Evidence for the role of polyphenols in preventing degenerative diseases, such as cancer, cardiovascular diseases or osteoporosis is emerging. Due to their antioxidant and anti-inflammatory potency, those molecules may actually help limit the damage associated with aging. However, the health effects of those micronutrients, although abundant in our diet, depend on both dietary intake and their bioavailability. Bioavailability differs from one polyphenol to another, possibly explaining why the most abundant are not necessarily those carrying the highest biological activity in target tissues, and are not necessarily the most likely to exert protective health effects.

Polyphenols found in fruit and vegetables

Several thousand polyphenols have been identified in plants, of which several hundred are found in edible plants. They are classified as phenolic acids, flavonoids, stilbenes and lignans, with respect to the number of phenol rings that they contain. Actually flavonoid molecules represent one of the most interesting classes of biologically active compounds with health-related properties. They are divided into 6 subclasses : flavonols (onions, kale, broccoli, blueberries…), flavones (mainly found in parsley and celery), anthocyanins (red wine, cereals and fruit), flavanols (catechins present in apricots, green tea and chocolate and proanthocyanidins found in grapes, peaches, apples, pears but also in several beverages), flavanones (citrus fruit) and isoflavones (found almost exclusively in leguminous plants, mainly soya).

Dietary intake in humans

Fruit and beverages (tea and red wine) constitute the main sources of polyphenols, but in most cases, they contain complex mixtures of these compounds, which unfortunately are often poorly documented. This may explain in part why dietary intake of one isolated polyphenol or of one subclass remains difficult to estimate. However, it has been well accepted that humans ingest around 1 gram of polyphenols per day (Scalbert & Williamson, 2000).

Effects of polyphenols on bone health Mechanisms are poorly understood

While it is established that calcium, vitamin D and micronutrients are essential for bone health, considerable research has also taken place to investigate the role of other compounds in food, in particular polyphenols (mainly flavonoids), which may modulate bone metabolism. However, when compared to other diseases or metabolism, little is known about the effects of polyphenols on bone health, with the exception of soy isoflavones. Evidence of protective effects of isoflavones in humans has been established in observational and intervention studies. Indeed, in Japanese women, consumption of soybeans is associated with greater bone mineral density. Moreover, several animal studies (rodents) have demonstrated the efficacy of supplementing of the diet with genistein, daidzein (aglycon or glycosides) or feeding soy proteins in preventing bone loss induced by ovariectomy. Those effects were attributed to the estrogen-like activity of isoflavones and their potency to bind estrogen receptors, explaining in part why those molecules were considered as possible alternative treatment to prevent osteoporosis. However, the mechanisms involved in these protective effects are poorly documented and not well understood, but available in vitro studies suggest that several mechanisms of action on bone may be considered. Finally, in humans, some intervention studies have shown prevention of bone loss in postmenopausal women with 80-90 mg isoflavones / day while others did not. Nevertheless, in practice, consumption of soy products is relatively low in Western countries as compared to the Asian population whose average daily intake is 20-40 mg/day. Thus, nutritional prevention of osteoporosis based essentially on soy-rich diet (with classic calcium and vitamin D intake) may be limited in Western countries and has led some researchers to focus on other polyphenols which may modulate bone metabolism.

Epidemiological evidence

Some epidemiological studies have suggested associations between consumption of fruit and vegetables and prevention of osteoporosis. Fruit and vegetables contain flavonoids, which are the most abundant polyphenol in our diets. Thus, these anti-oxidant flavonoids may be partially responsible for some of the positive links found between fruit and vegetable intake and higher bone mineral density in adults and children. Several animal studies have shown that consumption of some flavonoids inhibit ovariectomy induced bone loss in rats. For example, rutin, a quercetin glycoside found mainly in onions, but also resveratrol (present in red wine), phloridzin (apples) or euloropein (olive oil) were able to protect bone density when added to the diet. The flavanone hesperedin, found abundantly in citrus fruits, especially oranges, has been shown to inhibit bone loss in ovariectomized mice and rats while citrus juice consumption has recently been demonstrated to prevent bone loss in male orchidectomized rats. One interesting point is that hesperidin is much more abundant than isoflavones in the Western diet. Intakes range from 35-50 kg of orange per person per year. Indeed, in Finland, the flavonone consumption was estimated to be 28.3 mg hesperetin/day (contributing to 50% of total flavonoid intake), closer to the consumption of isoflavones by Asian women. Thus, these molecules, as well as other plant micronutrients may offer a good opportunity for nutrition research to investigate and develop new strategies for osteoporosis prevention.

Finally, consumption of green tea has been associated with higher bone mineral density in humans, suggesting an effect of catechins. The effects of these polyphenols have yet to be assessed in intervention studies.

Despite all evidence shared by animal studies, the mechanisms of action that explain the effects of flavonoids on bone remain unknown due to the difficulty of conducting well-designed studies. Indeed, the metabolites present in blood, resulting from digestive and hepatic processes, usually differ from the native compound and are not all known or available.

Conclusion

Human studies are needed to provide clear evidence of protective effects of polyphenols on bone health, where their potency on fracture risk prevention to date has not been evaluated.