Risk factors for bone mineral density at the calcaneus in 40–59 year-old male workers: A cross-sectional study in Korea

WHO recognizes osteoporosis as an important, global health problem that will increase in significance as the world population both increases and ages [1, 2]. Since most of the lifestyle aspects that affect osteoporosis are modifiable, lifestyle modification is important in the prevention of osteoporosis.

In recent studies, the prevalence of osteoporosis in men older than 49 years old was about 7% [3], and fatalities caused by femoral neck fracture were more common in men than in women [4]. In addition, osteoporosis in elderly men has become an important disease because one study found that 25% of men were in danger of fracture due to osteoporosis [5]. Moreover, a quick recovery from osteoporosis is not possible and osteoporosis increases the fracture risk [6], therefore increasing the importance of prompt treatment through prevention and early diagnosis. Therefore, the quality of life in Korea, which is an aging society, can be improved by focusing on the prevention of osteoporosis in the general population.

Previous studies of osteoporosis examined the risk factors of osteoporosis [7‐9], exercise and bone density [10, 11] and calcium and bone density [12, 13]. However, the study subjects were American or European women. Studies of lifestyles related to osteoporosis in women have been performed, especially climacteric women [14] and university and college students [15], but the epidemiological conditions and risk factors of osteoporosis in Korean men remain to be elucidated due to the absence of any studies of osteoporosis in Korean men. Moreover, most subjects of domestic and foreign studies were older than 47 years old [16‐23]. Therefore, the aim of this study was to assess the prevalence of osteoporosis, which is a major health problem, in 40–59 year-old male workers, and to obtain epidemiological data for the association between life style factors and bone density.

The general characteristics of study subjects are shown in Table 1. The mean age of the subjects was 47.1 years old, and the mean BMD was 0.60 ± 0.09 g/cm².

Correlation analysis was conducted to investigate the continuous variables related with BMD. BMD was correlated negatively with age (r = -0.18, P < 0.001), but positively with WHR (r = 0.15, P < 0.001), body fat (r = 0.10, P < 0.001), height (r = 0.26, P < 0.001), and weight (r = 0.43, P < 0.001) (Table 2).

Multiple linear regression analysis was performed to identify the related factors that affect BMD. Age, education level (<12 years vs. ≥ 12 years), physical activity (≤ 2/week vs. ≥ 3/week), WHR, BMI, drinking status (never vs. ever), and smoking status (never vs. ever) were selected from those subjects scoring less than 10.0 in the variance inflation factors, i.e., body fat, height and weight were excluded.

The independent parameters associated with BMD were age (coefficient = -0.002, P < 0.001), physical activity (≤ 2/week vs. ≥ 3/week; coefficient = 0.017, P < 0.001), WHR (coefficient = -0.796, P < 0.001), BMI (coefficient = 0.023, P < 0.001) and smoking status (never vs. ever; coefficient = -0.018, P < 0.001). The variance inflation factors in this regression model were less than 4.01 and the adjusted R² value was 20.7% (Table 3).

Osteoporosis is a cause of significant morbidity and mortality in both postmenopausal women and men [27]. At present, there are no sufficient data for epidemiological research on the bone density of healthy, middle-aged, male workers in Korea.

In a study with 152 healthy, middle-aged men [28], the prevalence of osteoporosis and osteopenia in the lumbar vertebra was 3.9% and 28.3%, respectively. In a study that investigated the bone density of the femoral neck of American men older than 49 years old, the prevalence of osteoporosis was 3~6% and that of osteopenia 28~47% [29]. In a study of Canadian men older than 49 years old, the prevalence of osteoporosis in the lumbar vertebra and femoral neck was 2.9% and 4.8%, respectively, giving a total of 6.6% [30]. In the present study, the prevalence of osteoporosis in the calcaneus was 3.0% and that of osteopenia was 22.8%, according to the diagnostic criteria of WHO.

Several studies have reported physical activity to be a relevant factor of osteoporosis [10, 31, 32]. Rikli & McManics [33] reported that weight load exercise was an effective training form. Hsu et al [34] reported that vigorous physical activity decreased osteopenia by 0.87-fold and osteoporosis by 0.74-fold. Consistent with these results of previous studies, physical activity (people who exercised three times or more a week) in the present study was positively associated with BMD.

Tobacco exposure has been implicated as a risk factor for decreased bone density, which might result in osteoporosis. Similar to previous studies, we observed negative associations between the smoking exposure and BMD. Byron and Jay [35] suggested that serum cotinine, as a marker for tobacco exposure, is a significant risk factor for decreased bone mineral content. In addition, the bone density of smokers may be low because smokers lack calcium uptake or tend to exercise less than never smokers [36]. In a study of 410 people aged from 61 to 73, the density of the lumbar vertebra of smokers was lower than that of never smokers [37]. In middle-aged men, there was a negative correlation between history of smoking and BMD, and this correlation was especially strong in current smokers [17, 20, 38].

Consistent with previous studies, there was a negative association between BMD and age. The most powerful predictor of osteoporosis was increased age [39, 40]. Eastell et al [41] reported that age-induced decrease of bone density could be the result of decrease of kidney function, deficiency of vitamin D, increase of parathyroid hormone, decrease of testosterone or decrease of both calcium uptake and absorption. Moreover, two studies reported that the odds ratio for fracture in men with osteoporosis was 2–2.7 compared to men with normal bone density, indicating that decreased bone density in men was also associated with an increased risk for fracture [42, 43].

BMI and WHR were used as parameters of general obesity and fat distribution, respectively. BMI was positively related to BMD, whereas WHR was inversely associated with BMD. Our results are consistent with those from other studies presenting a positive association of BMD with BMI [16, 17, 19, 41, 44‐46] and WHR [32, 47, 48].

We suggest that BMD of the calcaneus is associated negatively with smoke exposure and increased WHR, but positively with regular exercise and increased BMI.

The study limitation was that the workplace-based participants may not truly represent the general Korean population due to the selection bias known as the healthy worker effect.

This research provided epidemiological data on the BMD of Korean middle-aged men. We suggest that among 40 to 59 year-old male workers, BMD is negatively related to smoke exposure and increased WHR, but positively with regularly physical activity and increased BMI.