Osteoporosis, once viewed as an expected aspect of ageing among women, is no longer considered to be either age or gender-dependent. Many risk factors are, however, associated with the development of osteoporosis and osteoporotic fracture, including: low peak bone mass, hormonal factors, the use of certain drugs (eg, glucocorticoids), cigarette smoking, low physical activity, low intake of calcium and vitamin D, race, small body size, and a personal or family history of fracture.  Osteoporosis is now considered to be largely preventable, thanks to progress in the scientific understanding of its causes, diagnosis and treatment.
Balancing bone resorption with bone formation
The primary characteristic of osteoporosis is a reduction in bone mass due to an increase in bone resorption over bone formation. Most new bone is added during childhood and teenage years and continues until the age of around 30 years, at which time the peak bone mass is reached. After this age bone resorption slowly begins to exceed new bone formation leading to bone loss, which, for women, occurs fastest in the first few years after menopause. Whilst bone loss continues as we age, there are dietary and lifestyle factors that can positively influence the degree and rate of bone loss. Optimisation of bone health is therefore a process that must occur throughout the lifespan in order to reduce the risk of developing the disease.
The role of cytokines
Cytokines are secreted proteins derived from immune cells. They not only play a direct role in immunity and inflammation but also induce cells to proliferate and differentiate. Interleukin-1 (IL-1), interleukin-6 (IL-6) and tumour necrosis factor alpha (TNF- a) enhance the proliferation of hematopoietic osteoclast precursors. IL-1 and TNF-a then induce the differentiation of these cells into inactive, but more mature osteoclasts before rapidly inducing their activation. In addition, IL-1 and TNF-a also inhibit osteoclast cell death, thereby elongating the osteoclast lifespan. Thus, the rate of bone loss by osteoclasts is directly mediated by these inflammatory cytokines. Cytokines are, therefore, important biological mediators and overproduction contributes to pathogenesis of chronic and acute inflammatory, autoimmune, atherosclerotic and neoplastic diseases. Animal and human studies have shown, however, that production of cytokines can be reduced by certain dietary fatty acids, specifically those containing the long-chain omega-3 polyunsaturated fatty acids (PUFAs) and specifically eicosapentaenoic acid (EPA).[3, 4]
Fish consumption and bone health
It is known that fish consumption protects against bone loss in both men and women. The protective nature of fish consumption seems to be due to its fatty acid composition and, most notably, its high omega-3 content, of which oily fish such as salmon and mackerel are particularly rich sources. The latest data, derived from the Framingham Osteoporosis Study, indicates that individuals who consume three or more servings per week of oily fish have a greater mean baseline bone mineral density (BMD) than those who consume less than three servings per week.
Omega-3 and bone health
It is suggested that the omega−3 and omega−6 polyunsaturated fatty acids (PUFAs) influence bone health through several mechanisms, including enhancement of calcium transport and reducing urinary calcium excretion, their effect on osteoblastogenesis and osteoblast activity, change of membrane function, and decrease in inflammatory cytokines including IL-1, IL-6, and TNF-a. The reduction in cytokine production by dietary supplementation with fish oil is concomitantly associated with increasing calcium absorption, bone calcium and bone density. Evidence also points to the possible benefit of gamma-linolenic acid (GLA) in preserving bone density which, when combined with the omega-3 eicosapentaenoic acid (EPA), enhances calcium absorption, reduces excretion and increases calcium deposition in bone. [4, 6]
It is widely accepted that omega-3 fatty acids offer great potential, either alone or as an adjunct therapy for acute and chronic inflammation and for disorders which involve an inappropriately activated immune response. The benefits of increasing omega-3 intake in order to sustain good health and prevent the onset of disease should also be highlighted. Recommendations for dietary intake of long-chain omega-3 PUFAs required to maintain general health are currently set at 0.45mg/day (SACN/COT 2004), which corresponds to eating two portions of fish weekly, one of which should be white and one oily. Data obtained from the latest National Diet and Nutrition Survey shows that mean consumption of oily fish is currently well below the recommended one portion (140g) per week. While fish and shellfish are a direct dietary source of ‘preformed’ EPA, consumption of marine-sourced EPA is failing to meet the requirements for general health. It seems reasonable, therefore, to suggest that the consumption of purified omega-3 (EPA) supplements should be encouraged to ensure optimal bone health, especially in those individuals who may be at increased risk of developing osteoporosis.
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