Introduction
Osteoporosis is a critical public health problem due to its association with bone fragility and susceptibility to fracture [
1]. According to the U.S. National Institutes of Health, osteoporosis is defined as a systemic skeletal disorder characterized by compromised bone strength [
2]. Bone strength is not only determined by measures of bone density, such as mass and mineral density, but also by bone quality, including microarchitecture, turnover, accumulation of microdamage, mineralization, and quality of collagens [
2,
3]. Interestingly, patients with type 2 diabetes have an increased risk of fracture despite normal or high bone mineral density (BMD) compared with non-diabetic controls, suggesting poorer bone quality in diabetic patients [
4].
Accumulation of advanced glycation end-products (AGEs), which are often found in diabetic patients, in bone collagen has been proposed as a factor responsible for reducing bone strength with aging [
5], diabetes [
6,
7], and osteoporosis [
8‐
10]. AGEs are a diverse class of compounds resulting from non-enzymatic reactions between glucose and proteins. A common consequence of AGE formation is covalent cross-linking, mostly to proteins including collagen. Accumulation of AGEs in bone collagen decreases the mechanical properties of bone collagen [
11,
12]. In rats, an increase of AGE content in bone decreases the mechanical properties of bone despite normal BMD [
6]. In humans, the AGE concentration of cortical bone shows a significant increase with age and is negatively correlated with osteoporotic index (Singh score) [
5]. Moreover, the AGE content in bone is higher in patients with hip fracture than in subjects without fractures [
10].
In a population study, Shiraki et al. demonstrated that a high level of urinary pentosidine, a major AGE in vivo, was an independent risk factor for osteoporotic vertebral fractures in elderly women [
13]. Schwartz et al. reported that urinary pentosidine content was associated with increased fracture incidence in older adults with diabetes [
14]. The subjects of these studies were older adults who had an increased risk of life-related diseases, such as diabetes and osteoporosis. However, AGEs may accumulate before the onset of diabetes and even at a younger age. In non-diabetic Japanese subjects, serum AGE levels were independently correlated with insulin resistance, which may gradually cause diabetes [
15]. Pentosidine content in bone or serum increased with advancing age [
5]. Given that bone strength commonly peaks when a person is in his/her 20s and then gradually declines with advancing age, AGE accumulation may be associated with bone strength, if not with fractures, preclinically. Moreover, in men, the lifetime risk of any osteoporotic fracture has been assessed as being within the range 13–22% [
1], so osteoporosis is no longer a problem only for women and the elderly. Greater AGE accumulation may potentially be related to poorer bone strength in apparently healthy adult men.
Thus, in this study, we examined the association between skin autofluorescence (AF), which is associated with skin accumulation of AGEs, including pentosidine [
16], and quantitative ultrasound examination of calcaneal bone, which correlates with mechanical properties of the bone and may have a predictive value for hip fractures in men [
17], among apparently healthy adult men. We hypothesized that skin AF would have a negative association with quantitative ultrasound among adult men.
Results
Characteristics of the 193 study participants are shown in Table
1. Overall, median (interquartile range) OSI was 2.75 (2.59–2.93), and skin AF was 1.96 (1.78–2.14) AU. Median age was 45.0 years.
In the univariate analyses (Table
2), log-transformed OSI was significantly associated with age, BMI, calcium intake, high PA, former smoker, and skin AF. The association of log-transformed OSI with waist circumference, education level (college level and above), and MS were borderline significance, and there was no association of log-transformed OSI with fasting blood glucose, TG, LDL-C, HDL-C, BP, vitamin D intake, middle PA, current smoker, drinking status, depressive symptoms (SDS ≥ 45), desk work, and leg fracture. Among current smokers, Brinkman index was associated with OSI (
r = −0.16,
P = 0.04, data not shown).
