Green tea and bone metabolism

Abstract

Osteoporosis is a major health problem in both elderly women and men. Epidemiological evidence has shown an association between tea consumption and the prevention of age-related bone loss in elderly women and men. Ingestion of green tea and green tea bioactive compounds may be beneficial in mitigating bone loss of this population and decreasing their risk of osteoporotic fractures. This review describes the effect of green tea or its bioactive components on bone health, with an emphasis on (i) the prevalence and etiology of osteoporosis; (ii) the role of oxidative stress and antioxidants in osteoporosis; (iii) green tea composition and bioavailability; (iv) the effects of green tea and its active components on osteogenesis, osteoblastogenesis, and osteoclastogenesis from human epidemiological, animal, as well as cell culture studies; (v) possible mechanisms explaining the osteoprotective effects of green tea bioactive compounds; (vi) other bioactive components in tea that benefit bone health; and (vii) a summary and future direction of green tea and bone health research and the translational aspects. In general, tea and its bioactive components might decrease the risk of fracture by improving bone mineral density and supporting osteoblastic activities while suppressing osteoclastic activities.

Introduction

Osteoporosis is a degenerative bone disease characterized by low bone mass and microarchitectural deterioration of bone tissue that leads to bone fragility and an increased susceptibility to fractures, especially in the hip, spine, and wrist [1]. Osteoporosis research has also reported some gender disparities. Women are 4 times more likely than men to develop osteoporosis because of a decrease in their estrogen levels after menopause in conjunction with generally lighter and thinner bones [2]. The rapid decrease in bone mineral density (BMD) that occurs in the first 3 to 5 years immediately after menopause and the slower decrease that continues throughout the remainder of a woman's life markedly increase the risk of hip or vertebral fracture, which is a major cause of morbidity and mortality in older women [2], [3]. More than half of postmenopausal women will experience a bone fracture as the result of osteoporosis [4]. Similarly, 1 of 4 osteoporosis patients is male, and 30% of hip fractures occur in men [5]. The pathogenesis of osteoporosis in men is still poorly understood; it has been reported that approximately one third of osteoporotic men have an idiopathic disease [6].

Hip fracture is the most severe consequence of osteoporosis, leading to reduced activities of daily living, lowered quality of life, and increased mortality of patients [7], [8]. As the population ages worldwide, osteoporosis has become a serious health threat in many countries [7], [8]. It is estimated that almost 44 million American women and men 50 years and older have osteoporosis and low bone mass. By the year 2010, it is estimated that more than 52 million women and men in this same age category will be affected, and if current trends continue, the number will climb to more than 61 million by 2020 [9]. The economic costs due to hip fractures have increased tremendously in the past decade and are predicted to grow [10], [11], [12].

To predict the risk of fractures, clinical application of bone densitometry, such as dual-energy x-ray absorptiometry, is generally used to measure BMD at the spine, hip, and femoral neck. This method is also used to monitor the natural progression of diseases that affect BMD, or the therapeutic response to osteoporosis-specific treatments 1 to 2 years after beginning treatment [13], [14], [15], [16]. Low areal BMD is the most important risk factor for hip fractures [17].

Recent research has suggested that BMD is positively associated with tea consumption, which may optimize bone health. The bioactive components in tea may benefit bone health in terms of maintaining higher BMD [18], [19], [20], [21], [22], [23] and reducing the risk of fracture [24], [25]. Specifically, green tea seemed to benefit bone health more than other kinds of tea (eg, black and oolong), which may be due to decreased oxidative stress [26], [27], increased activity of antioxidant enzymes [26], and decreased expression of proinflammatory mediators [26], [27]. In this review, we discuss the beneficial osteoprotective effects of green tea and its bioactive components. In addition, the possible mechanisms of osteoprotection of green tea along with its bioactive components are discussed.