In our present study, coffee intake was found to decrease the risk of osteoporosis in women regardless of age, but green tea and soda drink were not.
Coffee, green tea, and soda are one of the most preferred beverages by many people and different ingredients in each beverage have variable effects on BMD. Therefore, studies for the association between these beverages and the risk osteoporosis have been conducted for a long time.
In a nationwide study in Sweden, high coffee intake (≥ 4 cups daily) was associated with modestly lower BMD compared to low coffee intake (< 1 cup daily) (P < 0.001) [16]. However, another study showed that highest quartile of coffee intake (≥ 2 cups daily) was significantly associated with increased BMD compared to non-coffee intake group among 4066 postmenopausal Korean women whose mean age was 62.6 years (adjust OR = 0.64, 95% CI = 0.43–0.95; P = 0.015) [24]. In longitudinal study performed in Taiwan, high coffee drinking was associated with higher T-score of BMD in men and premenopausal women (β = 0.237; P = 0.0067 and β = 0.233; P = 0.0355, respectively) [18]. There was no significant association between coffee drinking and BMD among 200 postmenopausal Turkish women over 40 years old [25].
A number of studies for the association between green tea drinking and BMD showed a positive or no correlation. The study of the multiethnic postmenopausal cohort form the Women’s Health Initiative Observational Study showed increased total body BMD with high tea drinking (≥ 4 cups daily) compared to low tea drinking (< 1 cup daily) (mean BMD = 1.029, 95% CI = 1.023,1.036, P = 0.03) [19]. In the study for the women age over 60 years performed in Japan, green tea drinking was associated with higher BMD [26]. However, another study showed that green tea intake was inverse associated with BMD in postmenopausal women (b = -3.42 g/ cm2 per cups/day, standard error = 1.49) [27]. In the study of Minnesota Green Tea Trial, There were no difference in BMD for 12 months (-0.006 ± 0.002 g/cm2 in green tea intake group compared with − 0.003 ± 0.002 g/cm2 in control group; P = 0.49) [20].
These results are thought to be due to the different effects of various components such as caffeine and polyphenol and other materials in coffee and green tea on bone metabolism. Caffeine is main component in coffee and is contained in various amounts depending on the type of coffee ranging from 58 to 259 mg/dose [28]. Caffeine has various actions on calcium metabolism in body and bone cells such as osteoblast and osteoclast. Caffeine has proven to increase the loss of bone mineral density by triggering apoptosis in osteoblasts via activation of mitochondria-dependent cell death signaling and inactivation of the survival signal [10]. Another experimental study has also showed that caffeine enhance the rate of osteoblast apoptosis and decreases osteoblast formation [29]. Calcium level in the body is regulated by modulating calcium intestinal absorption, rates of bone resorption and deposition, as well as calcium reabsorption in the kidneys [30]. In previous studies, different patterns of renal excretion of calcium were shown according to caffeine intake. Heavy caffeine intake increases the urinary excretion of calcium, whereas moderate coffee intake (1 ~ 2 cups per day) does not have a significantly impact on calcium imbalance in postmenopausal women. This result showed that heavy coffee intake was associated with decreasing BMD compared to low coffee intake [31]. In Australian double-blind clinical study, high dose caffeine intake (800mg) results in doubled the amount of calcium lost in the urine [32].
Oxidative stress involves in the pathogenesis of bone loss by osteoblast apoptosis and favoring osteoclastogenesis [33]. Coffee and green tea contain polyphenols of chlorogenic acid and catechin, respectively which have antioxidant and anti-inflammatory activity. Chlorogenic acid in coffee is shown to have inhibitory effects on osteoclastogenesis by suppressing the expression of NFATc1 (Nuclear Factor Of Activated T Cells 1), a key transcription factor for the induction of osteoclastogenesis [34]. Catechin enhances osteogenic differentiation of mesenchymal stem cells by increasing osteoblastic proliferation and differentiation, and attenuating osteoclastogenesis by enhancing apoptosis of osteoclast and prohibiting bone resorption [35].