Table 2
Univariate linear regression models of skin AF and other factors with OSI
Age (years) | −0.26 | <0.01 |
BMI (kg/m2) | 0.20 | <0.01 |
Waist circumference (cm) | 0.13 | 0.06 |
SBP (mm Hg) | 0.03 | 0.67 |
DBP (mm Hg) | 0.01 | 0.91 |
Fasting blood glucose (mg/dL) | −0.10 | 0.16 |
TG (mg/dL) | −0.10 | 0.92 |
LDL-C (mg/dL) | 0.03 | 0.72 |
HDL-C (mg/dL) | −0.01 | 0.85 |
Calcium intake (mg/day·2,000 kcal) | 0.15 | 0.03 |
Vitamin D intake (mg/day·2,000 kcal) | 0.03 | 0.64 |
High PA (median values, 48.0 METs h/week)a
| 0.15 | 0.03 |
Middle PA (median values, 12.0 METs h/week)a
| −0.07 | 0.30 |
Smoking statusb
| | |
Current | −0.03 | 0.69 |
Former | −0.15 | 0.03 |
Drinking statusc
| | |
7 drinks/week | −0.06 | 0.42 |
≥1 drinks/week | 0.09 | 0.18 |
Depressive symptoms (SDS ≥ 45) | −0.05 | 0.49 |
Education (≥college) | 0.12 | 0.07 |
Desk work | 0.06 | 0.42 |
Leg fracture | 0.08 | 0.22 |
MS (JASSO) | 0.13 | 0.05 |
Skin AF | −0.25 | <0.01 |
To determine whether skin AF was independently associated with OSI, we performed a multiple linear regression analysis using skin AF and other variables associated with OSI in the univariate analyses (Table
3). Although waist circumference had a tendency to associate with OSI in the univariate model, waist circumference was not included in the multivariate model since it was strongly correlated with BMI. After adjustment for age, BMI, calcium intake, PA level, smoking status, education level, and MS, log-transformed skin AF had a negative association with log-transformed OSI (
β = −0.218, SE = 0.069,
P < 0.01). Table
4 shows the relationship of the tertiles of skin AF with log-transformed OSI using ANCOVA. The adjusted geometric mean (95% CI) of log-transformed OSI across the tertiles of skin AF was 2.81 (2.75–2.87) for the lowest tertile, 2.81 (2.74–2.87) for the middle tertile, and 2.66 (2.61–2.73) for the highest tertile; thus, participants in the highest tertile had 5.0% lower OSI than those in the lowest and middle tertiles (Bonferroni-corrected
P value < 0.01).
Table 3
Multiple linear regression models of log-transformed skin AF with log-transformed OSI
Participants (n = 193) | | | |
Log-skin AF | −0.218 | 0.069 | <0.01 |
Table 4
Relationship of the tertile of skin autofluorescence (AF) with log-transformed OSI among adult Japanese men
Number of participants | 65 | 64 | 64 |
Crude | 2.83 (2.76–2.90) | 2.78 (2.71–2.85) | 2.68 (2.61–2.74)*
|
Adjusteda
| 2.81 (2.75–2.87) | 2.81 (2.74–2.87) | 2.66(2.61–2.73)*,**
|
Discussion
The present study examined the relationship between skin AF associated with AGE accumulation and OSI, a quantitative ultrasound measure, among non-diabetic adult Japanese men. Consistent with our hypothesis, our results showed that levels of skin AF were independently associated with OSI, suggesting that participants with higher skin AF had lower OSI.
In previous population studies, the relationship between AGE accumulation and fracture risk has been controversial. Some studies reported that there was no association between urinary pentosidine and fracture risk after adjustment in non-diabetic older Caucasian [
14] and among postmenopausal Caucasian women [
27]. On the other hand, in elderly Japanese women, a high level of urinary pentosidine was an independent risk factor for osteoporotic vertebral fractures [
13]. Possibly in line with these findings, we found a negative association between skin AF with OSI among adult Japanese men after adjustment for potential confounders, given that lower OSI may lead to higher fracture risk. Although the reasons for this discrepancy are unknown, racial differences may potentially explain the inconsistent results of the studies. While Japanese have twice the incidence of the methylenetetrahydrofolate reductase polymorphism (C677T) compared with Caucasians, Japanese subjects are predisposed to mild hyperhomocysteinemia [
28‐
30]. Indeed, hyperhomocysteinemia caused a reduction in bone toughness through the accumulation of pentosidine in bone in rabbit models [
31]. Other explanation could be diet, which is a major source of exogenous AGEs [
32]. AGEs are especially high in Western diet, since heat-treated process enhances the formation of AGEs [
32], and the concentration of urinary pentosidine in Caucasian women [
27] was more than twofold greater than that in Japanese women [
13]. Therefore, Japanese who may be ingesting less dietary AGE might be more susceptible to the adverse effect of AGE accumulation.
Skin AF measurement is a noninvasive, rapid, and highly reproducible method, which effectively measures tissue AGE accumulation. This method has been validated to correspond to specific AGE skin levels, including pentosidine [
16]. As for the clinical significance of skin AF measurement, however, we still have a limited number of prospective studies in which Skin AF was shown to predict developments of diabetic complications [
33], and was associated with all-cause mortality [
34] in type 2 diabetes in a prospective study with a follow-up period of 3.1 years. Therefore, more prospective studies with larger sample size and longer follow-up period are necessary to establish its clinical significance.
