The Association of Red Blood Cell n3 and n6 Fatty Acids with Bone Mineral Density and Hip Fracture Risk in The Women's Health Initiative
The Association of Red Blood Cell n3 and n6 Fatty Acids with Bone Mineral Density and Hip Fracture Risk in The Women's Health Initiative
Year: 2013
Authors: Orchard, T.S. Ing, S.W. Lu, B. Belury, M.A. Johnson, K. Wactawski-Wende, J. Jackson, R.D.
Publication Name: J Bone Mineral Res.
Publication Details: Volume 28; Number 3; Pages 505-515
Abstract:
N3 and n6 polyunsaturated fatty acids (PUFA) in red blood cells (RBCs) are an objective indicator of PUFA status and may be related to hip fracture risk. The primary objective of this study was to examine RBC PUFAs as predictors of hip fracture risk in postmenopausal women. A nested case-control study (n of 400 pairs) was completed within the Womens Health Initiative (WHI) using 201 incident hip fracture cases from the Bone Mineral Density (BMD) cohort, along with 199 additional incident hip fracture cases randomly selected from the WHI Observational Study. Cases were matched on age, race, and hormone use with non–hip fracture controls. Stored baseline RBCs were analyzed for fatty acids using gas chromatography. After removing degraded samples, 324 matched pairs were included in statistical analyses. Stratified Cox proportional hazard models were constructed according to case-control pair status; risk of fracture was estimated for tertiles of RBC PUFA. In adjusted hazard models, lower hip fracture risk was associated with higher RBC alpha linolenic acid (tertile 3 [T3] hazard ratio [HR]: 0.44; 95 percent confidence interval [CI], 0.23–0.85; p for linear trend 0.0154), eicosapentaenoic acid (T3 HR: 0.46; 95percent CI, 0.24–0.87; p for linear trend 0.0181), and total n3 PUFAs (T3 HR: 0.55; 95percent CI, 0.30–1.01; p for linear trend 0.0492). Conversely, hip fracture nearly doubled with the highest RBC n6/n3 ratio (T3 HR: 1.96; 95 percent CI, 1.03–3.70; p for linear trend 0.0399). RBC PUFAs were not associated with BMD. RBC PUFAs were indicative of dietary intake of marine n3 PUFAs (Spearman’s rho of 0.45, p 0.0001), total n6 PUFAs (rho of 0.17, p 0.0001) and linoleic acid (rho of 0.09, p 0.05). These results suggest that higher RBC ALA, as well as eicosapentaenoic acid and total n3 PUFAs, may predict lower hip fracture risk. Contrastingly, a higher RBC n6/n3 ratio may predict higher hip fracture risk in postmenopausal women. (Authors abstract)
Postmenopausal osteoporosis is a major public health problem that contributes to an estimated 1.45 million fragility fractures, including 222,000 hip fractures annually in U.S. women. The n3 FAs produce eicosanoids that are generally less inflammatory than n6 FA metabolites and lead to lipid mediators important in resolution of inflammation. They also may act to decrease proinflammatory cytokines that impact critical regulators of bone turnover. Additionally, n3 FAs may positively affect calcium absorption and excretion and modulate transcription factors involved in regulation of bone turnover(8) and stem cell differentiation. Analysis of PUFA levels in biological samples may more objectively assess the relationship of these fatty acids to skeletal outcomes. PUFAs have frequently been measured in various blood fractions (serum, plasma, red blood cells [RBCs]) and adipose tissue in an effort to predict disease outcomes.
Thus, based upon these data suggesting that RBC samples are useful surrogates reflecting PUFA exposure, and because hip fracture is associated with the greatest morbidity and mortality of all osteoporotic fractures, the primary goal of this study was to investigate n3 and n6 FAs in RBCs as predictors of hip fracture risk in postmenopausal women. Secondary goals were to (1) examine the association between major classes of fatty acids (saturated, monounsaturated, and polyunsaturated) in RBCs and hip fracture risk, (2) investigate the relationship between serial measures of BMD across varying levels of RBC n3 and n6 FAs,
and (3) examine the correlation of RBC FAs to self-reported dietary intake of FAs in this cohort.
RBC ALA, EPA, and total n3 FAs were significantly inversely associated with risk of hip fracture in these WHI participants. Conversely, women with an n6 to n3 FA ratio in the highest tertile had nearly twice the risk of hip fracture compared to those in the lowest tertile. In this nested case-control study within WHI, there was no significant relation of total RBC n6 FAs to hip fracture. Nevertheless, it is interesting to note that the direction of association was toward increased risk and this appears to be primarily driven by RBC AA. Total RBC n3 FAs were significantly inversely associated with risk of hip fracture in these postmenopausal women. A similar significant inverse linear trend was noted for individual RBC ALA and EPA. Women in the highest tertile of RBC ALA had a 56 percent lower risk of hip fracture than women in the lowest tertile. Women with highest RBC EPA in this case control study had a 54 percent lower risk of hip fracture. RBC EPA may be directly impacted by intake of marine sources of EPA and indirectly impacted by the metabolism of ALA to EPA. This study examined the association of the AA to EPA to DHA ratio in RBCs to hip fracture risk and observed results similar to those seen for the total n6 to n3 FA ratio in RBCs; namely, higher values for either ratio were associated with increased risk of hip fracture. RBC PUFAs were not associated with baseline BMD or change in BMD in this cohort.
The primary strengths of this study are the use of an objective biospecimen reflecting n3 and n6 FA status, the quality of the hip fracture phenotype, and the availability of data on multiple
covariates contributing to fracture risk. Additionally, mean n3 FA levels were typical of older U.S. women and controls were not specifically selected for fracture risk, which may make the results of this study more broadly generalizable to postmenopausal women in the United States. These results suggest that higher RBC total n3 FAs may predict lower risk of hip fracture. This beneficial association was noted with either ALA or EPA, but not DHA. Whether this reflects conversion of ALA to EPA requires further research. It may be especially important to investigate the mechanisms behind these relationships in light of the relatively large amount of ALA consumed in Western diets compared to the marine n3 FAs and the potential impact on skeletal health that increasing RBC ALA or EPA may have on vulnerable groups such as postmenopausal women. (Editors comments)
