Tea flavonoids for bone health: from animals to humans

Laboratory animals

There are two major types of osteoporosis: primary osteoporosis and secondary osteoporosis.18 Primary osteoporosis includes estrogen deficiency-induced bone loss in females and aging-induced bone loss in females and males with advanced age.18 Secondary osteoporosis includes juvenile osteoporosis, localized osteoporosis, drug-induced osteoporosis, and disease-induced osteoporosis.18

Osteoporotic animal models should reflect the underlying clinical scenarios, be that primary or secondary osteoporosis.19 Animal models are widely used to investigate the pathogenesis of osteoporosis and for the preclinical testing of antiresorptive drugs.19 Especially, rats and mice are increasingly used to investigate the pathophysiology of osteoporotic fractures and to develop prevention and treatment strategies.19 The goals of using animal models to assess the impacts of bioactive compounds on skeletal health are to maintain bone strength through reducing bone matrix loss or microstructural deterioration. These can be mediated through suppression of bone turnover rate (especially resorption rate), elevation of anabolic local factors and systemic hormones, and reduction of catabolic bone local factors and systemic hormones, as a result of improved bone mechanical strength.15 Published animal studies have shown bone loss and microstructural deterioration in a variety of bone loss animal models due to estrogen deficiency,20 ,21 testosterone deficiency,22 aging,20 ,22 chronic inflammation,23 obesity,24 caloric restriction,25 binge drinking,26 and others.27

The osteoprotective effects of black tea, green tea, and their flavonoids have been demonstrated in estrogen deficiency-induced bone loss animal models (through ovariectomy, OVX). For instance, Das et al28–30 found that black tea extract has preservative and restorative effects against OVX-induced mononuclear cell oxidative stress and associated bone loss in rats, as shown by decreased numbers of active osteoclasts and bone turnover rate, and increased bone ash mineral content and strength. Recently, Das et al31 further reported that black tea extract may be a prospective adjunct for calcium supplement to prevent early menopausal bone loss in female OVX rats. The efficacy of black tea extract in maintaining skeletal health is close to that of 17 β-estradiol, which may be attributed to its phytoestrogenic efficacy in regulating the intestinal absorption of calcium, the mechanisms of which may be either modulating the activities of mucosal calcium transferring enzymes or a direct action on intestinal estrogen receptor or both.31 On the other hand, Karmakar et al32 demonstrated that black tea extract significantly ameliorated the high-fat diet-induced skeletal dysfunction in urinary, serum, and bone parameters as well as calcium homeostasis in wistar rats.

In an estrogen deficiency-induced bone loss model, Shen et al20 ,21 reported that when the 14-month-old F344xBFN1/NIA OVX and sham female rats were administered with green tea polyphenols (GTP), consisting of epigallocatechin-3-gallate (EGCG), epicatechin-3-gallate (ECG), epicatechin (EC), epigallocatechin (EGC), and catechin, at 0.1% and 0.5% (weight/volume, wt/vol) in drinking water for 16 weeks, the animals had increased urinary ECG and EC concentrations in a dose-dependent and time-dependent manner. GTP supplementation mitigated bone loss (based on BMD measurement) and deterioration of bone microarchitecture (based on histomorphometric analyses), resulting in improved bone strength (based on bone mechanical properties).20 ,21 Such osteoprotective effect of GTP in OVX animals was mediated by an increase in antioxidant capacity (ie, glutathione peroxidase activity, superoxide dismutase-1, and ATP synthase protein expression), a decrease in oxidative stress DNA damage (ie, 8-hydroxydeoxyguanosine, 8-OHdG), and a decrease in estrogen-associated protein expression of catecho-O-methyltransferase.20 ,33 ,34 In the same OVX animal study, the results of blood chemistry and liver activities showed that the dosages of GTP at 0.1% and 0.5% (about 1–2 and 4–6 cups per day equivalence in human consumption, respectively) did not cause any adverse effect.20 In addition, Chen et al35 corroborated the finding of Shen et al by reporting that intraperitoneal treatment with EGCG at 3.4 mg/kg/day for 3 months can mitigate bone loss and improve bone microarchitecture in OVX rats, probably through increasing the expression of bone morphogenetic protein 2. In androgen-deficient aged rats, a model of male osteoporosis, Shen et al22 concluded that GTP supplementation (0.5% wt/vol in drinking water) attenuated trabecular and cortical bone loss through increasing bone formation while suppressing bone resorption in male rats, due to GTP's antioxidant capacity.

