficiency as a serum 25(OH)D level less than 30 nmol/L and insufficiency as between 30 and 50 nmol/L [
6], the cut-off values set by the Endocrine Society were significantly higher (deficiency: less than 50 nmol/L; insufficiency: 50–74 nmol/L) [
5]. The use of different cut-off values would alter the prevalence of deficiency and insufficiency significantly. The arguments on the scientific merits of these definitions were presented elsewhere [
22,
23]. The IOM definitions were adopted in this study because previous studies showed that there was a congruence of bone beneficial effects at a serum 25(OH)D level between 40 and 50 nmol/L, but not higher [
22].
The proportion of insufficiency reported in this study (22.2%) was considerably lower compared to previous studies. Moy and Bulgiba (2011) reported that the prevalence of vitamin D insufficiency (<50 nmol/L) in Malaysian Malay adult men (mean age: 49.6 ± 5.7 years) was 41.4% [
13]. However, their subjects might not be representative of the Malay population, because they were limited to employees within a university campus. Hawkins (2013) found that in the Singaporean population (median age: 36 years (range: 19–71)), 30% of Chinese and 48% of Malay men had vitamin D insufficiency (<50 nmol/L) [
24]. None of the subjects were found to have vitamin D deficiency (<30 nmol/L) [
24]. It should be noted that the sample size in his study was small (40 men per ethnicity). In contrast to these studies, the current study recruited a larger sample size of free living Malaysian Chinese and Malay men. Thus, the subjects were more reflective of the population.
Racial discrepancy in vitamin D status as observed in this study had been found in other multiracial populations. Forrest and Stuhldreher (2011) reported that suboptimal vitamin D level (<50 nmol/L) was most commonly found in African Americans, followed by Hispanics and non-Hispanic whites in the United States aged 20 years and above [
17]. In this study, the Malay men had a lower serum 25(OH)D level compared to the Chinese men. The darker pigmentation of the Malay men could have contributed to this observation. Previous studies had established that subjects with darker skin pigmentation had a lower 25(OH)D level after UVB radiation exposure [
25,
26]. This was because melanin absorbed and competed with 7-dehydrocholesterol for UVB photons [
27]. This explanation on the discrepancy of vitamin D status between the Chinese and the Malay men remained speculative, because their skin pigmentation was not quantified in this study. However, other factors were less possible, because racial differences in clothing and adiposity were not apparent. The Malay subjects were significantly younger and more physically active, yet they had a significantly lower 25(OH)D level compared to the Chinese subjects.
The serum 25(OH)D level was found to be associated positively and significantly with age in this study. Previous studies on the association between age and serum 25(OH)D level were inconclusive. Several studies found that serum 25(OH)D level decreased with increasing age in men [
9,
10,
28]. This complied with the observation that aging decreased the capacity of the skin to produce 7-dehydrocholesterol [
29]. However, there were also studies showing that younger male subjects had a lower serum 25(OH)D level compared to older subjects [
13,
14,
30,
31]. This was attributed to lifestyle and behavioral changes across generations, such as a tendency to work indoors and avoidance of sunlight exposure in the younger generation. The exact reason for the positive association between serum 25(OH)D and age in this study was not known.
Body mass index was associated negatively with the serum 25(OH)D level of the subjects in this study. This was a universal finding across different populations [
9,
10,
13,
17,
30,
32]. The percentage of body fat and waist circumference were not significant predictors of serum 25(OH)D level. This was different from the observations of several studies, whereby a significant and negative relationship was found between serum 25(OH)D and waist circumference [
9,
10,
13,
33]. The prevailing explanation for the inverse relationship between vitamin D and obesity was that vitamin D, a fat-soluble vitamin, was sequestered in the adipose tissue [
34]. A prospective study also reported that a lower vitamin D level was associated with a higher risk of developing metabolic syndrome, with obesity being one of the components [
13,
33]. As the current study was cross-sectional, it was impossible to determine the causal relationship between obesity as indicated by BMI and serum 25(OH)D in the subjects.
Physical activity status was found to be significantly and positively associated with the serum 25(OH)D level in this study. The association was more prominent in subjects with a high physical activity status compared to subjects with low physical activity status. Similarly, previous studies also demonstrated that physical activity, as determined using various different tools, was associated with vitamin D level [
10,
32,
35]. It was speculated that subjects who were physically active also tended to spend more time under the sun. However, sunlight exposure was not quantified in this study. We also did not discriminate between outdoor and indoor physical activity. Previous studies had established that outdoor physical activity was more relevant to the vitamin D status of the subject [
35,
36].
The relationship between serum 25(OH)D and intact PTH level was significant and negative. This inverse association was more prominent between the lowest and the highest quartile of PTH. Previous studies showed that the relationship between 25(OH)D and PTH was inverse and linear [
10,
14]. Others found a transition point, whereby a further decrease of vitamin D caused an accelerated increase in PTH level [
37,
38]. However, this was not observed in the current study, probably because there were few subjects with a very low 25(OH)D level. The variation in serum 25(OH)D was not associated with serum total calcium and inorganic phosphate level. This was contributed to by the fact that serum calcium and inorganic phosphate levels were tightly regulated in the body and none of our subjects had medical conditions that could affect calcium and phosphate homeostasis. Furthermore, blood collection was performed after an overnight fast; thus, individual variation was minimized.
The bone health of men as determined by calcaneal SOS was not associated with the serum 25(OH)D level. This was different from previous studies, which found a significant relationship between the 25(OH)D level and bone health, indicated by bone mineral density (BMD) or QUS indices [
39,
40]. This discrepancy was probably because the subjects were generally healthy, and their serum 25(OH)D level did not explain much of the variation in their bone health status. On the other hand, there were more important factors contributing to bone health in these subjects, such as age [
41], BMI [
21], physical activity [
21], testosterone [
42,
43] and sex hormone-binding globulin [
42].
Several limitations should be considered when interpreting the results of the current study. The study population was limited to Chinese and Malay men in Klang Valley, Malaysia. Indian men were not recruited, due to logistical difficulties. The results should not be generalized to the whole Malaysian population. However, Chinese and Malay are the two largest ethnic groups in Klang Valley and in Malaysia [
44]. Besides, men who visited the health screening session might be more health conscious and healthier. The bone health of the subjects was determined using a calcaneal QUS device, which generated SOS as the single determinant. Another QUS index, broadband ultrasound attenuation, which was shown to be associated with hip fracture risk independent of BMD [
45], could not be generated by this device. Calcaneal SOS had been shown to correlate strongly with BMD assessed using dual-X-ray absorptiometry [
46]. On the other hand, this study filled the gap in the knowledge of the vitamin D status in Malaysian men and its correlates. It showed that Malay men and the younger generation should be given emphasis in preventing the progression of vitamin D insufficiency and its associated health problems.