It is estimated that one in two women and one in five men over the age of fifty in the United Kingdom will have an osteoporotic fracture¹, and the prevalence of osteoporosis and its related fractures is increasing². Poor nutrition is likely to have a role in the pathogenesis of osteoporosis, and nutritional research on bone health has concentrated on the nutrients calcium and vitamin D; although more recently there has been interest in the role of protein, and the nutrients found in fruit and vegetables. One family of compounds commonly found in fruit and vegetables are the flavonoids.

Dietary Flavonoids

All foods of plant origin potentially contain flavonoids, and over 5,000 different flavonoids have been characterised. However, the majority of these are not commonly eaten in the Western diet, nor have they attracted attention in terms of their health benefits. The flanonols, flavones, catechins, procyanidins and flavanones are dietary flavonoids which are regularly eaten by humans. Fruit contains flavonols (in apples and stone fruit), flavanones (in citrus fruit) and catechins (in red berries). Flavonols are abundant in most vegetables (onions and the Brassica family are rich sources), flavones are found in celery, lettuce and capsicum peppers, and the catechin and procyanidin family have been detected in legumes such as green and broad beans. Flavonols are found in fruit juice and tea, though catechins are the most abundant dietary flavonoids found in beverages due to their high concentrations in tea.

Flavonoid intakes across the world vary and this is to be expected as different populations obtain their flavonoids from different food sources; apples are major contributors to Finnish intakes, and in Italy red wine is important. Principal sources of flavonoids within countries will change as dietary patterns vary, as may be seen in Japan where onions were the principal sources of flavonols and flavones; whereas previously green tea provided the majority of flavonoids to the diet.

Associations between Bone Health and Flavonoid Intakes

Animal models have shown positive associations between flavonoid intakes and better bone health³. Human studies have mostly concentrated on tea, as tea has been reported to protect against hip fracture⁴, however the relationship between tea and bone mineral density (BMD) is unclear with studies reporting conflicting results. The first epidemiological study to examine the influence of dietary flavonoids on BMD and bone turnover was performed in Scotland, and it included over 3,000 postmenopausal women who were all members of a longitudinal study⁵. They had bone mineral density scans of their lumbar spine (LS) and hips and provided urine for measurement of bone resorption markers (deoxypyridinoline (DPD) and pyridinoline (PYD). Diet was assessed using a validated food frequency questionnaire, and intakes of flavonols, flavones, catechins, procyanidins and flavanones were calculated.

Mean total flavonoid intake of the diet was 307 mg/day. Tea was the main contributor of flavonoids to the diet (57%), and fresh fruit and fruit juice also contributed significant amounts of flavonoids to the diet (18% and 12% respectively). A significant association was seen between total flavonoid intakes and BMD at the hip and lumbar spine. Dietary flavanones were shown to have a negative correlation with bone resorption markers; and catechins and procyanidins were associated with annual change in hip and LS BMD.

A sub-study which only included flavonoids from fruit and vegetable sources showed that these were associated more strongly with lumbar spine BMD than flavonoids from all sources. Bone resorption markers were negatively associated with intakes of total flavonoids, flavonols, and flavanones from fruit and vegetable sources, thus showing that dietary flavonoids from fruit and vegetable sources alone were still associated with decreased bone resorption.

Conclusions

This study of over 3,000 women showed associations between greater intakes of dietary flavonoids and improved bone health, however these associations were not strong. Dietary flavonoids may be a marker of a healthy diet and this must lead us to the conclusion that although fruit and vegetables may be good for bone health, and flavonoids are part of this jigsaw, the puzzle is still incomplete.