It is well known that estrogen in women has a protective effect on BMD and estrogenic effect of coffee can modulate bone metabolism [36]. Trigonelline which is present in large amount in coffee bean is known as phytoestrogens which elicit estrogenic response [37, 38]. It was reported that trigonelline was significantly related to coffee intake and also significantly related to BMD [39]. Schliep et al. demonstrated the difference in the serum estrogen level in various races according to caffeine intake. Asian women who consumed an average of 200 mg or more of caffeine a day had elevated estrogen levels, whereas white women who consumed the same amount of caffeine had slightly lower estrogen levels than women who consumed less [40].
Our study showed that coffee intake was associated with lower OR for osteoporosis in women with all age. These results can be presumed to be due to the anti-inflammatory, antioxidant or estrogenic effects of components other than caffeine described above. Unlike men, in women, menopause and the accompanying loss of estrogens are associated with the progressive decrease in BMD [41]. If some of the components contained in coffee show a protective effect on BMD as a phytoestrogen, it can be assumed that coffee intake can have a protective effect for BMD. Moreover, as reported by Schliep et al., estrogen concentration increased after caffeine intake in Asian women, unlike White women, suggesting racial differences in the effect of caffeine on estrogen. However, coffee intake was associated with a reduced incidence of osteoporosis in women younger than 53 as well as women older than 53, mostly postmenopausal women. Since the risk of osteopenia increases after the age of 40 in women as estrogen decreases, it can be assumed that BMD is more affected by estrogen in women than in men [42]. More researches for the association between coffee intake and the risk of osteoporosis related to estrogen metabolism are needed.
Our study showed that green tea intake was not associated with the risk of osteoporosis and it was consistent with the study results of dostal et al.. They reported that favorable effect of green tea on BMD was observed in individuals with low BMD, not with high BMD.
Soda is known to contain high level of phosphoric acid which interferes with calcium absorption and enhancing loss of calcium [43]. Furthermore, excessive intake of soda can reduce the intake of healthy drinks such as milk, leading to a lower intake of calcium [44]. The calcium/phosphorus imbalance caused by the soda drink intake may increase the risk of osteoporosis and osteoporotic fractures [14, 45]. However, in the study of American-Indian postmenopausal women, daily soda drink intake was not associated with increased risk of osteoporosis [21]. Our study showed that soda drink intake was not associated with the risk of osteoporosis. It is well known that a high BMI reduces the progression of bone loss and excessive soda drink intake is associated with weight gain and obesity [46, 47]. It can be hypothesized that this fact offsets the effect of reducing bone mineral density by increasing the risk of weight gain caused by soda drink intake.
Some limitations exist in this study. First, the diagnosis of osteoporosis and participants’ information was obtained based on self-reported questionnaires by interviewers. These diagnostic methods using questionnaires may cause recall bias and underestimate or overestimate the diagnosis. However, the interview was conducted through well-structured questions by a well-trained interviewer and self-reported disease history using questionnaires is widely accepted in large-scale epidemiologic studies Second, because the degree of coffee, green tea, and soda drink intake was classified according to the frequency of beverage consumed using the questionnaire, it may contain various amount of caffeine depending on the type of beverage, and other ingredient such as sugar, milk, or cream powder may be added. Previous studies showed that the frequency of caffeine intake is important when determining the relationship between coffee intake and bone health, and the frequency of caffeine intake through the questionnaire was used [18, 48]. Third, because our study is a cross-sectional study, there may be no clear causal relationship between coffee, green tea, and soda drink intake and osteoporosis. In spite of the above limitations, our study has strengths. This study is the first large-scale study conducted in Korea to investigate the association between commonly consumed beverages such as coffee, green tea, and soda and osteoporosis on a population basis. Large-scale study can represent its generalizability to the Korean population. We used the KoGES HEXA data used for large epidemiologic studies conducted by the Korean government, and calculated accurate and reliable results from data collected by trained and quality-controlled surveyors and well-trained interviewers.