Implications of dietary a-linolenic acid in bone health.

Recent evidence implies the benefit of u-3 polyunsaturated fatty acids in bone health. Although eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), present in fish oil, have been extensively researched, much less is known about the influence of alpha-linolenic acid (ALA; present in flaxseeds), a metabolic precursor of EPA and DHA, on bone. Our objective was to evaluate the published literature and distinguish between the individual effects of flaxseed oil and flax lignans on bone to elucidate the exact role of ALA in skeletal biology. The search was conducted in several databases resulting in 129 articles of which 30 were eligible for inclusion in this review. The studies showed that consumption of whole flaxseeds did not lead to a marked improvement of osteoporotic bones in humans and animals. However, when combined with estrogen therapy, flaxseed supplementation offered an extra benefit to bone in animal models.  Similar results were found in studies conducted with flaxseed oil (predominantly ALA), but the favorable role of flaxseed oil was more obvious in various pathologic conditions (kidney disease, obesity with insulin resistance), resulting in improved bone properties. In contrast, despite a marginal estrogenic effect, the consumption of flax lignans resulted in little benefit to bone and the effect was limited to early life of females only in animal models. Based on the available studies, it could be concluded that supplementation with flaxseeds may contribute to some improvement in osteoporotic bone properties but the bone-protective effect may be attributed to ALA, not to the lignan fraction of flaxseeds. (Author's Abstract)
There is limited research published regarding the role of ALA on bone. Flaxseeds and flaxseed oil are good sources of ALA and most studies in humans and animals have been performed using these foods instead of pure ALA. The objective of this review was to systemically evaluate the published literature and distinguish between the individual
effects of flaxseed oil and flax lignans on bone to elucidate the exact role of ALA in skeletal biology. Based on this review, the authors concluded that for older adults (particularly postmenopausal women), supplementation with flaxseeds or flaxseed oil appears to have a marginal benefit to bone, possibly by inhibiting bone resorption. In most animal studies (regardless of growing stages or physiologic conditions), feeding flaxseed/oil resulted in improved lipid profiles in plasma, bone, and other tissues by increasing n-3 PUFA and decreasing n-6 PUFA levels. This subsequently improved bone properties. In animals with secondary osteoporosis due to PKD, IBD, or obesity with insulin resistance, the positive role of flaxseed/oil was reflected in a marked improvement of bone strength, BMD, and BMC, suggesting a possible use of flaxseed/oil as a dietary supplement in those conditions. The effect of flax lignans on bone is controversial; however, it seems that younger females are more sensitive to dietary flax lignans, although the effect, whether positive or negative, is minimal and obscure. Overall, the consumption of flaxseed oil may be more beneficial for bone health, especially for individuals with osteoporosis and those with certain diseases contributing to bone loss. The authors suggest that there is a need to further elucidate how ALA plays a role in regulating bone metabolism. Studies comparing ALA with longer-chain n-3 PUFAs, such as EPA and DHA, should be conducted to establish optimal guidelines for the general public. (Editor's comments)