Much of the research on omega-3 fatty acids, found in oily fish, has focused on their benefits for the heart and mental health. But a study on rats adds to mounting evidence of their role in bone health.
The researchers found that diets with a low ratio of omega-6 fatty acids to omega-3 fatty acids minimized the bone loss typically brought on by the low levels of oestrogen in women after menopause.
"Our lab and others have shown that omega-3 fatty acids help promote bone formation," said Bruce Watkins, professor and director of Purdue University's Center for Enhancing Foods to Protect Health.
"We also have shown that higher intakes of omega-6 fatty acids lead to an increased production of compounds associated with bone loss."
The findings, still to be confirmed in humans, could offer an important prevention strategy for osteoporosis, currently second only to cardiovascular disease in terms of global healthcare burden, according to the World Health Organisation.
The condition affects some 200 million people today but the number of sufferers is set to increase steadily with growing numbers of elderly living longer, and obesity adding extra strain on bones.
The new study, in press in the Journal of Nutritional Biochemistry, assessed bone mineral content and bone mineral density in female rats. These measurements are used as indicators of bone mass and bone strength, respectively.
Half the rats in the study had their ovaries removed, leading to a rapid drop in oestrogen levels and therefore mimicking menopause.
"Bone loss due to oestrogen depletion in the adult female rat is very similar to that which occurs in post-menopausal women," said Mark Seifert, a professor of anatomy and cell biology at the Indiana University School of Medicine and the study's co-author.
The rats were fed diets containing different ratios of omega-6 to omega-3 polyunsaturated fatty acids. While both types of fats are essential for human health, diets with a high ratio of omega-6 fatty acids to omega-3 fatty acids are often associated with cardiovascular disease, cancer and inflammatory and autoimmune diseases.
A low ratio of omega-6 to omega-3 fatty acids, however, is believed to promote cardiovascular health, improve memory and, as the current study shows, protect bone health.
After 12 weeks, rats with the lowest ratio of omega-6 to omega-3 fatty acids in their diet experienced significantly less bone loss than rats in the other dietary groups.
"We saw in this study that omega-3 fatty acids are associated with a better blood profile of bone health, and with higher bone mineral density, in the absence of oestrogen," Watkins said.
"A 5-to-1 dietary ratio of omega-6 to omega-3 fatty acids led to a conservation of bone mineral content that we didn't see with a 10-to-1 ratio," Watkins said.
Seifert added that the previously seen anti-inflammatory effects of omega-3s may be responsible for minimizing bone loss with oestrogen deficiency.
Inflammation is caused by a number of compounds, including cytokines. These compounds also stimulate bone breakdown, a natural part of a body process known as the bone cycle in which they are continuously broken down and rebuilt.
Oestrogen blocks some of the inflammatory compounds associated with bone resorption, which may explain why osteoporosis typically progresses after oestrogen levels fall with the onset of menopause.
Watkins' previous studies have shown omega-3 fatty acids may reduce the production of these same inflammatory compounds, accounting for their bone-protective effect.
"Omega-3s change the behaviour of cytokines in a way that is consistent with the role of omega-3s in mitigating cardiovascular disease," he explained.
While the new study results cannot yet be translated to humans, scientists have for some time been warning that the typical Western diet includes more omega-6 fatty acids than in the past, partly due to the reliance on high omega-6 grains like corn to feed livestock, which then alters the fats in the human diet.
The average American dietary ratio of omega-6 to omega-3 fatty acids is approximately 10 to 1.