Impact of recreational physical activity on bone mineral density in middle-aged men

Abstract

Physical activity is known to exert beneficial effects on general health status of young, adult and elderly populations. Exercise (aside from genetic, hormonal, nutritional and pathological factors) also influences bone mineral density (BMD). Unfortunately, the association between physical exercise and BMD in adult population is controversial. Our aim was to assess relations between recreational physical activity and BMD in middle-aged men. We performed densitometry and hormonal measurements (total testosterone, free testosterone, dehydroepiandrosterone sulfate, estradiol) in a homogenous group of 38 subjects. Among them, we distinguished 22 who had not engaged in any physical activity, and 16 who had recreationally exercised for about 10 years. Both groups did not differ in regard to hormonal status. Similarly, densitometry did not reveal any statistically significant differences in BMD between both groups of men. Upon our observation, we can hypothesize that recreational physical activity does not affect bone mineral density in middle-aged men.

Keywords:

• Bone mineral density
• middle-aged men
• physical activity

Introduction

Bone mineral density (BMD) is determined by genetic, hormonal, nutritional and pathological factors. Physical exercise is also known to influence BMD [1],[2]. Although the positive role of recreational physical activity on overall human health is doubtless, effects of different loads and intensity of exercise on bone mineralisation are still discussed [3],[4].

The impact of physical exercise on bone mineralisation is most evident between puberty and the age of 25 years. This is a period of an intense growth of both the muscular and the skeletal systems, which is remarkably dependent on hormonal stimuli [4],[5]. In older populations the influence of physical exercise on BMD is less pronounced, although opinions on this matter are controversial [6].

Our objective was to assess bone mineral density in recreationally exercising, middle-aged men.

Methods

The study material comprised 38 men (aged 45–58 years) from within a cohort described in detail previously [7]. They hailed from the same socio-economical background and were brought up in a similar material status; thus, their nutritional habits (average calcium intake in Polish population varies from 300 mg/daily to 500 mg/daily, lower than that recommended by WHO) [8] and leisure activities (including physical exercise) did not differ significantly. All of them attended the same schools in post-war Wrocław.

The examined men were ex-smokers (for at least 10 years), did not suffer from any chronic diseases and had a BMI between 25.8 and 27.2. All of them were academic teachers or held business/administrative positions. They led a hygienic lifestyle and occasionally drank alcohol. They had travelled to work by car daily, for at least 20 years. In each of them presence of andropausal symptoms was excluded, according to Morley and Heinemann scales.

Among the studied subjects, 22 had not engaged in any recreational physical activity since graduation (including holidays). They are referred to as the inactive group. Sixteen men who have exercised on average for 1.0–1.5 hours (swimming, basketball, volleyball or bicycle riding), twice a week, for about ten years prior examination (three men from within this group jogged regularly, two to three times a week) were selected for the active group.

Assessment of bone mineral density was performed by peripheral quantitative computed tomography (pQCT) densitometry (Stratec 960) at the peripheral end of the radius of the non-dominant hand. Cortical, trabecular and whole bone mineral density (g/cm³) were estimated. This method measures bone density with a high degree of precision [9]. It is a good predictor of vertebral fracture status and a reliable indicator of general age-related skeletal deterioration [10],[11]. Bone mineral densities at the distal radius were found to be useful in diagnosis of osteopenia and osteoporosis [12].

Blood samples were obtained between 8.00–9.00 a.m. from the ulnar vein. After coagulation, they were centrifuged for 10 minutes with the power of 2000×g. Obtained plasma was frozen in −20°C until it was analysed. Serum hormone levels were determined with the use of commercial RIA kits: total testosterone (TT), free testosterone (FT), dehydroepiandrosterone sulphate and estradiol (DPC, Los Angeles, USA). The intraserial and interserial variability coefficients were: 6.5% and 6.7% for total testosterone, 6.2% and 8.5% for free testosterone, 4.4% and 7.7% for dehydroepiandrosterone sulphate, 5.8% and 7.4% for estradiol, respectively.

The results were analysed with parametric tests (student t-test) and differences were acknowledged as significant at p < 0.05.

Results

Serum concentrations of hormones in the two studied groups are presented in Table I. We have found no statistically significant differences of total testosterone, free testosterone, dehydroepiandrosterone sulphate and estradiol between groups of physically active and physically inactive middle-aged men.

Table I.  Bone mineral density (BMD) of the radius in the groups of physically active and physically inactive men (mean ± SD). In all cases, differences of BMD between the groups were statistically insignificant (ns).

BMD (g/cm^3)	Physically active (n = 16)	Physically inactive (n = 22)
Cortical	180 ± 40	177 ± 39
Trabecular	1284 ± 213	1188 ± 225
Whole bone	380 ± 57	374 ± 56

Densitometry results are shown in Table II. They revealed no statistically significant differences of cortical (including subcortical), trabecular and whole bone BMD between the two studied groups.

