In this current meta-analysis, a total of 9 studies with 147,950 individuals were included. The quantitative synthesis of these observational studies demonstrated that tea consumption showed neither protective nor harmful effect on the incidence of fractures. Meanwhile, the subgroup analysis by the study designs, study sites, genders and fracture types displayed no statistical significant results. The cumulative meta-analysis showed that an inverse but not significant association between tea intake and risk of fractures since 1992. No significant association was detected in the dose–response meta-analysis.
Fractures are producing more and more physical ills and economic loss in the whole world. The studies on the detection of the risk factors of the fractures would provide us better knowledge on the prevention of the incidence of fractures. Several lifestyles and dietary factors, such as the tobacco exposure, coffee intake, calcium and vitamin D intake, were associated with the risk of fracture as mentioned above. Tea is regarded as the most frequently consumed beverage and the protective or harmful effects on the health are substantial public issues. As a modifiable factor, the understanding of the relationship between tea drinking and risk of fractures would provide potentially effective preventative measures of the fractures.
The association between tea consumption and BMD has been studied in several studies. However, PA collected the physical and lifestyle data from 62 postmenopausal women and the intake of tea was reported to be associated with increased BMD measurements . Hegarty VM et al. compared the BMD measurements of 1,134 tea drinkers and 122 non-tea drinkers; and higher BMD was detected in the tea drinkers . However, as a beverage that contains caffeine, tea drinking was also considered to be risk factors of bone loss. In animal models, high doses of caffeine could induce calcium loss and influence the normal development of bone . Nowadays tea, regarded as a beverage with multiple nutrients and caffeine, is a positive factors in the bone health .
Several epidemiological studies were also conducted to study the association and risk of fractures directly. A cross-sectional study in Sichuan China observed the association between habits of smoking, alcohol consumption, tea consumption and exercise and osteoporotic fracture among very old people. Among all the lifestyles, tea consumption was reported to be unrelated with the incidence of osteoporotic fracture . Besides, the observational studies, including cohort and case–control studies, were conducted as well to detect the etiological effect of tea consumption on the incidence of fractures. In this current meta-analysis, we studied the association between tea drinking and risk of fractures. Through pooling the published observational studies together, we found that tea consumption was not associated with the risk of fractures. This result was similar with the most included studies. Among all the included studies, only a case–control study demonstrated that tea drinking was a risk factor of the incidence of fracture . Through the comparison of 100 cases and 100 age and sex matched controls, tea drinkers was reported to have a higher risk of hip fractures (OR, 22.8; 95% CI, 3.73-139.43). The ethnic and regional differences might explain part of the result differences. However, the relative smaller amount of participants would result in more unsteadiness of the result. Contrarily, three case–control studies demonstrated that tea consumption produced a protective effect on the incidence of fracture [25, 30, 31]. However, these studies might not be powerful enough to influence the general conclusion in this meta-analysis.
In the subgroup analysis by the study designs, no association was detected between tea consumption and risk of fractures in cohort study group and case–control study group. The cohort study design could avoid the potential recall bias and would provide more credible conclusions. In our meta-analysis, the results of the subgroups from both study designs were accordant and thus the conclusion was quite substantial. Besides, the sensitivity analysis when the studies with high quality were included showed a slight but not significant result (OR, 0.88; 95% CI, 0.78-1.00). Moreover, the cumulative meta-analysis showed that the association between tea drinking and incidence of fracture was not statistically significant; however, a potential trend that indicated the protective effect from tea consumption couldn’t be ignored.
The data about the tea or tea extract on the bone metabolism is abundant. The tea contains several kinds of antioxidants, including flavan-3-ols, flavanols and flavones, and these polyphenol produced multiple effects on the bone metabolism. At a low concentration, polyphenol would increase osteoblast proliferation, while at a high concentration, polyphenol would inhibit its proliferation inversely . Oka Y et al. reported that polyphenols, particularly epigallocatechin-3-gallate (EGCG), which were extracted from the green tea, could inhibit MMPs expression and activity. Meanwhile the tea polyphenols could inhibit osteoclast formation and differentiation in vitro . Choi EM reported that treatment with catechin increased alkaline phosphatase activity which could indicate the activity of osteoblastic and reduced osteoblastic apoptosis induced by the tumor necrosis factor (TNF) . Oxidative stress is regarded as a pivotal physiopathological process in the age-related bone loss . Das AS et al. showed that the black tea extract would protect the mononuclear cells from the oxidative stress and thus slowed the progression of bone loss . Apart from the polyphenol, the other contents in tea, such as fluorine and caffeine, also play important roles in the bone health .
To our best knowledge, this is the first meta-analysis involving the relationship between tea consumption and risk of fracture. There are some strengths of this work. Firstly, a comprehensive literature search and advanced consulting the relevant references make available included in this meta-analysis. Secondly, the included studies are mainly of high quality and the sensitivity analysis showed that the results are quite tough, and thus the conclusions are credible.
However, there are some limitations could not be ignored in this study. Firstly, most of the studies followed a case–control study design, and therefore there were recall and selection bias which are inherent to retrospective studies. Even through the subgroup analyses by the study designs were conducted, the efficiency was limited by the absence of enough cohort studies. Secondly, tea is mainly grouped into green tea and black tea. The functional contents and effects were not identical for there two tea subtypes and thus the potential bias should be noted. The following epidemiological might need to take this into consideration.