Reduction in estrogen level is the major cause of bone loss in postmenopausal osteoporosis [22
]. The ovariectomised rats are the recommended animal model for investigating preclinical therapies for postmenopausal osteoporosis [23
], since the bone changes in ovariectomy and postmenopausal state are similar. The reduction in endogenous estrogen levels in both situations causes an increase in the bone turnover which leads to enhanced bone loss and a decrease in the bone mineral density [24
Postmenopausal osteoporosis is associated with oxidative stress and inhibition of the antioxidant defense system [22
], resulting in the imbalance between osteoblast and osteoclast activities. Previously, we demonstrated that virgin coconut oil significantly improved the bone histomorphometric parameters, including the trabecular number, trabecular thickness, and trabecular separation in ovariectomised rats (unpublished data). The positive findings in the histomorphometric study in the bone directed us to further investigate the effect of VCO on oxidative status in the bone of osteoporotic rat model as an attempt to understand the role of VCO in enhancement of the body defense system against oxidative stress and free radicals.
The results of the present study showed significant improvement in the bone antioxidant status after VCO supplementation by a significant increase in the levels of glutathione peroxidase in OVX-VCO group compared to OVX-control group, with an increased trend of SOD levels. The positive effect on the antioxidant enzymes was supported by a low level of MDA in OVX-VCO group. In the same way, the significant increase in the levels of GPX and SOD in the ovariectomised-control rats represented the endogenous release of antioxidant enzymes, in response to oxidative stress and the high free radical activity in bone. Nonetheless, this elevation in the antioxidant enzymes was unable to suppress the lipid peroxidation which explained the significantly high level of MDA in OVX-control group.
Our results showed some similarities with the studies involving postmenopausal osteoporotic women, whereby the antioxidant parameters such as the total antioxidant capacity (TAC), plasma activity of SOD, catalase, and glutathione reductase were significantly increased compared to normal women, but this physiological elevation in the antioxidants was not enough to prevent the development of osteoporosis [26
]. In contrast, several studies indicated that the activity of GPX in the plasma was significantly reduced in postmenopausal women compared to healthy women [28
]. Glutathione peroxidase has an important role in reducing lipid hydroperoxide, which breaks down the oxidation chain and suppresses the free radicals release [30
]. Dreher et al. [31
] reported that the reduction in GPX expression could interfere with osteoblast functions and enhance the bone loss leading to osteoporosis. In addition, GPX expression by osteoblast was increased in response to oxidative stress [32
The role of VCO in preventing oxidative stress was also manifested in other organs as well. VCO was shown to have superior suppressive effect on microsomal lipid peroxidation compared to copra oil and groundnut oil [14
]. VCO stimulated the antioxidant enzymes activity and decreased the MDA and glutathione levels in healing wounds. This inhibition in lipid peroxidation promoted fibroblast proliferation, neovascularization, and healing process [33
]. In addition, blending of VCO with groundnut oil or olive oil was proven to be effective in inhibiting LDL oxidation, and stimulating the activity of hepatic antioxidant enzymes [34
The antioxidant activity of VCO is due to the high composition of polyphenol compounds in the oil [11
]. Marina et al. estimated the total phenolic content of VCO to be in the range of 7.78–29.18
g oil, which is significantly higher than the refined, bleached, and deodorized coconut oil [35
]. The major polyphenols in VCO are ferulic acid and p-coumaric acid [35
]. Seneviratne and Dissanayake (2008) also detected the presence of ferulic acid, p-coumaricacid, and caffeic acid in the commercial and traditional VCO [36
]. Polyphenols are stronger as antioxidants than vitamins C and E in vitro
on the molar basis [37
The antioxidant properties of ferulic acid have been established. Ferulic acid belongs to phenoxy carboxylic acid family [38
]. Toda et al. [39
] have proven that ferulic acid has the ability to scavenge the superoxide radical and suppress the lipid peroxidation induced by superoxide anion. Superoxide radicals can enhance bone resorption by degrading matrix proteins, making the bones weak and easily digested by enzymes [5
]. Ries et al. [40
] have reported that a superoxide radical scavenger such as Desferal-manganese complex can reduce superoxide production and decrease bone resorption by osteoclast.
The effects of ferulic acid and superoxide dismutase as antioxidants were equal in magnitude, and this characteristic made it superior to caffeic acid and p-coumaricacid as an antioxidant [39
]. In addition, the effect of ferulic acid as inhibitor of lipid peroxidation was similar to the effect of α
]. Castelluccio et al. [41
] reported that ferulic acid was more potent as an antioxidant against LDL oxidation than ascorbic acid. It seems that VCO derives most of its effects from the free-radical scavenging and antioxidant properties of ferulic acid.
The antioxidant power of ferulic acid is due to its ability to effectively end the terminal radical chain reactions, since any free radical colliding with ferulic acid molecule can easily extract a hydrogen atom from the phenolic hydroxyl group to form a phenoxy radical which is considered a highly stable compound [38
]. This phenoxy radical is unable to initiate or propagate the reactive chain reaction. This stability belongs to easy formation and lack of reactivity of phenoxy radical. Moreover, there is extended conjugation in the unsaturated side chain of phenoxy radical, and the unpaired electron may not be attached to oxygen atom, but it can move throughout the entire molecule [38
There are few studies that investigate the effects of phenolic acids on bone loss. Sassa et al. [42
] reported that ferulic acid enhanced bone remodeling process by stimulating osteoblasts to compensate for the bone loss by osteoclasts, and ferulic acid raised serum level of estrogen, progesterone in postmenopausal osteoporotic rat model. Zych et al. [43
] showed that caffeic acid and p-coumaricacid increase serum estrogen levels in estrogen deficiency rat model. The authors suggested that phenolic acids such as caffeic acid may affect the metabolic pathway which regulates the extraovarian estrogen release [43
]. Folwarczna et al. [44
] investigated the effects of phenolic acids on bone loss in postmenopausal osteoporotic rat model, and they reported that p-coumaricacid had a positive effect on the bone mass/body-mass ratio and bone mineral mass/body-mass in bone. On the other hand, phenolic acids have no effect on bone mineral mass/bone mass in the bone of ovariectomised rats.