Table II.  Serum concentrations of total testosterone (TT), free testosterone (FT), dehydroepiandrosterone sulphate (DHEA-s) and estradiol (E₂) in physically active and physically inactive men (mean ± SD). In all cases, differences between the groups were statistically insignificant (ns).

Hormone (serum)	Physically active (n = 16)	Physically inactive (n = 22)
TT (ng/ml)	6.8 ± 3.1	6.5 ± 3.8
FT (pg/ml)	18.6 ± 5.2	15.0 ± 5.3
DHEA-S (ng/ml)	1982 ± 685	1943 ± 630
E2 (pg/ml)	28 ± 9.1	32 ± 10.0

Normal range – TT: 1.8–7.7 ng/ml, FT: 7.2–23 pg/ml, DHEA-S: 800–5600 ng/ml, E₂: 0–44 pg/ml.

Discussion

The relationship between hormonal status and bone structure is well documented in both men and women. However, in male long-distance runners with lowered BMC, hormone metabolism turned out to be unrelated to training [13]. In a study of 140 men aged 53–62 years, aerobic threshold, or the change in aerobic threshold, were also not associated with sex hormones or SHBG levels [14]. Hormonal parameters that could potentially affect bone mineral density (as well as other parameters, e.g. symptoms of andropause) did not differ between the active and the inactive men in our study. Although the level of physical activity in the studied subjects was low and did not conform to current recommendations, both time and type of exercise were typical for the Polish male urban population. The amount of time spent on exercising was essentially the most important feature distinguishing these two groups.

The influence of physical activity on bone is due to forces exerted through passive parts of the motor system and muscles tensions during movements. These forces lead to cellular changes, e.g., through piezoelectric mechanisms and subsequent remineralisation. Exercise suppresses osteoclastic bone resorption [15]. Increased cortical thickness seems to be a result of endocortical and periosteal apposition [16]. The positive role of exercise-induced bone tensions on geometrical reorganization of the bone without considerable increases in bone mineral mass has also been postulated. According to this theory, physical activity could enhance the strength of the skeletal system without considerable increases in bone mineral mass.

Human bones are especially sensitive to exercise before puberty. In this period moderate weight-bearing exercise leads to, for example, an increase in femoral BMD. On the other hand, several studies have found lowered bone mineral content in young [17],[18] and older males undergoing physical training (extreme levels of exercise, at more than 200 min/week) [18]. It was hypothesized that bone mineral loss observed in male athletes could be due to calcium imbalance during training [19]. However, fast demineralisation of the skeleton under immobilization proves the role of exercise in bone maintenance most evidently [20].

Recreational physical activity is often recommended as a means of preserving BMD in the adult population. Our observation suggests there is no correlation between recreational exercise and bone mineral density in middle-aged Polish men. This stands in concordance with the outcome of a four-year, controlled, randomized trial in Finnish middle-aged men; long-term, regular aerobic physical activity appeared to have no effect on the age-related loss of femoral BMD [14]. Also, Huuskonen et al. found no correlation between regular aerobic exercise and BMD in men aged 53–62 [14]. Similarly, physical activity was not related to improvement of BMD in men and women studied by Brahm et al. [21].

On the other hand, exercise was found to contribute to bone mass in a prospective Leuven longitudinal study on lifestyle, physical fitness and health (126 males, 27 year follow-up) [22]. Duppe et al. showed that, in men aged 21–42 years, BMD was higher by 9% than in those who spent more time exercising [23]. Nguyen et al. demonstrated that physical activity may have a positive impact on BMD and may reduce the risk of osteoporosis in men and women aged 69 ± 6.7 years [2]. Also, Yoshimura implied that strength exercise (unlike moderate or low intensity aerobic one) might stimulate BMD increase in adults [24]. It is worthy of note that peripheral quantitative computed tomography allows the measurement of the actual densitometric parameters separately in cortical and trabecular compartment of bone structure (and whole bone) [25]. This method is often applied in evaluation of associations between various factors and BMD in human and animal studies [26],[27].

Regarding conflicting reports in the literature and our own results, we see an urgent need for establishing the precise type (strength or resistance) and amount of exercise required for bone mass preservation in middle-aged men. As a decrease in BMD is an expected process during this period of life, preventive measures (such as recreational exercise) should focus on its prevention, as well as reduction of falls.

It should be noted here that regular physical activity improves the function of multiple organs, especially the muscular and the vascular systems. It also warrants hormonal homeostasis. Even if exercise has no direct impact on BMD, better physical performance, improved strength and resistance are essential for keeping general fitness until late senility. Thanks to greater muscle strength and joint flexibility one can avoid falls, bone fractures and their sometimes-fatal consequences.