Calcium, vitamin D, and protein are among the dietary essentials for skeletal health. Concerns about recent changes in the dietary balance of n-6 to n-3 fatty acids in the United States

Dietary fat is an important source of energy for mammals, but it was not until the advent of intravenous feeding that the importance of essential fatty acids was widely accepted. “Essential” fatty acids were identified in research with patients on total parenteral nutrition or with malabsorption syndromes, as they developed dermatitis, changes in visual and neural function, and immunomodulatory disorders. The fatty acids can be divided into two categories: saturated fatty acids (solid at room temperature) and unsaturated fatty acids (liquid at room temperature). The unsaturated fatty acids are of two classes: monounsaturated fatty acids (in olive and canola oil), with only one double bond, and polyunsaturated fatty acids (PUFAs, in corn, sunflower, flax seed, and other oils). Cold-water fish, fish oils, walnuts, wheat germ, and organ meats are good sources of omega-3 PUFAs. The omega-3 family has its first double bond located at the 3rd carbon from the methyl end (designated as omega-3, ω-3, or n-3). Chain length is designated by the number of carbons, with long-chain PUFAs having 20 and 22 carbons. Mammalian cells cannot introduce the double bond at the n-3 and n-6 positions; hence, those fatty acids are “essential” and their precursors must be derived from the diet. Omega-3 PUFAs are α−linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), having 18, 20, and 22 carbons respectively. The omega-6 fatty acids are linoleic acid, gammalinolenic acid (GLA), and arachidonic acid (AA). On the basis of observational and randomized trial data, as well as plausible mechanisms for benefit, the American Heart Association (AHA) [1] and the Institute of Medicine [2] recommend that all adults eat fish, particularly fatty fish, regularly (twice/week) to reduce cardiovascular disease risk. The AHA advises patients with coronary heart disease to consume ~1g/d of EPA and DHA combined. The typical U.S. adult, however, consumes only 100-200 mg/d of EPA+DHA [3]. Many people find it difficult to eat two fish meals per week, and the recommendation of ~1g/d of EPA+DHA translates to more than one fish meal daily. In addition, concerns have been raised about environmental contamination of the fish supply. Thus, fish oil supplements may be a preferable way to achieve compliance. Calcium, vitamin D, and protein are among the dietary essentials for skeletal health. Concerns about recent changes in the dietary balance of n-6 to n-3 fatty acids in the United States [4] has raised interest in differential cellular effects of dietary fatty acids and in their potential skeletal effects. ANIMAL INTERVENTION STUDIES More information is available from animal than clinical intervention studies. Dietary fish oil protected mice from bone loss following ovariectomy, and also prevented the elevation of RANKL that was seen in T-cells from ovariectomized mice supplemented with corn oil [5]. That study supports the view that one of the mechanisms by which dietary n-3 fatty acids reduce bone loss in ovariectormized mice is by inhibition of osteoclast generation and activation. Other research shows direct evidence of beneficial effects on bone formation. Dietary fish oil protected retired breeder rats from bone loss following ovariectomy, with a marked increase in mineral apposition rate [6]. In many animal studies, n-3 fatty acids or a low ratio of n-6 to n-3 fatty acids show a positive influence on bone. In 1994, EPA was shown to inhibit bone loss in ovariectomized rats that were maintained on a low calcium diet [7]. Shortly thereafter, it was reported that both EPA and DHA prevented the increase in bone fragility in diabetic rats, and that EPA prevented osteopenia even in diabetic rats fed a low zinc diet, which was used as a potent accelerator of diabetic osteopenia [8]. Subsequently, it was shown that n-3 PUFA deficiency caused severe osteoporosis in rats. Further, when deficient animals were replenished with n-3 PUFA, the ratio of n-3 to n-6 PUFA in bone compartments was restored and the process of bone degradation was reversed [9]. The beneficial effects of n-3 fatty acids appear to be associated with downregulation of PGE2 formation with a net enhancement of bone formation OMega-3 faTTy acidS and bOne healTh