There is now ample evidence to suggest that smoking adversely affects bones. The bone mass achieved in young adults is of importance for bone density in later life (1). Thus, although the majority of osteoporotic fractures occur in elderly people, the risk may be profoundly affected by events in early life. In young men, the adverse effects of smoking have been demonstrated in both bone mass and bone morphology. Ortego-Centeno et al(2) reported that smoking by healthy young males aged 20–45 years is associated with decreased bone mineral density (BMD). A prospective cohort study showed that not smoking is important in achieving maximum peak bone mass in adolescents and young adults (20–29 years) (3). Lorentzon et al(4) observed a 4–5% decrease in femoral neck and lumbar spine areal BMD and in distal tibia volumetric BMD and cortical tibial area in healthy young male smokers compared with non-smokers, confirming that smoking affects bone at the tissue level. Similar results are obtained for the radius of men aged ~40 years (5). The adverse effects of smoking on bone mass and bone morphology appear to be more pronounced in men than in women (3,6).

Although smoking is considered as a threat to bone health, the meaning of the association and the mechanism remain poorly understood. Tobacco smoke contains >4000 compounds. The metabolism and detoxification of tobacco smoke toxins are essential mechanisms to decrease the adverse effects of smoking. Glutathione S-transferases (GSTs) are an important superfamily of phase II xenobiotic metabolizing enzymes involved in a wide range of these detoxification processes (7). Certain genetic variants of GSTs have been identified. The most commonly investigated variants include the complete deletions (null polymorphisms) of the GSTM1 and GSTT1 genes. GSTM1 and GSTT1 homozygous deletions (null genotypes) result in the absence of the respective functional enzyme activity (8,9,10). Moreover, GSTM1 is absent in 35–60% of the human population, and GSTT1 is absent in 10–65% of individuals (11,12). Thus, GSTM1 or GSTT1 null polymorphism may have a greater effect on health.

The aim of this study was to examine whether the association between tobacco smoking and bone status, as assessed by calcaneus quantitative ultrasound (QUS), differs according to GSTM1 or GSTT1 null polymorphisms in young men. An interaction with genes involved in tobacco smoke detoxification would support the toxicity effect of smoking on bone and help us to understand the related mechanisms.

In our study population, the frequencies of GSTM1 and GSTT1 null genotypes were 54.1 and 48.1%, respectively. The prevalence of smoking was 36.4%. Age and physical activity were significantly associated with QUS parameters (P<0.03) (data not shown). However, we did not detect an association of BMI with QUS outcomes (P>0.2) (data not shown). Smokers had lower BUA (P=0.001), SOS (P=0.01) and BQI (P=0.001) compared with non-smokers (data not shown).

In the population, the genotypic distribution of GSTM1 or GSTT1 did not differ according to smoking status, and GSTM1 or GSTT1 polymorphisms were not associated with QUS outcomes (Table I).

After adjusting for confounding variables, smoking was inversely associated with SOS and BQI (β = −0.28, 95% CI −34.01 to −8.65 and β = −0.30, 95% CI, −16.41 to −4.49, respectively) among subjects with a null genotype for GSTM1, but not among subjects with the allele present (β = −0.02, 95% CI −15.01 to 12.59 and β = −0.09, 95% CI, −8.75 to 3.02, respectively) (P for interaction = 0.04 for SOS and 0.08 for BQI) (Table II). No interaction between smoking and GSTT1 polymorphism was observed (Table II).

In the present study, GSTM1 null polymorphism modified the association between smoking and calcaneus QUS in young men. Individuals with the GSTM1 null genotype who had a smoking habit were more at risk of adverse QUS. These results support the toxic effect of smoking on bone in young men. Although our study population was small, the interaction between smoking and GSTM1 polymorphism suggested in the study adds to the plausibility of a biological interaction between GSTs and tobacco smoke components in bone detoxification.

In this study, GSTM1 and GSTT1 genotype distributions are comparable with those previously reported in other normal Chinese populations (15). GSTM1 and GSTT1 null variants are prevalent in the human population (11,12). Common genetic variants of relevant detoxification enzymes may determine the individual susceptibility to environmental toxicants. In particular, the GSTM1 null genotype has been identified as a susceptibility factor of common smoking-related cancers, respiratory and cardiovascular diseases. To the best of our knowledge, this is the first study to assess the influence of common genetic polymorphisms on the effects on bone induced by smoking. Tracking down the genetic polymorphisms that regulate metabolic pathways of relevance to environmental exposures and diseases may provide better understanding of causation.

We did not find an interaction between smoking and GSTT1 genotype in this study. The reason for this is not clear. Although GSTT1 is predominantly involved in the detoxification process of xenobiotics, certain toxic compounds may be activated through the GSTT1 pathway (16). Similar to our results, previous studies on diseases did not observe an interaction between GSTT1 null polymorphism and smoking, but reported the modification effect of GSTM1(17,18).

GSTs are important in the metabolism processes of many toxic compounds contained in cigarette smoke, and therefore may have a function in the protection of bone. However, which components of cigarette smoke adversely affect bone remains unclear. Nicotine may be involved in the toxic process. In vitro studies have shown decreased proliferation and impaired collagen synthesis in osteoblast-like cells exposed to high nicotine concentration (19,20). The adverse effect of nicotine is further supported by a study on adult rats demonstrating that nicotine may adversely affect bone formation and reduce body storage of vitamin D (21). Another explanation for the mechanism by which GSTs protect bone against smoking may be related to their important roles in defending against oxidative stress (22). It is well-known that smoking increases the level of oxidative stress. Increased oxidative stress is associated with reduced bone density (23). Increased reactive oxygen species stimulate osteoclast formation and activity, and therefore enhance bone resorption (24). Moreover, elevated reactive oxygen species suppress osteoblast differentiation and promote osteoblast apoptosis (25). Oxidative stress may be a pivotal pathogenetic mechanism of bone loss and strength attenuation (26). Individuals with GSTM1 null genotype, due to the absence of corresponding functional enzyme activity, may therefore be more susceptible to the toxic effect of tobacco smoke exposure.

Certain potential limitations in the present study should be considered and our results should be interpreted with caution. First, this was a cross-sectional study and our study population was small in size. Furthermore, our analyses have a potential problem of multiple comparisons. Therefore, subsequent large and prospective studies are required to confirm our results. The gene-gene interaction was not analysed in view of the small sample sizes in the present study. Second, we did not have data from all selected men, which made selection bias possible. Third, we cannot exclude the possibility that our results may be confounded by other genetic variants that are in linkage disequilibrium with the present variant. Certain other confounding factors, such as calcium intake, were not included in this study. In addition, we did not use dual-energy X-ray absorptiometry or peripheral quantitative computed tomography, which may be better methods in certain respects, to assess bone.

In conclusion, GSTM1 deficiency may increase the adverse effect of smoking on bone, as shown by calcaneus quantitative ultrasound in young men, suggesting an interaction between metabolic genes and tobacco smoke in bone detoxification.