Sell et al. have shown an exponential increase in pentosidine accumulation across the age in skin collagen [
35]. In a separate study, Odetti et al. have shown a similar exponential increase in pentosidine accumulation across the age in bone collagen [
5]. Interestingly, the level of pentosidine per unit collagen is higher in the bone as compared to the skin. This difference well corresponds to the result obtained in a cadaver study in which post-mortem bodies of human were analyzed [
36]. They showed that pentosidine level per milligram of collagen was more than 60% higher in the bone tissue as compared to the skin tissue. Taken together, skin and bone pentosidine levels are likely to have a positive correlation. Further study is necessary to establish this relationship, but we believe that skin AF may not only correspond to skin pentosidine accumulation, but also bone pentosidine accumulation. In rats, the accumulation of pentosidine in bone was significantly associated with the reduction of bone stiffness [
7]. Although the cause–effect relationship cannot be established in this cross-sectional design, we believe that skin AF may be associated with bone strength. Further prospective study is, therefore, required to establish the prospective value of skin AF on bone strength.
In the present study, we used OSI as an index of bone strength. Although OSI is not widely used to assess bone strength, quantitative ultrasound (QUS) parameters including OSI may reflect not only bone mass but also bone quality. A previous study found that impaired bone mechanical properties in diabetic rats coincided with impaired enzymatic cross-link formation and increases in glycation-induced pentosidine, despite the lack of reduction in BMD [
7], therefore, it is possible that AGE accumulation may more clearly be associated with OSI rather than BMD which measures bone density. In this study, OSI was 5.0% lower for the highest skin AF compared with the lowest and middle skin AFs after adjustment for confounders. Njeh et al. showed that patients with hip fractures had 8.0% lower OSI compared with control subjects [
37]. Therefore, the influence of preclinical AGE accumulation in bone tissue may not be negligible.
Our finding is consistent with the observation in in vitro studies that increased AGE level in bone collagen reduces bone mechanical properties [
11,
12]. AGE accumulation significantly alters the quantity and morphology of microdamage and results in reduced fracture resistance [
38]. Moreover, in spontaneously diabetic rats, decreased mechanical properties of femoral bone are accompanied by increased accumulation of pentosidine, and the pentosidine content is significantly associated with the mechanical properties of bone [
6]. Indeed, in several studies, patients with hip fractures had higher bone pentosidine content [
9,
10] or serum AGEs [
8] compared with subjects without fractures.
In addition to the adverse effects of AGEs on the material properties of bone collagen, AGE accumulation may potentially influence bone cells. AGE-modified bone collagen has detrimental effects on osteoblastic function [
7,
39]. The effects of AGEs on osteoclastic bone resorption are controversial. Receptor-of-AGE (RAGE) knockout mice have significantly higher bone mechanical strength, probably due to decreased number of osteoclasts compared with wild-type mice [
40]. Furthermore, Miyata et al. showed that AGEs increased the number of resorption pits in cultured mouse bone cells as well as when AGE-accumulated bone particles were implanted subcutaneously in rats [
41]. In contrast, Valcourt et al. reported that bone resorption was inhibited in an in vitro study using rabbit and human mature osteoclasts seeded on AGE-modified slices [
42]. AGEs also inhibited the proliferation of human mesenchymal stem cells and cognate differentiation into bone [
43].
Although there was no association between smoking status and OSI in this study, Brinkman index was negatively associated with OSI among current smokers. Therefore, smoking may have harmful effect on bone strength among healthy adult men. As for drinking status, we found no association with OSI. A previous study reported that, among Korean men, four to seven cups of soju (the most popular liquor in Korea) is associated with the risk of reduced QUS parameters [
44]. When the amount of alcohol intake in our population was converted to alcohol amount per a cup of soju, only less than 20% of our participants drank the amount of alcohol corresponded to four or more cups of soju (data not shown). Thus, it is possible that the alcohol intake in our study was small in the previous study [
44].
This study has some limitations. First, although we adjusted for confounders such as lifestyle factors and disease, we could not exclude the possibility that bone strength was affected by other factors associated with lifestyle or disease. Moreover, because this study was a cross-sectional study, we could not conclude whether AGE accumulation in skin tissue reduced bone strength. A larger population-based prospective study should be performed to further confirm the causal relationship between skin AGE accumulation and bone strength.
In conclusion, in apparently healthy adult Japanese men, skin AF was independently associated with OSI, suggesting that the participants with higher skin AF had a lower OSI. Further studies are needed to confirm the causal relationship between skin AGE accumulation and bone strength.