September 1, 2012
Mark S. McMahon, BS, MSc, MD
Abstract
To the Editor:
Over the past several years, interest has increased in the effect of consumption of dietary fish oil.1 Beneficial effects of omega-3 polyunsaturated fatty acids on bone metabolism have been described. A common recommendation for the general population is to consume at least 2 weekly portions of fatty fish.
Nutritionally important omega-3 fatty acids include alpha-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid, all of which are polyunsaturated. The human body cannot synthesize omega-3 fatty acids de novo, but it can form eicosapentaenoic acid and docosahexaenoic acid from alpha-linolenic acid. These conversions occur competitively with omega-6 fatty acids, which are closely related chemical analogues derived from alpha-linolenic acid. The synthesis of omega-3 fatty acids from alpha-linolenic acid within the body is slowed by the omega-6 analogues.
Modern Western diets typically have ratios of omega-6 to omega-3 in excess of 10 to 1; the optimal ratio is thought to be 4 to 1 or lower. The higher omega-6/omega-3 ratio in dietary oils has been implicated in causing osteoporosis.
Studies have shown a beneficial effect of omega-3 fatty acids on bone mineral density. Sham and ovariectomized mice were fed diets containing 5% corn oil or 5% fish oil. Significantly increased bone mineral density loss was observed in the ovariectomized mice fed corn oil, whereas mice fed fish oil were significantly less osteoporotic. Omega-3 fatty acids added in vitro caused a significant decrease in osteoclastic tartrate-resistant acid phosphatase activity in bone marrow cells compared with omega-6 fatty acids. Omega-3 fatty acids also inhibited bone marrow macrophage nuclear factor kappa B activation induced by the receptor activator of nuclear factor kappa B ligand in vivo.
In regard to bone formation, supplementing the diets of growing rats with omega-3 fatty acids results in greater bone formation in rats. Omega-3 fatty acids also increased alkaline phosphatase activity in osteoblastic cells in culture.
An additional study used the fat-1 mouse, a transgenic model that synthesizes omega-3 fatty acids from omega-6 fatty acids to directly determine whether the outcome of bone health was correlated with omega-3 fatty acids. Fat-1 ovariectomized mice exhibited significantly lower levels of receptor activator of nuclear factor kappa B ligand and tartrate-resistant acid phosphatase in their serum and higher bone mineral density than ovariectomized controls.
The fat-1 mouse was also used in a study to determine whether the fat-1 gene modulates the fatty acid composition of bone phospholipids. The authors concluded that dietary fatty acid incorporation into bone alters its metabolism through changes in the fatty acid composition of membrane phospholipids. Alteration of the membrane phospholipid fatty acid composition was thought to influence bone cell signaling and bone mineralization.
A more recent study evaluated whether a diet enriched in eicosapentaenoic or docosahexaenoic acid for the entire adult life of mice could improve bone microstructure and strength.
Eicosapentaenoic acid reduced the age-related decline in osteocalcin and increased leptin and insulin-like growth factor-1 levels. These data indicate that the long-term intake of omega-3 fatty acids may improve cortical bone properties by an increase in leptin and insulin-like growth factor-1 levels.