In a systemic chronic inflammation lipopolysaccharide (LPS)-induced bone loss model of adult rats, Shen's team demonstrated GTP supplementation at 0.5% wt/vol for 12 weeks resulted in higher values for femur bone mineral content (BMC), BMD, and serum osteocalcin (bone formation marker), but lower values for serum tartrate-resistant acid phosphate (TRAP, bone resorption marker), as a result of improved bone strength, through reducing oxidative stress-induced damage and inflammation.36 ,37 In conjunction with GTP and α-calcidol on LPS-induced bone loss model, there was a synergistic osteoprotective effect of both active components on bone properties including increased bone mass, restored LPS-induced detrimental changes in femur, proximal tibia and endocortical tibial shaft, sustained bone microarchitecture and femoral strength, and decreased serum TRAP.38 ,39 Such syngestic effect on bone health was mediated through suppression of urinary 8-OHdG and tibia mRNA expression of tumor necrosis factor-α (TNF-α).38

In a high-fat diet-induced bone deterioration model, Shen et al40 ,41 found GTP supplementation benefited body composition (as shown by higher percentage of fat-free mass) and improved bone properties (as shown by improved BMD, bone microarchitecture, and strength) in obese rats through suppressing bone formation and erosion, resulting in a larger net bone volume. Interestingly, such a similar bone-protective impact of GTP in obese rats was also sustained in the obese rats after switching to a caloric-restricted diet, as shown by increased femoral mass and strength, trabecular thickness and number, and cortical thickness of tibia, as well as decreased trabecular separation, formation rate, and eroded surface at proximal tibia, and insulin-like growth factor-I and leptin. Shen et al25 noted significant interactions (high-fat diet vs caloric-restricted diet×no GTP vs GTP) on osteoblast surface/bone surface, mineral apposition rate at periosteal and endocortical bones, periosteal bone formation rate, and trabecular thickness at femur and lumbar vertebrate.

In a binge alcohol-induced bone deterioration model, Shen et al26 demonstrated that 6 weeks of binge alcohol administration lowered BMC, BMD, and bone strength in femura; reduced proximal tibial trabecular bone volume and lumbar vertebrae; and decreased cortical thickness at the tibial mid-diaphysis. Supplementation of GTP (0.5% wt/vol) in the drinking water increased femoral BMD and tibial cortical thickness at the mid-diaphysis. There was an interaction between the alcohol administration and GTP dosage, and such interaction impacted serum TRAP and several parameters of bone strength, which reduced the negative effect on alcohol-induced bone modeling. Importantly, GTP supplementation in drinking water improved overall bone quality in young binge alcohol-treated male rats through suppressing bone turnover rate.26

Several groups have investigated the osteoprotective impacts of green tea on alveolar bone resorption in the model of periodontal disease. For example, Nakamura et al42 reported that oral administration of green tea catechin suppressed LPS-induced alveolar bone resorption in BALB/c mice by lowering interleukin-1 β production or by directly inhibiting osteoclastogenesis. Gennaro et al43 also demonstrated that green tea intake attenuated TNF-α expression in periodontal disease and mitigated alveolar bone resorption in type 1 diabetic rats. Jin et al44 further demonstrated that EGCG inhibited titanium particle-induced osteolysis in mouse calvaria by suppressing TNF-α expression and osteoclast formation.

It was noted that green tea extract may act as a pro-oxidant at high doses harming bone, instead of an antioxidant benefiting bone. For instance, Iwaniec et al45 reported that supplementation of green tea extract (1% and 2%, wt/wt in diet) for 6 weeks was harmful to bone growth in growing male mice, supported by an evidence of decreased bone accumulation accompanied by shorter bone length, reduced cortical bone volume and thickness, and compromised BMC.

Based on the results of animal studies, moderate intake of tea has shown to benefit bone health as shown by mitigation of bone loss and microstructural deterioration as well as improvement of bone strength and quality, suggesting a potential prophylactic role of tea and its flavonoids in human bone health, especially bone health of postmenopausal women with high risk for osteoporosis.

