The Phytoestrogens, Calcitonin and Thyroid Hormones: Effects on Bone Tissue

The skeleton is a metabolically active organ that undergoes remodeling throughout life. This involves a complex process by which old bone is continuously replaced by new tissue. Bone remodeling refers to the sequential, coupled actions of osteoclasts and osteoblasts. In conditions of sex hormone deficiency during advancing age, after the menopause or andropause, the rate of remodeling increases and bone formation is reduced relative to resorption. These alterations can cause microarchitectural deterioration of bone tissues, which increases bone loss as a predisposition to the occurrence of osteoporosis (Rehman et al., 2005). However, in contrast to postmenopausal osteoporosis in women, age-related bone loss in men is less well defined. Numerous studies attest to the importance of estrogen in bone remodeling, evident from the finding that hormone replacement therapy (HRT) administered in a dose-dependent manner effectively prevented bone loss in postmenopausal women (Lindsay et al., 1976, 1984). However, in addition to protective effects on bone, HRT is associated with an increased risk for breast, endometrial, ovarian or prostate cancers (Davison & Davis, 2003; Loughlin & Richie, 1997; Nelson et al., 2002). Therefore, it is important to examine alternative approaches for prevention and treatment of osteoporosis without side effects. It is well known that the incidence of osteoporosis-related fractures is significantly lower in Southern and Eastern Asian women than in Western women (Tham et al., 1998). One possible reason for this difference is a high intake of phytoestrogen-rich plants, which Asian people eat more often than Western people (Ho et al., 2003). As a result, over the past decade a number of clinical trials for prevention of bone loss have assessed the effectiveness of plant derived non-steroidal phytoestrogens found in a wide variety of foods, most notably soybean. Isoflavones, which include daidzein and genistein are a class of phytoestrogens that act like estrogens. Since these compounds bind to estrogen receptors (ERs) and have estrogen-like activity (Branca, 2003), they have attracted much attention because of their potential benefit in the prevention and treatment of osteoporosis. In addition to the phytoestrogen-mediated protective mechanisms against bone loss, recent evidence suggests that daidzein may also act on rat bone tissue through enhancement of thyroid C cell activity (Filipović et al., 2010). Namely, thyroid C cells produce the hormone, calcitonin (CT), which lowers plasma calcium concentration by suppressing osteoclast activity. Synthesis of CT and its release from C cells were decreased in conditions of gonadal hormone