Abstract

The association of red blood cell n-3 and n-6 fatty acids with bone mineral density and hip fracture risk in the women’s health initiative.

Omega-3 (n-3) and omega-6 (n-6) 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 = 400 pairs) was completed within the Women’s 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 1:1 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 α-linolenic acid (tertile 3 [T3] hazard ratio [HR]: 0.44; 95% confidence interval [CI], 0.23-0.85; p for linear trend 0.0154), eicosapentaenoic acid (T3 HR: 0.46; 95% CI, 0.24-0.87; p for linear trend 0.0181), and total n-3 PUFAs (T3 HR: 0.55; 95% CI, 0.30-1.01; p for linear trend 0.0492). Conversely, hip fracture nearly doubled with the highest RBC n-6/n-3 ratio (T3 HR: 1.96; 95% 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 n-3 PUFAs (Spearman’s rho = 0.45, p < 0.0001), total n-6 PUFAs (rho = 0.17, p < 0.0001) and linoleic acid (rho = 0.09, p < 0.05). These results suggest that higher RBC α-linolenic acid, as well as eicosapentaenoic acid and total n-3 PUFAs, may predict lower hip fracture risk. Contrastingly, a higher RBC n-6/n-3 ratio may predict higher hip fracture risk in postmenopausal women.

Orchard TS, Ing SW, Lu B, Belury MA…
J. Bone Miner. Res. Mar 2013
PMID: 23018646 | Free Full Text

The full text has a nice chart showing the hazard ratios for the various fatty acids they looked at.

The Omega-6:Omega-3 ratios and their respective hazard ratios were:

[Hazard Ratios] for hip fracture by tertiles of RBC FAs with multivariate adjustment for risk factors per Robbins and colleagues37 are reported in Table 3. No significant associations were found between RBC total SFA, MUFA, or PUFA and risk of hip fracture. However, there was a significant inverse linear association between hip fracture risk and total n-3 FAs in RBCs (p for linear trend 0.0492). When examining individual n-3 FAs, there was a 56% lower relative risk of hip fracture with highest RBC ALA (tertile 3 [T3] hazard ratio [HR]: 0.44; 95% CI, 0.23–0.85; p for linear trend 0.0154), and a 54% lower hip fracture risk with highest EPA levels (T3 HR: 0.46; 95% CI, 0.24–0.87; p for linear trend 0.0181) compared to T1. Neither DHA nor the n-3 index was significantly associated with risk of fracture. In contrast, hip fracture risk nearly doubled in women in the highest tertile of the n-6/n-3 FA ratio (HR T3: 1.96; 95% CI, 1.03–3.70; p for linear trend 0.0399). Because the n-6/n-3 FA ratio in RBCs primarily reflects the ratio of AA to EPA and DHA, we further examined the relation of the AA/EPA + DHA ratio to hip fracture risk. Similar to the n-6/n-3 FA ratio, a higher AA/EPA + DHA ratio produced higher HR for hip fracture, but the association was not significant (T3 HR: 1.69; 95% CI, 0.86–3.31; p for linear trend 0.1242). Although the direction of association between total n-6 FAs, AA, and hip fracture was toward harm, there was no significant relation of either total n-6 FAs or AA with hip fracture. There was an inverse direction of association between LA and hip fracture risk, but again, this was not statistically significant (T3 HR: 0.77; 95% CI, 0.40–1.49; p for linear trend 0.5140). Inclusion of additional potential confounders (alcohol consumption, total energy intake, total calcium intake, total vitamin D intake, and multivitamin use) in the model produced similar results….