Epidemiological observational studies

Epidemiological studies have reported positive, insignificant, and negative impacts on BMD at multiple skeletal sites and risk of fracture in humans with habitual tea consumption.15 ,16 For example, Hegarty et al46 reported that after adjusting for age and body mass index, the mean BMD of postmenopausal women (aged 65–75 years, N=1256) at the lumbar spine, greater trochanter, and Ward's triangle was significantly higher in tea drinkers than those in non-tea drinkers. A positive relation between tea drinking and BMD was also reported among Canadian postmenopausal women,47 British older women (age 65–76 years),46 Australia older women (age 70–85 years),48 Danish perimenopausal women,49 Japanese elderly women (age≥60 years),50 Chinese Women,51 and Asian older men and women.52

In an Investigation of Prevalence of Postmenopausal Osteoporosis in Turkey (IPPOT study), Hamdi Kara et al53 reported the T-scores of women who consumed tea on a regular basis tended to be higher than non-consumers (−1.51±1.68 vs −1.09±1.66; p=0.070), when smokers, those who received hormonal therapy, and those >65 years were excluded. In a cross-sectional study at an osteoporosis outpatient clinic, Muraki et al50 reported that patients with a habit of green tea drinking had significantly higher BMD at the lumbar spine than those without the habit, after adjusting for age, body mass index, and other variables related to lifestyle. In a case study, Wang et al51 reported that consuming at least one cup of oolong tea (a traditional Chinese black tea) per day was associated with a higher BMD at the greater trochanter and Ward's triangle. In Keramat et al54 study on Iranian women and Indian women, it was concluded that regular consumption of seven cups of tea per day and more was a significant protective factor in Iranian women. In a 4-year prospective longitudinal study, Devine et al48 showed mitigated loss of BMD in tea drinker relative to those in non-tea drinker based on a 24-hour dietary recall.

In contrast to positive findings in postmenopausal women with tea drinking habit, no association between habitual consumption of tea and BMD was reported in Iranian men (age 20–76 years),55 young Greek men (age 18–30 years),56 and middle-aged men (age 45–65 years).57 In general, onset of bone loss occurs in older men (age 75 years and older), not in young men, explaining the lack of positive relationship between tea consumption and BMD in young men. An inverse relation between tea consumption and BMD was found in a study among premenopausal and perimenopausal women (age 50–60 years) in the USA,52 but premenopausal and perimenopausal women may still have estrogen protecting against bone loss.

In terms of risk of hip fracture, the Mediterranean Osteoporosis (MEDOS) Study showed that tea drinking was associated with 30% reduction in the risk of hip fractures in women58 and men59 over 50 years of age. A prospective analysis over 4 years in a study suggested that tea drinking is associated with preservation of hip structure in elderly Australian women.48 In a prospective cohort study of women aged >75 years (n=1188), Myers et al60 recently reported that higher intake of black tea and particular classes of flavonoids were associated with lower risk of fracture-related hospitalisation in elderly women at high risk of fracture. On the other hand, Chen et al61 reported that based on a longitudinal follow-up study, tea consumption in the USA resulted in such a weak effect on BMD that was unlikely to have any significant impact on fracture risk among the US postmenopausal women. Tavani et al62 found that there was no association between tea consumption and risk of hip fracture in Italian women after adjusting for age, menopausal status, education, smoking status, total alcohol drinking, and calcium intake. Insignificant impact of tea consumption on fracture was also reported in Canadian women (50–84 years)63 and Chinese elderly men and women >9064 and 55–80 years of age.65 In contrast, a case–control study reported that tea drinkers have a higher risk of hip fracture (OR 22.8; 95% CI 3.73 to 139.43) in an Indian urban population.66

The majority of these questionnaire-based epidemiological observational studies are lack of objective data characterizing tea consumption (such as categories of tea, method of tea drink preparation, frequency and volume of tea consumption, length of habitual tea consumption, and quantitative characterization of components (eg, GTP, EGCG, ECG, caffeine) in tea), incomplete adjustment of the confounding factors of lifestyle, and different skeletal sites for BMD measurement.